Science book class 8

Modern Concept Science and Environment – 8 143

11.2 Electronic Configuration of an Atom

Systematic arrangement of electrons of Nucleus Distribution
an atom in its shells and sub-shells is of electrons
called electronic configuration. Electrons
are arranged outside the nucleus in Shells
different elliptical orbits but for our
convenience we draw a circular orbit to
arrange electrons. The force of attraction

between a nucleus and electrons holds Electronic configuration of argon
the electrons in their orbit. The number

of electrons that an orbit can hold increases as their distance from the nucleus increases. The

orbits are named using English alphabets like K, L, M, N, etc. The distance between the orbit

and nucleus increases as it moves from the first orbit (K) to other orbits like L, M, N, etc.

2n2 rule for electronic configuration

The maximum number of electrons that can be arranged in a shell of an atom is given by
a formula known as 2n2 rule. ‘n’ in the formula denotes the shell number. Generally, the
outermost shell of an atom holds maximum 8 electrons. So, this rule is applicable in special
cases only.

Arrangement of the electrons in different shells according to 2n2 rule is shown below:

Shell Symbol n 2n2 No. of electrons

First shell K n=1 2.12 2

Second shell L n=2 2.22 8

Third shell M n=3 2.32 18

Fourth shell N n=4 2.42 32

2n2 rule is not applicable for all 118 elements. This rule can be applied in the shells K, L, M and
N only. It cannot be applied in the shells O, P and Q. This is because in these shells we can
arrange 32, 18 and 8 electrons respectively.

The name of elements, symbol, atomic number, atomic weight, number of protons, neutrons
and electrons and electronic configuration of first 20 elements from hydrogen to calcium is
given in the following table:

Atomic Name of Symbol Number of Atomic Electronic Valency
Number Elements Weight Configuration
H P+ n0 e– (p++ n0) 1
He 1 01 K L MN 0
Li 2 22 1
1. Hydrogen Be 3 43 1+0 = 1 1 × × × 2
2. Helium B 4 54 3
3. Lithium 5 65 2+2 = 4 2 × × ×
4. Beryllium
5. Boron 3+4 = 7 2 1 × ×

4+5 = 9 2 2 × ×

5+6 = 11 2 3 × ×

144 Matter

6. Carbon C 6 66 6+6 = 12 2 4 × × 4
7. Nitrogen 3
8. Oxygen N 7 77 7+7 = 14 2 5 × × 2
9. Fluorine 1
10. Neon O 8 88 8+8 = 16 2 6 × × 0
11. Sodium 1
12. Magnesium F 9 10 9 9+10 = 19 2 7 × × 2
13. Aluminium 3
14. Silicon Ne 10 10 10 10+10 = 20 2 8 × × 4
15. Phosphorus 3
16. Sulphur Na 11 12 11 11+12 = 23 2 8 1 × 2
17. Chlorine 1
18. Argon Mg 12 12 12 12+12=24 2 8 2 × 0
19. Potassium 1
20. Calcium Al 13 14 13 13+14 = 27 2 8 3 × 2

Si 14 14 14 14+14 = 28 2 8 4 ×

P 15 16 15 15+16 = 31 2 8 5 ×

S 16 16 16 16+16 = 32 2 8 6 ×

Cl 17 18 17 17+18 = 35 2 8 7 ×

Ar 18 22 18 18+22 = 40 2 8 8 ×

K 19 20 19 19+20 = 39 2 8 8 1

Ca 20 20 20 20+20 = 40 2 8 8 2

The atomic structure and electronic configuration of first twenty elements is given below:

1. Hydrogen (H) 2. Helium (He)

Shell K L MN Shell K L MN
No. of e– 1××× No. of e– 2×××

KK

1p+ 2p+
0n0 2n0

Hydrogen Helium

3. Lithium (Li) 4. Beryllium (Be)

Shell K L MN Shell K L MN
No. of e– 21×× No. of e– 22××

3p+ L 4p+
4n0 K 5n0 K L
Beryllium
Lithium

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5. Boron (B) 6. Carbon (C)

Shell K L MN Shell K L MN
No. of e– 23×× No. of e– 24××

5p+ KL 6p+ KL
6n0 6n0

Boron Carbon

7. Nitrogen (N) 8. Oxygen (O)

Shell K L MN Shell K L MN
No. of e– 25×× No. of e– 26××

7p+ KL 8p+ KL
7n0 8n0

Nitrogen Oxygen

9. Fluorine (F) 10. Neon (Ne)

Shell K L MN Shell K L MN
No. of e– 27×× No. of e– 28××

9p+ K L 10p+ K L
10n0 10n0

Fluorine Neon

11. Sodium (Na) 12. Magnesium (Mg)

Shell K L MN Shell K L MN

No. of e– 281× No. of e– 282×

11p+ KLM 12p+ KLM
12n0 12n0

Sodium Magnesium

146 Matter

13. Aluminium (Al) 14. Silicon (Si)
Shell
Shell K L MN No. of e– K L MN
284×
No. of e– 283×

13p+ KLM 14p+ KLM
14n0 14n0

Aluminium Silicon

15. Phosphorus (P) 16. Sulphur (S)

Shell K L MN Shell K L MN
No. of e– 285× No. of e– 286×

15p+ KLM 16p+ KLM
16n0 16n0

Phosphorus Sulphur

17. Chlorine (Cl) 18. Argon (Ar)

Shell K L MN Shell K L MN
No. of e– 287× No. of e– 288×

17p+ KLM 18p+ KLM
18n0 22n0

Chlorine Argon

19. Potassium (K) 20. Calcium (Ca)

Shell K L MN Shell K L MN
No. of e– 2881 No. of e– 2882

Modern Concept Science and Environment – 8 147

19p+ KLMN 20p+ KLMN
20n0 20n0

Potassium Calcium

ACTIVITY 1

Collect a thin metallic wire and 18 spherical candy of three different colours (6 of one colour). Cut the
wire into two pieces of different lengths. Poke 2 identical candy by the smallest wire. Bend the wire
to make a circle and keep the candy apart from each other. Similarly, poke other 4 candy of the same
colour by another wire. Bend the wire to make a circle again and keep the candy apart from each other.
Take a cardboard and fix the larger circular wire on the board. Then, fix the smaller circular wire inside
the larger wire. At last, fix 6 candy of one colour at one side of the centre and the remaining 6 candy at
other side of the centre.

CONCLUSION : In this way, a model of carbon atom becomes ready.

11.3 Symbol

In chemistry we use one or two letters to represent the name of an element. This representation
is called a symbol.Thus, the abbreviation of full name of an element is called a symbol. A
symbol also represents the atom of that element. It is tedious and time consuming to write the
exact names of the elements in chemistry. Therefore, symbols are used as the short, quick and
convenient way to write the name of elements.

Name of many elements have English words while the name of few other elements come from
Latin words. Thus, the symbols of elements are derived either from first or first and second
or first and third letter of their English or Latin names. The symbol always have first letter as
capital letter and if it has second letter it is always small. Therefore, the symbol of an element
is either one or two English alphabet which is shorter than the name of element.

Some elements are represented by single capital letter. For example,the symbol of Hydrogen is
“H”. Similarly, the symbol of Carbon is “C”. Some other examples are given in the table below:

Elements Symbol Elements Symbol Elements Symbol

Hydrogen H Nitrogen N Phosphorous P

Boron B Oxygen O Sulphur S

Carbon C Fluorine F Iodine I

The symbol ‘H’ is already used for Hydrogen. Therefore, the symbol of Helium is “He”. Here
Hydrogen and Helium are English words. Similarly, Chlorine and Chromium both have “Ch”
as their first and second letter. The symbol of both elements cannot be the same. So, they have
symbol “Cl” for Chlorine and “Cr” for Chromium. Such kind of some elements are given in
the table below:

148 Matter

Elements Symbol Elements Symbol Elements Symbol

Helium He Chlorine Cl Manganese Mn

Lithium Li Chromium Cr Zinc Zn

Berylium Be Magnesium Mg Zirconium Zr

On the basis of above discussion, the symbol of Sodium should be “S” or “So” but it is actually
“Na”. Similarly, the symbol of Potassium is “K”. It is because symbol of Sodium and Potassium
are derived from their Latin words “Natrium” and “Kalium” respectively. Such kinds of other
examples with their Latin words are given in the table.

English Names Latin Names Symbol
Sodium Natrium Na
Potassium Kalium K
Iron Ferrum Fe
Gold Aurum Au
Copper Cuprum Cu
Silver Argentum Ag

11.4 Valency

The combining capacity of an atom or radical with other atom or radical to form a new molecule
(compound) is called valency. In other words, valency of an atom is the number of electrons
gained, lost or shared during chemical combination. The valency of an atom is determined by
the number of electrons in the outermost shell (valence shell). Thus, the number of electrons
which are present in the valence shell of an atom are called valence electrons. Most of the
atoms of different elements tend to gain or lose or share valence electrons in order to attain
stable electronic configuration. To be in stable electronic configuration, an element should
have eight electrons in its outermost shell (except K shell as outermost shell). Therefore, an
atom has tendency to be stable by fulfilling eight electrons in its outermost shell (two electrons
if K shell is the outermost shell). For this, they take part in chemical combination with other
atoms or radicals and become stable.

FACT WITH REASON

Elements with 0 valency are stable elements, why?

Elements with 0 valency are stable elements because they neither gain nor lose electrons in normal
conditions.

11.5 Duplet and Octet State

The state in which an atom contains 2 electrons in its outermost K shell is called a duplet state.
A helium atom is in duplet state. The rule of an atom to be stable by making two electrons in
its outermost shell (K-shell) either by gaining or losing or sharing of electrons is called duplet
rule. Similarly, The state in which an atom contains 8 electrons in its outermost shell is called

Modern Concept Science and Environment – 8 149

an octet state. For example, the atoms of elements: neon, argon, krypton, xenon and radon
are in octet state. The tendency of an atom to make 8 electrons in its outermost shell either by
gaining or losing or sharing of electrons is called octet rule.

A sodium atom has one electron in its outermost shell. So, it has a tendency to lose one electron
to get an octet state. Similarly, a chlorine atom has seven electrons in its outermost shell. So,
it is easy for a chlorine atom to accept one electron from other atoms to make eight electrons
in its outermost shell. Therefore, to form sodium chloride (NaCl), a sodium atom loses one
electron and chlorine atom gains one electron. This makes eight electrons in the last shell of
each atom. That is, there are 8 electrons in each L-shell of the sodium atom and M-shell of the
chlorine. Due to this reason, both sodium and chlorine have valency of 1.

Sodium Chlorine

Atoms of the elements having one, two, and three electrons in their outermost shell usually
lose electrons to gain an octet state. So, their valency is 1, 2, and 3 respectively. Generally,
the atoms having four electrons in their outermost shell share their electrons to other atoms
to gain an octet state. So their valency is 4. Similarly, the atoms having five, six, and seven
electrons accept electrons from the atoms of other elements. So, their valency is 3, 2, and 1
respectively.

FACT WITH REASON

The valency of both oxygen and magnesium is 2, why?

Valency of both oxygen and magnesium is 2 because an oxygen atom satisfies the octet rule by gaining two
electrons and a magnesium atom satisfies the octet rule by losing two electrons.

According to duplet rule, a helium atom has two electrons in its K-shell. It is in a duplet
state. Which makes a helium atom stable. It does not take part in chemical combination with
other atoms. Elements like neon, argon and krypton, etc. are stable as their atoms already
contain eight electrons in the outermost shell. Therefore, they do not take part in chemical
combination with other elements. Elements that do not take part in chemical reaction are
called noble elements. Their valency is 0.

FACT WITH REASON

Neon does not take part in chemical reaction, why?
Neon does not take part in chemical reaction because its atom is in stable state by octet rule. It does not
lose, gain or share electrons with other atoms.
Argon is also called a noble gas, why?
Argon is called a noble gas because its atom has 8 electrons in the outermost shell. Which is an octet state
and makes argon stable. So, argon does not take part in chemical reaction and called a noble gas.

150 Matter

11.6 Radicals

When sodium chloride is kept in water, it ionizes into sodium ion (Na+) and
chloride ion (Cl-). These opposite species are called radicals. Thus, the charged
atoms or groups of atoms that behave as a single unit during chemical reactions
are called radicals. They are also known as ions. They bear charge on them based
on their ability to gain or loss electrons. They are either positively charged or
negatively charged. Due to the presence of charge in them they are unstable and
react with other opposite charged particles to form stable compounds. Based on
their charges, radicals are of two types. They are electropositive radicals (basic
radicals) and electronegative radicals (acidic radicals).

Electropositive radicals

Positively charged atoms or group of atoms are called electropositive radicals. They are also
known as basic radicals or cations. Examples of electropositive radials with their valency are
given in the table below:

Radicals having Radicals having Radicals having Radical having
valency 1 valency 2 (Bivalent) valency 3 (Trivalent) valency 4

(Monovalent) Beryllium (Be++) Boron (B+++) (Tetravalent)

Hydrogen (H+) Stannic (Sn++++)

Lithium (Li+) Magnesium (Mg++) Aluminium (Al+++) Plumbic (Pb++++)

Sodium (Na+) Calcium (Ca++) Ferric (Fe+++) Silicon (Si++++)

Potassium (K+) Strontium (Sr++) Auric (Au+++)
Rubedium (Rb+) Barium (Ba++) Chromium (Cr+++)

Caesium (Cs+) Cupric (Cu++) Manganic (Mn+++)
Cuprous (Cu+) Mercuric (Hg++)

Mercurous (Hg+) Stannous (Sn++)

Ammonium (NH4+) Zinc (Zn++)
Aurous (Au+) Nickel (Ni++)

Manganous (Mn++)

FACT WITH REASON

Ammonium is an electropositive radical, why?
Ammonium is an electropositive radical because it is a positive charged group of atoms (NH4+) which
behaves as a single unit during chemical reaction.

Electronegative radicals

Negatively charged atoms or group of atoms are called electronegative radicals. They are also
known as acid radicals or anions. Examples of electronegative radicals with their valency are
provided in the table below:

Modern Concept Science and Environment – 8 151

Radicals having valency 1 Radicals having valency 2 Radicals having valency 3

(Monovalent) (Bivalent) (Trivalent)
_ __ ___

Fluoride (F ) Oxide (O ) Nitride (N )
_ __ ___

Chloride (Cl ) Sulphide (S ) Phosphide (P )
_ __ ___
Sulphite (SO3 ) Phosphite (PO3 )
Bromide (Br ) _ _ _
_ _
Iodide (I –) Sulphate (SO4 ) Phosphate (PO4 )
_
Nitrite __
(NO2 ) Carbonate (CO_ _3 )

_
Nitrate (NO3 ) Zincate (ZnO–2– )

_ Silicate (SiO3_ _ )
Cyanide (CN )
_
Hydroxide (OH ) Peroxide (O2 ) _ _
_
Chlorate (ClO3 ) _ Dichromate (Cr2O7_ )

Bisulphate (HSO4 _
) Thiosulphate (S2O3 )
_
Bicarbonate (HCO3 )

_
Metaluminate (AlO2 )

FACT WITH REASON

Nitrate is an electronegative radical, why?
Nitrate is an electronegative radical because it is a negatively charged group of atoms (NO3– ) which behaves
as a single unit during chemical reaction.

11.7 Molecular Formula

The symbolic representation of a substance that shows the actual number and kinds of atoms
present in it is called a molecular formula. H2O is a molecular formula of water. It shows that
two atoms of hydrogen and one atom of oxygen are present in it. In case of inert gases, i.e. He,
Ne, Ar, Kr, Xe and Rn, the single atom represents atom as well as molecule because they are
monoatomic molecules. Elements like hydrogen, nitrogen, oxygen, chlorine, bromine and
iodine have two atoms in their molecule, viz. H2, N2, O2, F2, Cl2, Br2 and I2 respectively. So, they
are called diatomic molecules.

Molecular formulae of some other molecules are shown in the table below:

1. Ammonia NH3
2. Carbon dioxide CO2
3. Sodium chloride NaCl

152 Matter

4. Glucose C6­H12O6
5. Calcium carbonate CaCO3
6. Calcium chloride CaCl2
7. Sulphuric acid H2SO4
8. Nitric acid HNO3
9. Magnesium sulphate MgSO4
10. Caustic soda NaOH

Differences between symbol and molecular formula.

SN Symbol SN Molecular formula

1 The abbreviation of the full name of an 1 The symbolic representation of a
element is called a symbol. substance that shows the actual number
and kinds of atoms present in it is called
2 It represents one atom of an element. 2 a molecular formula.
Examples: H, Na, K, etc.
It represents one molecule of an element
Way of writing a molecular formula or a compound. Examples: H2, NaCl,
H2SO4, etc.

We should follow the following steps to write the correct molecular formula of a molecule.

a) Write the symbol of basic (positive) and acidic (negative) radicals side by side.

b) Write the valency of each radical on their upper right corner.

c) Exchange the valency of these radicals. Take HCF if it is necessary.

d) Combine radicals with their exchanged valency.

Examples:

Modern Concept Science and Environment – 8 153

Information obtained from a molecular formula

a) Molecular formula represents one molecule of a substance.

b) It indicates total number of atoms of the same or different element/s in each molecule.

c) It indicates percentage composition of each element present in the compound.

d) The valency or combining capacity of each element can be found from the molecular
formula.

Example: In water molecule (H2O), the valency of hydrogen is 1 and that of
oxygen is 2.

e) We can calculate molecular weight from the molecular formula.

For example,

The molecular weight of water (H2O) = 2 × H + 1 × O = 2 ×1 + 1 × 16 = 18 amu.

11.8 Molecular Weight

The total weight of the molecule of a substance is called molecular weight. It is calculated by
adding the atomic weights of all the atoms present in a molecule. So, the sum of atomic weights
of the atoms present in a molecule is called molecular weight. One molecule of hydrogen (H2)
contains two atoms of hydrogen. So the molecular weight of hydrogen molecule is 2, since
atomic weight of a hydrogen atom (H) is 1.

The molecular weights of some common molecules are given below:

S.N. Molecules Molecular Weights
1. Sodium chloride (NaCl) NaCl = 1 × Na + 1 × Cl

2. Water (H2O) = 1 × 23 + 1 × 35
= 58 amu
3. Carbon diodixde (CO2) H2O = 2 × H + 1 × O
= 2 × 1 + 1 × 16
= 18 amu
CO2 = 1 × C + 2 × O
= 1 × 12 + 2 × 16
= 44 amu

154 Matter

4. Calcium carbonate (CaCO3) CaCO3 = 1 × Ca + 1 × C + 3 × O
= 1 × 40 + 1 × 12 + 3 × 16

= 100 amu

5. Magnesium chloride (MgCl2) MgCl2 = 1 × Mg + 2 × Cl
= 1 × 24 + 2 × 35

= 94 amu

6. Calcium chloride (CaCl2) CaCl2 = 1 × Ca + 2 × Cl
= 1 × 40 + 2 × 35

= 110 amu

7. Ammonium sulphate (NH4)2SO4 = 2 × N + 8 × H + 1 × S + 4 × O
[(NH4)2SO4] = 2 × 14 + 8 × 1 + 1 × 32 + 4 × 16

= 132 amu

11.9 Periodic Table

Scientists have discovered 118 elements so far. Among them, 92 elements are found in nature
and remaining 26 elements are synthesized artificially in laboratories. It is difficult to study all
these elements one by one. So, scientists have arranged these elements into various categories
like groups, periods, blocks, etc. on the basis of their similarities and differences.

a. Mendeleev’s periodic table

Russian chemist Dmitri Ivanovich Mendeleev studied the physical and chemical properties
of 63 known elements and their compounds. After the study, he arranged all the known
elements on the basis of increasing atomic weights. As a result, he found that elements with
similar properties occur at regular intervals. On the basis of above observation, Mendeleev
formulated a periodic law in 1869 AD. According to Mendeleev’s periodic law, “The physical
and chemical properties of elements are a periodic function of their atomic weights.” Periodic
function means, if elements are arranged in an order of increasing atomic weights, the
properties of the elements go on changing with atomic weights. But after a certain interval,
they repeat the properties of previous elements and fall one below another in the same vertical
column. With the help of above periodic law, Mendeleev arranged elements according to
increasing atomic weights. Hence, he found a table known as Mendeleev’s periodic table. In
Mendeleev’s periodic table, elements are classified into horizontal rows called periods and
vertical columns called groups.

Group I A part of Mendeleev’s periodic table
Period 1 H
Period 2 Li Group II Group III Group IV Group V Group VI Group VII Group VIII
Period 3 Na
Be B C N O F

Mg Al Si P S Cl

Modern Concept Science and Environment – 8 155

Period 4 K Ca 1* Ti V Cr Mn Fe Co
Period 5 Rb Cu Zn 2* 3* As Se Br Ni
Period 6 Cs Rh
Sr Y Zr Nb Mo 4* Ru Pd
Ag Cd In Sn Sb Te I Ir
Pt
Ba La Hf Ta W Re Os
Au Hg Th Pb Bi Po At

Merits of Mendeleev’s periodic table
a) Mendeleev’s periodic table was the first scientific and systematic study of all
known 63 elements.

b) In Mendeleev’s periodic table, there were many gaps for undiscovered elements.
After their discovery, they got the proper place in the table.

c) With the help of Mendeleev’s periodic table, properties of several elements were
studied correctly.

Demerits of Mendeleev’s periodic table
a) Mendeleev could not arrange hydrogen properly because it showed the properties
of both alkali metals (IA) and halogens (VIIA).

b) Mendeleev could not arrange Lanthanides and Actinides.

c) In Mendeleev’s periodic table, some elements do not obey the periodic law. In
some places, elements having more atomic weight were placed before those of
less atomic weight. For example, 39.9Ar was placed before 39.1K. Also, 58.927Co was
placed before 58.628Ni.

d) In Mendeleev’s periodic table, more reactive alkali metals (Li, Na, K, etc.) and less
reactive coinage metals (Cu, Ag, Au) are placed together in the same group.

e) Inert gases or noble gases are absent.

b. Modern periodic table

A group of chemists led by Henery Moseley studied the chemical and physical properties
of all known elements. They found that properties of elements depended more correctly on
atomic number rather than atomic weight. They said that atomic number is the fundamental
property of the atom. Afterward, they proposed a new law which is called modern periodic
law. Modern periodic law states that, “The physical and chemical properties of elements are
a periodic function of their atomic numbers.”After the discovery of modern periodic law,
elements were arranged on the basis of increasing atomic numbers. As a result, they found
that elements having similar properties fall one after another in the same vertical column
called group. The elements having gradual change in the properties were in the horizontal
row called period. Hence, a more correct table than Mendeleev’s table was obtained which is
known as modern periodic table. So, “The table which is obtained after arranging elements on
the basis of increasing atomic number is called modern periodic table.”

s-Block Modern Periodic Table 0/18 156 Matter
IA/1
p-Block
1
IIIA/13 IVA/14 VA/15 VIA/16 VIIA/17
IIA/2

2

d-Block

3

IIIB/3 IVB/4 VB/5 VIB/6 VIIB/7 VIII/8 VIII/9 VIII/10 IB/11 IIB/12

4

5
6

7

Lanthanide series
Actinide series

f-Block

Alkali Alkaline Transition Basic Metalloids Non Halogens Noble Lanthan- Actinides
metals earth metals metals metals gases ides
metals

Modern Concept Science and Environment – 8 157

Differences between Modern periodic table and Mendeleev’s periodic table

SN Modern periodic table SN Mendeleev's periodic table

1 It is based on increasing atomic numbers. 1 It is based on increasing atomic weights.

2 In this table, there are eighteen vertical 2 In this table, there are eight vertical

columns. columns.

3 Inthistable,thereisfixedpositionforhydrogen, 3 In this table, there is no fixed position for

noble gases, lanthanides and actinides. hydrogen, noble gases, lanthanides and actinides.

11.10 Periods

In modern periodic table, there are seven horizontal rows, which are called periods. In a
period, elements having gradual change in properties are placed one after another. According
to the number of elements, periods of modern periodic table are divided into four groups.
They are very short, short, long and very long.

S.N. Period Number of elements Types of period
1. First 2 Very short period
2. Second 8 Short period
3. Third 8 Short period
4. Fourth 18 Long period
5. Fifth 18 Long period
6. Sixth 32 Very long period
7. Seventh 26 Very long but incomplete period

11.11 Groups

In modern periodic table, there are a total of 18 vertical columns (IUPAC system) which are
called groups. In a group, elements having similar properties are placed one below another.

In modern periodic table, representative elements are kept in groups IA, IIA, IIIA, IVA, VA,
VIA and VIIA. These elements are called representative elements or normal elements as they
have only one incomplete outer shell. Noble gas elements are kept in 0 (zero) group or group
VIIIA. Similarly, transitional elements are kept in groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and
VIII. The group VIII is further divided into three vertical columns.

Differences between periods and groups

SN Periods SN Groups

1 The horizontal rows of a periodic table 1 The vertical columns of a periodic table

are called periods. are called groups.

2 In periods, atomic size of elements 2 In groups, atomic size of elements increases

decreases while moving from left to right. while moving from top to bottom.

3 Valency of elements increases first 3 Valency of elements remains the same in

(upto group IV) and then decreases a group.

(upto group 0).

4 The elements of the same period have 4 The elements of the same group have

different properties. similar properties.

158 Matter

11.12 Chemical Reaction

When hydrogen gas (H2) burns in oxygen (O2),
it forms water (H2O). Here, hydrogen combines
with oxygen and forms water. When calcium

carbonate (CaCO3) is heated, it decomposes
or breaks down into calcium oxide (CaO) and

carbon dioxide (CO2). Similarly, when zinc
(Zn) reacts with sulphuric acid (H2SO4), it
displaces hydrogen from sulphuric acid and

forms zinc sulphate (ZnSO4) and hydrogen
(H2) gas. So, the combination, decomposition or displacement that occurs in the molecules of
matter during a chemical change is called chemical reaction.

Examples:

a) When hydrogen gas burns in oxygen gas, it forms water, i.e.

Hydrogen + Oxygen Water

H2 + O2 H2O

b) When calcium carbonate is heated, it forms calcium oxide and carbon dioxide, i.e.

Calcium carbonate Calcium oxide + Carbon dioxide

CaCO3 CaO + CO2

c) When zinc reacts with sulphuric acid, it forms zinc sulphate and hydrogen gas, i.e.

Zinc + Sulphuric acid Zinc sulphate + Hydrogen

Zn + H2SO4 ZnSO4 + H2

A chemical reaction is expressed in word equation and chemical equation or formula equation.

11.13 Word Equation

The chemical reaction expressed by writing the full names of reactants and products is called
a word equation.

Examples: Water
Hydrogen + Oxygen Sodium chlorine
Sodium + Chlorine Calcium oxide + Carbon dioxide
Calcium carbonate
Zinc + Hydrochloric acid Zinc chloride + Hydrogen

Modern Concept Science and Environment – 8 159

11.14 Chemical Equation

The chemical reaction expressed by writing symbols and molecular formulae of reactants and
products is called a chemical equation. A chemical equation is more informative than a word
equation.

Examples: 2H2 + O2 ∆ 2H2O
1. 2Na + Cl2 ∆ 2NaCl
2. CaCO3
3. Zn + 2HCl CaO + CO2
4. ZnCl2 + H2

11.15 Reactants and Products

The chemical substances which take part in a chemical reaction are called reactants. The
chemical substances which are produced after chemical reaction are called products. Reactants
are written on the left side of the arrow whereas products are written on the right side of the
arrow while writing a chemical equation.

Examples:

NaOH + HCl H2O + NaCl

Reactants Products

11.16 Unbalanced or Skeleton Chemical Equation

The chemical equation in which the total number of atoms of each element in reactants and
products are not equal is called unbalanced or skeleton chemical equation.

Examples:

H2 + O2 H2O

In above equation, the number of oxygen atoms in reactant and product sides is not equal, so

it is called an unbalanced chemical equation. Some more examples of unbalanced chemical

equations are as follows:

Mg + HCl MgCl2 + H2

KOH + H2SO4 K2SO4 + H2O

H2O2 MnO2 H2O + O2
CaCl2 + H2O
Hydrogen peroxide Water Oxygen

HCl + Ca(OH)2

11.17 Balanced Chemical Equation

The chemical equation written by balancing the total number of atoms of each element in
reactants and products is called balanced chemical equation. In this chemical equation, the
number of atoms of each element is equal in reactants and products. It gives more information
than the unbalanced chemical equation. Above unbalanced chemical equations can be
balanced as follows:

160 Matter

Examples:
2H2 + O2 ∆ 2H2O

Mg + 2HCl MgCl2 + H2

2KOH + H2SO4 K2SO4 + 2H2O
2H2O + O2
2H2O2 MnO2

2HCl + Ca(OH)2 CaCl2 + 2H2O

In the above chemical equations, the numbers of atoms of the same element in reactant and

product sides are equal. So, they are called balanced chemical equations.

Methods of writing balanced chemical equation

Following points should be remembered while balancing the chemical equation:

a) First of all, the chemical change is written correctly in the form of a word equation.

For example: Hydrogen + Oxygen Water

b) The word equation is written correctly in the form of formula equation or chemical
equation.

For example: H2 + O2 H2O

c) The number of atoms of each element are balanced by using suitable coefficient without

changing the molecular formulae of reactants and products.

For example: 2H2 + O2 2H2O

d) The number of atoms in the biggest molecule should be balanced before balancing the

number of hydrogen and oxygen atoms.

This method of balancing chemical equation is called a hit and trial method.

Some more examples of balanced chemical equation

a) Word equation : Sodium + Chlorine Sodium chloride
Unbalanced formula equation : Na + Cl2 NaCl
Balanced formula equation: 2Na + Cl2
2NaCl
b) Word equation : Potassium + Oxygen
Unbalanced formula equation : K + O2 Potassium oxide
Balanced formula equation: 4K + O2 K2O
2K2O
c) Word equation : Magnesium + Oxygen
Unbalanced formula equation : Mg + O2 Magnesium oxide
Balanced formula equation: 2 Mg + O2 MgO
2MgO
d) Word equation : Zinc + Hydrochloric acid
Unbalanced formula equation: Zn + HCl Zinc chloride + Hydrogen
Balanced formula equation: Zn + 2HCl ZnCl2 + H2
ZnCl2 + H2

Modern Concept Science and Environment – 8 161

e). Word equation: Sulphuric acid + Sodium hydroxide Sodium sulphate + Water

Unbalanced formula equation: H2SO4+NaOH Na2SO4 + H2O

Balanced formula equation: H2SO4 + 2NaOH Na2SO4 + 2H2O

Information obtained from a balanced chemical equation

Following pieces of information can be obtained from a balanced chemical equation.

a) The names of reactants and products
b) The symbols and molecular formulae of reactants and products
c) The total number of atoms or molecules of reactants and products
d) The ratio of molecular weight of reactant and product molecules
e) The type of chemical reaction

Limitation of a balanced chemical equation

A balanced chemical equation cannot provide information about

a) The physical state of reactants and products
b) Concentration of reactants
c) Conditions required for the reaction like heat, light, pressure, catalyst, etc.
d) The duration of a chemical reaction

11.18 Modification of Chemical Equation

To make the chemical reaction more informative following modifications are done.

a) The physical state of reactants and products are denoted by ‘s’ for solid, ‘l’ for liquid, ‘g’
for gas and ‘aq’ for aqueous solution.

b) Concentration of reactants are denoted by ‘dil.’ for dilute and ‘conc.’ for concentrated solution.

c) The conditions like temperature, pressure, light, catalyst, etc. are written above or below
the arrow.

d) A double-way arrow ( ) is used for reversible reaction and a single way arrow

(→) is used for the irreversible reaction.

For example,

2Na(s) + 2H2O (l) 2NaOH (aq) + H2↑

N2(g) + 3H2(g) 2NH3↑

2HgO (s) 2Hg (l) + O2↑

11.19 Reversible Reaction

The chemical reaction in which the products can recombine to give back the reactants is called
a reversible reaction.

For example,

H2 + I2 2HI

When hydrogen (H2) and iodine (I2) are heated, hydrogen iodide (HI) is
formed. When hydrogen iodide is heated in a closed vessel, it also forms

hydrogen and iodine. Therefore, the given reaction is a reversible reaction. Reversible reactions

are written by giving a double-way arrow between reactants and products as follows:

162 Matter

H2 + I2 2HI

N2 + 3H2 2NH3

2H2 + O2 2H2O 2H2O 2H2 O2

11.20 Irreversible Reaction

The chemical reaction in which the products cannot recombine to give back reactants is called
an irreversible reaction. A single way arrow is used to denote an irreversible reaction.

Examples:

CaCO3 CaO + CO2↑

2Na + 2H2O 2NaOH + H2↑

2KClO3 2KCl + 3O2↑

ANSWER WRITING SKILL

1. What is 2n2 rule?
Ans: 2n2 is a rule to calculate the maximum number of electrons in a particular shell of an atom. For

example, in M-shell, the maximum number of electrons becomes 18.
2. State modern periodic law.
Ans: According to modern periodic law ‘the physical and chemical properties of elements are periodic

function of their atomic number’.
3. What are the terms used for vertical columns and horizontal rows in periodic tables?
Ans: Vertical columns of the periodic table are termed as groups and horizontal rows of the periodic

table are termed as periods.
4. Write down the charge, mass and position of different sub-atomic particles.

Sub-atomic particles Charge Mass Position

Proton Positive 1 amu Nucleus

Electron Negative 1/1837 amu Shells

Neutron Chargeless 1 amu Nucleus

5. Atoms are electrically neutral, why?

Ans: Atoms are electrically neutral because they consist of equal number of positively charged protons
and negatively charged electrons which cancel each other to make atoms neutral.

6. Chemical equation is more informative than word equation, why?

Ans: Chemical equation is more informative than word equation because from the chemical equation,
we can know the symbol of elements and radicals with their valency. It makes easy to balance the
chemical equation which is not possible in word equation.

Modern Concept Science and Environment – 8 163

7. Write down any two differences between elements and compounds.
Ans: Differences between elements and compounds are:

S.N. Elements S.N. Compounds

1 The simplest and pure form of a 1 The pure chemical substances whichare

substance which cannot be broken obtained by the chemical combination

into simpler substance are called of two or more than two elements are

elements. called compounds.

2 There are 118 elements. Examples: 2 There are millions of compounds.

Hydrogen, Oxygen, Magnesium. Examples: Ammonia, Nitrous oxide.

8. Write down the importance of a periodic table.
Ans: The importance of a periodic table are:

i) Periodic table makes study of elements easier and systematic.
ii) It helps to predict the properties of new elements.
iii) It explains the gradual change in properties of elements.
iv) With the help of a periodic table, we can compare the properties of different elements.

STEPS EXERCISE

STEP 1

5. Fill in the blanks with appropriate words.
a) O2 represents one ……………… of oxygen.
b) Atomic number is the number of ……………… present in an atom.
c) The electrons present in the outermost shell of an atom are called …………
electrons.
d) ……………… is the sum of mass of atoms present in a molecule.
e) The valency of argon is………………
f) Modern periodic table has……………periods.
g) The substances which are formed as a result of chemical reaction are called
…………
h) Horizontal rows of a periodic table are called ……………

6. Write True for the correct and False for the incorrect statements.
a) Compounds are impure substances.
b) The valency of Aluminium is 3.
c) Mendeleev’s periodic table is based on increasing atomic number.
d) An atom is electrically neutral.
e) The size of atoms decreases downwards in the groups.
f) Changing the magnesium ribbon into magnesium oxide is a chemical change.

164 Matter

g) The molecular weight of calcium carbonate is 100.
h) The valency of helium is 2.
i) Precipitated products are symbolized by downward arrow.

STEP 2

7. Answer the following questions in one word.
a) How many short periods are there in a modern periodic table?
b) How many elements are there in very short periods of modern periodic table?
c) Who introduced modern periodic table?
d) What is the valency of inert gases?
e) What is the name of tiny, rigid and solid centre of an atom?

8. Write any two differences between:
a) Atoms and molecules
b) Elements and compounds
c) Mendeleev’s periodic table and modern periodic table
d) Periods and groups
e) Molecular mass and atomic mass
f) Electropositive radicals and electronegative radicals
g) Protons and electrons
h) Word equation and formula equation
i) Balanced equation and skeleton equation

9. Give reasons.
a) Argon is also called a noble gas.
b) The valency of both oxygen and magnesium is 2.
c) H2 is a molecule but H is an atom.
d) Neon does not take part in a chemical reaction.
e) Ammonium is an electropositive radical.
f) Nitrate is an electronegative radical.

10. Write down the molecular formula and name of the elements present in following
compounds:
a) Sulphuric acid
b) Calcium carbonate
c) Nitric acid
d) Glucose
e) Ammonia
f) Magnesium sulphate

Modern Concept Science and Environment – 8 165

STEP 3

11. Answer the following questions.
a) Define atomic number and atomic mass. Which can exist freely in nature H or H2?
b) What is electronic configuration? Show electronic configuration of calcium and
chlorine.
c) What is 2n2 rule? How many electrons does M shell accommodate?
d) What are noble elements?
e) Explain octet and duplet rule with an example of each.
f) Define radicals and write down their types with any four examples of each.
g) What is a periodic table? How many elements are discovered so far?
h) What is Mendeleev’s periodic law? List out the merits and demerits of Mendeleev’s
periodic table.
i) What is modern periodic law? List out the characteristics of modern periodic table.
j) Write down the molecular formula of the following molecules:
hydrochloric acid, water, calcium chloride, ammonia and nitric acid
k) What is atomic number of the following elements?
potassium, chlorine, magnesium, sulphur, aluminium and nitrogen
l) Calculate the molecular mass of the following molecules.
CH4 , NaCl, CaO, Ca(OH)2, MgO and CO2
m) What is a chemical reaction?
n) Write down the pieces of information that are not provided by a balanced chemical
equation.
o) List out any three major modifications in chemical equation that provide complete
information.
p) Define reversible and irreversible chemical equation with an example of each.
q) Complete and balance the following chemical equations.

i) KClO3 ……………. + 3O­2

ii) NaOH + …………… NaCl + ……………

iii) Zn + HCl ……………+ H2

iv) KClO3 KCl + ……………

v) ……………+ Ca(OH)2 CaSO4 + H2O

12. Draw the electronic configuration of the following atoms:

a) Helium b) Carbon c) Neon

d) Sodium e) Phosphorous f) Calcium

166 M i x tu r eEstimated teaching periods Theory Practical
3 2
UNIT

12 Mixture

Syllabus issued by CDC Paper Chromatography

 Introduction to do distillation
 Types of distillation
 Simple distillation
 Fractional distillation
 Chromatography and its principle
 Application of chromatography

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 explain and demonstrate distillation and fractional distillation.
 explain and demonstrate chromatography.

Key terms and terminologies of the unit

1. Mixtures : Mixture is a substance which is obtained by the combination of two or more

substances in any proportion by weight if they do not react with each other.

2. Distillation : The method which is used to separate liquids from solids or from other liquids

through vapourization (evaporation) followed by condensation is called distillation.

3. Simple distillation : The distillation process which is used to separate a pure substance from its

mixture that dissolves another substance in it is called simple distillation. Or the

distillation process which is used to separate a mixture of two miscible liquids

having a wide range boiling point is called simple distillation.

4. Fractional distillation : The distillation process which is used to separate the mixture of two or more

liquids having close boiling points is called fractional distillation.

5. Chromatography : Chromatography is a modern technique for separating components of a

mixture by passing the mixture through a media which absorbs the components

at different rates.

6. Paper chromatography : Paper chromatography is a process of separating the coloured components of a mixture

with the help of a paper which can absorb different substances at different rates.

7. Column chromatography : Column chromatography is a process of separating the components of a

mixture by passing a the mixture through a column of silica gel, aluminium

oxide or chalk powder.

12.1 Introduction

We come in contact with various substances in our daily life. Most of these substances are
impure while some are pure. In chemistry, pure substances are elements and compounds
while impure substances are mixtures.

Modern Concept Science and Environment – 8 167

Mixture is a mass which is obtained by the combination of two or more substances in any
proportion by weight if they do not react with each other. Milk, muddy water, coffee, tea,
salt solution, sugar solution, water and ink, camphor and sand, etc. are some examples of
mixtures. Mixtures may be heterogeneous or homogeneous. They may have one or more
components. But sometimes we need to separate components of a mixture for our use. To
separate components of a mixture, we use various physical and chemical processes according
to the properties of components of the mixture.

FACT WITH REASON

Compound is a pure substance but mixture is an impure, why?
A compound is a pure substance because it contains only one type of molecules. The elements
in a molecule are chemically bonded in a fixed proportion by weight. But in a mixture two or more
substances are not chemically bonded and they are in any random proportion by weight. So, mixture
is an impure substance.

Different component of mixtures have different physical properties like size, state, solubility,
colour, density, boiling point and freezing point. These properties are used to separate the
components from their mixture. Filtration, sedimentation and decantation, distillation,
chromatography, crystallization, etc. are some of the methods toseparatecomponents of
mixtures based on their properties. Among these processes, distillation and chromatography
have been briefly discussed in this unit to separate homogeneous mixtures.

12.2 Distillation

The method which is used to separate liquids Thermometer

from solids or from other liquids through Hot water out
vapourization (evaporation) followed by
condensation is called distillation. The pure Stand
liquid that is formed by the condensation of
Liebig's condenser Cold water in

vapour is called distillate. The set of apparatus Round bottom
flask

used in distillation process is called still. The Wire gauze Conical flask
still consists of a Round Bottom Flask (boiler), Mixture (reciever)
Liebig’s condenser and a collecting vessel
Bunsen burner

or receiver (beaker). It is the most popular

process of separating two or more components Distillation

present in a homogenous mixture. However, the components of the mixture to be separated

by distillation process should have different boiling points. Distillation method can be used

to separate solution of solid and liquid or two or more miscible liquids having different

boiling points. Mixtures like solution of salt and water, iodine and alcohol, alcohol and water,

potassium chloride and water, crude oil, etc. can be separated by using distillation process.

FACT WITH REASON

Distillation is the best method to purify a substance from a mixture, why?

Distillation is carried out by heating a mixture at certain temperature so that only particular matter will evaporated
and be collected in another vessel. So, distillation is the best method to purify a substance from a mixture.

168 Mixture

Types of distillation Memory Tips

There are two types of distillations. They are: The distillation process which is
a) Simple distillation used to separate a mixture of two
b) Fractional distillation. miscible liquids having a wide
range boiling point is called simple
a) Simple distillation distillation.

The distillation process which is used to separate a pure substance from its mixture that
dissolves another substance in it is called simple distillation. The simple distillation is more
appropriate for separating the components of solutions having large or significant differences
in their boiling points. Sea water contains high amount of salt in it. When the sea water is
distilled, pure water vapourizes and collected in the receiver while the salt remains behind in
the flask separating salt from the water.

FACT WITH REASON

Drinking water in Arabian countries is more costly, why?

People of Arabian countries and sea-shore countries use distillation process to get pure water from sea
water. So, drinking water in such areas is costly than other areas.

ACTIVITY 1

OBJECTIVE : To separate pure water from salt solution. Thermometer

REQUIREMENT : Round bottom flask, stand, Liebig’s Stand

condenser, collecting vessel, clamp, burner, thermometer,

wire gauze, tripod stand and salt solution. Water out

PROCEDURE : Liebig's
1. Take a RB flask of about 500ml and pour about Water bath condenser

200ml salt solution in the flask. Mixture of
2. Clamp the flask to the stand and place wire salt and water
Water in

gauze in its base. Burner Beaker
3. Clamp the condenser with delivery tube in Tripod stand (reciever)

another stand.

4. Place one end of the delivery tube inside the flask through the rubber cork and another end to

the collecting vessel.

5. Put a thermometer in another hole in the cork to maintain the temperature.

6. Turn on the burner.

7. Gradually flow cold water through one hole of the condenser after the solution is heated.

OBSERVATION : Vapour from the heated solution gets raised and passes to the condenser through
delivery tube. Continuous inflow of cold water through the condenser cools the vapour inside it.
After cooling, the vapour changes into water droplets and collected into the vessel. After complete
vapourization, the salt is left in the bottom of the flask.

RESULT : Pure water and salt are separated from the mixture by using simple distillation process.

CONCLUSION : Therefore, distillation process can be used to separate components of mixture having
different boiling points.

Modern Concept Science and Environment – 8 169

FACT WITH REASON

The mixture of the substances having the same boiling points cannot be separated by distillation, why ?
Distillation is carried out by heating the mixture at a certain temperature. So that only a particular
matter will be evaporated and be collected in the receiver at a particular temperature. If both
components have the same boiling point both of them will evaporate together and cannot be separated.

b) Fractional distillation Refinery gas

Simple distillation process cannot separate the Fractionating Gasoline
components of miscible mixtures properly if the boiling column
points of the components are closer to each other. In this Kerosene
case, a complex form of distillation process is used. It is Storage
called fractional distillation. Thus, the distillation Diesel fuel
process which is used to separate the mixture of two or Crude oil Heater Lubrication
more liquids having close boiling points is called oils
fractional distillation.
Steam

In this process, the mixture is heated in a boiler and Asphalt

then passed to a fractionating column. Fractionating Oil refining

column consists of various layers of plates inside it. Memory Tips

The bottom of the column has higher temperature and it 1. Fractional distillation is used
to separate different kinds of
lowers as the height of the column increases. Each plate petroleum products from the
crude oil.
in the column has a tube perpendicular to the column.
2. Asphalt which is used in
In the fractionating column more volatile component road surfacing is one of the
by-products of the crude
rises higher in the column and passes through the tube oil obtained as a result of
fractional distillation.
at that layer. At the same time, less volatile component

remains lower in the column and passes through

respective column. The vapour from each of the column

is condensed to obtain pure liquid. For example, the

mixture of alcohol and water can be separated by using

fractional distillation. Alcohol boils at Thermometer
Water out
780C while water boils at 1000 C. When the Stand
Fractional column
mixture is heated at or near 780 C, alcohol

evaporates faster and passes through the Liebig's condenser

column and gets collected in the receiver.

Water also evaporates but very less at Mixture of Stand Water in
alcohol and
that temperature. So, water vapour comes Round bottom flask water Reciever
Burner

back into the boiler after entering into the Fractional distillation

fractionating column at low temperature.

Fractional distillation can be used to get a concentrated and pure alcohol from the mixture of
alcohol and water. Similarly, aviation fuel, diesel, petrol, kerosene and asphalt are obtained
from the hydrocarbons by using this method.

170 Mixture

Differences between simple distillation and fractional distillation.

SN Simple distillation SN Fractional distillation

1 The distillation process which is used 1 The distillation process which is used

to separate a pure substance from to separate the mixture of two or more

its mixture that dissolves another liquids having close boiling points is

substance in it is called simple called fractional distillation.

distillation.

2 The simple distillation is more 2 Fractional distillation process can

appropriate for separating the separate the components of the miscible

components of solutions having large liquids properly even if the boiling

differences in their boiling point. points of the components are closer to

each other.

12.3 Chromatography

Chromatography is a modern technique Stationary phase

for separating components of a mixture by

passing the mixture through a medium which

adsorbs the components at different rates. This

technique is based on the principle of selective Mobile phase

adsorption of components of mixture. The

mixture used in this process is normally in a liquid or gaseous state. This mixture is allowed to

move over the surface of a solid or a liquid medium. The moving substance is called the mobile

phase and the medium is called the stationary phase. As the mobile phase moves, it separates

out into its components on the stationary phase. Then, we can identify them one by one.

Different substances have different speed in different media. So, the components of a mixture

can be separated by using various mobile phase and stationary media in chromatography.

FACT WITH REASON

Chromatography cannot separate a mixture if the two components have the same speed through an
adsorbing medium, why?

Chromatography separates a mixture on the principle that different substances have different speed
through an adsorbing medium. So, if two components of a mixture have the same speed through an
adsorbing medium, mixture cannot be separated by chromatography.

Types of chromatography Memory Tips

There are different types of chromatography. They are: The word chromatography has
a) Paper chromatography been derived from two Greek
words kroma and graphy. Here,
b) Column chromatography
kroma means colours and graphy

c) Gas chromatography means process of writing or

We will discuss paper chromatography and column recording. So, different colours
can be separated by using
chromatography in this unit.
chromatography technique.

Modern Concept Science and Environment – 8 171

a) Paper chromatography

Paper chromatography is a process of separating the coloured components of a mixture with
the help of a paper which can adsorb different substances at different rates. Different coloured
mixtures like red, blue, black, etc. can be separated by using this technique. Filter paper is
used as a medium to separate colours in paper chromatography.

FACT WITH REASON

Simple filter paper can be used to demonstrate paper chromatography, why?
Simple filter paper can be used to demonstrate paper chromatography because filter paper is of
a porous and fibrous structure. Through such filter paper the components of a mixture travel with
different speed and get separated.

ACTIVITY 2

OBJECTIVE : To separate coloured components of a black marker ink.
REQUIREMENT : Filter paper, black marker, plate, distilled water in a dropper
PROCEDURE : Take a circular filter paper or a white tissue paper.
Mark a dark point at the centre of the paper using a black marker. Put
the paper in a plate. Pour some drops of distilled water on the black
spot at the centre of the filter paper using a dropper and wait.
OBSERVATION: After about 30 minutes, you will see different
coloured rings on the filter paper.
RESULT: Black colour ink of the marker is a mixture of various colourful components.
CONCLUSION: Different coloured components of a mixture have different speed on the filter paper. So,
different coloured rings can be observed on the filter paper.

ACTIVITY 3

OBJECTIVE: To separate mixtures of different coloured inks using a filter paper.

REQUIREMENT: Filter paper, mixture of red and blue ink, glass rod, beaker, distilled water

PROCEDURE: Glass rod
1. Take a beaker of volume 250ml and pour about 50ml of

water in it. Beaker
2. Add about 3 drops of red ink and 3 drops of blue ink in the Filter paper

beaker.

3. Cut a rectangular piece of filter paper that should have Mixture of red
breadth less than half of the diameter of the beaker. and blue ink

4. Place a delivery tube or glass rod horizontal on the

surface of beaker.

5. Clip or attach the piece of filter paper to the rod or delivery tube so that a little portion of the

filter paper should be under distilled water.

6. Leave the experiment for about 30 minutes.

172 Mixture

OBSERVATION: We can see a band of blue ink and red ink on the filter paper.

RESULT: Red and blue colours get separated from their mixture.

CONCLUSION: Therefore, the coloured components of a mixture having different speed through an
adsrobing medium can be separated on a same filter paper by using paper chromatography.

b) Column Chromatography

Column chromatography Memory Tips

is a process of separating Column chromatography or
the components of a adsorption chromatography is
mixture by passing the
mixture through a column based on the principle that an
of silica gel, aluminium adsorbent can adsorb different
oxide or chalk powder. In substances to different extent.

column chromatography, Column chromatography

column of adsorbent like silica gel, chalk powder, alumina, cellulose powder, etc. is taken

and kept in a vertical glass tube. Different colours have different speed. So, the colour with

high speed or mobility passes downwards and gets adsorbed by the adsorbent near to the

bottom. Similarly, the colour with lower speed gets adsorbed to less extent. Finally, the colour

components get separated as colourful layers in the column of adsorbent.

Write any two differences between paper chromatography and column chromatography.

SN Paper chromatography SN Column chromatography

1 Paper chromatography is a process of 1 Column chromatography is a process

separating the coloured components of separating the components of the

of a mixture with the help of a paper mixture by passing the mixture through

which can adsorb different substances the column of silica gel, aluminium oxide

at different rates. or chalk powder.

2 In paper chromatography, a filter 2 In column chromatography, a column of

paper can be used as a medium to adsorbent like silica gel, chalk powder,

separate colours. alumina, cellulose powder, etc. can be

taken as an adsorbent.

ACTIVITY 4

OBJECTIVE : To separate a mixture of red, blue and black ink by column chromatography.

REQUIREMENT: Transparent glass or plastic tube, silica gel, a stand with Stand Mixture of ink
clamp, collecting vessel, extract of flower Solvent
Glass tube

PROCEDURE : Adsorbent
(silica gel)

1. Take a glass or a plastic tube of about 8-10 cm. Beaker

2. Heat one end of the tube and make its mouth narrow. Solvent
3. Use silica gel to fill about 4-6 cm depth of the tube.
4. There should be some gaps of about 5 cm on the upper wide end of the tube.
5. Clamp the tube to the stand properly so that the narrow end of the tube facing downwards.

Modern Concept Science and Environment – 8 173

6. Keep a collecting vessel below the narrow mouth of the tube.
7. Take a mixture of extract of flowers and pour from the open end of the tube
8. Wait for some times to move the mixture towards the bottom. The time depends upon the

mixture we have chosen.

OBSERVATION: We can see one of the colours from the mixture oozing out through the narrow mouth of
the tube. It is collected in the vessel below. After some time, liquid of another colour starts to ooze out
through the mouth. Take another collecting vessel and collect it. Repeat the same process to collect all
the colours.

RESULT: Colum chromatography separates different colours present in the extract of flowers.

CONCLUSION: In this way, the coloured components of a mixture can be separated by using column
chromatography.

Application of chromatography

Chromatography is used to separate the components of a mixture for different purposes.
Following are some common applications of chromotography:
a) It is used for detecting the presence of drugs in urine, blood, soil, etc.
b) It is used to separate colours from various natural and artificial dyes.
c) It is used for studying the complex mixtures of food, perfumes, petrochemical and

pharmaceutical productions.
d) It is used in forensic science to identify the samples taken from the crime acts.
e) It is used in pollution monitoring for identifying small concentrations of unknown

pollutants present in the sample of air and water.

ANSWER WRITING SKILL

1. Which method of separation of mixture is used to produce table salt?
Ans: Evaporation of sea water is done to produce table salt.
2. Which process of separation of mixture is used to separate a mixture of miscible liquids having

close boiling points?
Ans: Fractional distillation process is used to separate a mixture of miscible liquids having close boiling

point.
3. Which process of separation of mixture is used to separate red and blue ink from the black ink?
Ans: Paper chromatography is used to separate blue ink and red ink from the black ink.
4. What is distillate?
Ans: A pure liquid collected in a vessel by condensing evaporated liquid is called distillate.
5. Chromatography cannot separate the components of a mixture if they move with the same speed

through an adsorbing medium, why?
Ans: Chromatography separates the mixture on the principle that different substances have different

speed through an adsorbing medium. So, if the two components of a mixture have the same speed
then both will reach at the same distance at the same time. So, it is impossible to separate the
components of a mixture if they move with the same speed through an adsorbing medium.

174 Mixture

6. Distillation is the best process for purification of the sea water, why?

Ans: Distillation is carried out by heating the mixture at a certain temperature so that only a particular
matter will get evaporated and be collected in another vessel. So, distillation is the best method for
purification of the sea water.

7. Differentiate between distillation and chromatography.

Ans: Differences between distillation and chromatography are:

S.N. Distillation S.N. Chromatography

1 Distillation is used to separate 1 Chromatography is a technique to

liquids from solids or miscible liquids separate the coloured components of a

through vaporization followed by mixture with the help of an adsorbing

condensation. medium.

2 It is used in purification of liquids. 2 It is used to identify the components of
a mixture.

8. What are the advantages of chromatography?

Ans: Advantages of chromatography are:

i) Chromatography is a simple and cheap method to separate the coloured substances.

ii) It takes very less time to separate mixtures.

iii) It is used to separate different colours if the amount of sample is very less.

iv) Modern technique of chromatography can separate colourless particles too.

STEPS EXERCISE

STEP 1

1. Fill in the blanks with appropriate words.
a) The pure liquid that is formed by the condensation of vapour is called …………
b) Fractional distillation is the process of separating the mixing components having
close…………….. points.
c) Chromatography is based on the principle of selective ……………. of the
components of a mixture.
d) Coloured components of a mixture can be separated by …………….
e) The boiling point of water is ………and that of alcohol is……

2. Write True for the correct and False for the incorrect statements.
a) In a mixture, two or more substances are mixed in any proportion by mass.

b) The components of a mixture do not undergo any chemical change.

c) Fractional distillation is not useful for two miscible liquids.
d) In column chromatography we can use silica gel to separate colours.
e) We can separate cream from milk by chromatography.

Modern Concept Science and Environment – 8 175

STEP 2

3. Answer the following questions in one word.
a) Name the method to separate salty water.
b) Name the apparatus used to boil a mixture in distillation.
c) Which process of separation of mixtures separates a mixture of miscible liquids?
d) Which method of separation is used to separate red and blue ink from the black ink?
e) Which method is used to separate drugs from blood?

4. Write any two differences between:
a) Simple distillation and fractional distillation
b) Paper chromatography and column chromatography
c) Distillation and chromatography

5. Give reasons.
a) A compound is a pure substance but mixture is not.
b) Distillation is the best process of separation and purification of the sea water.
c) Chromatography cannot separate a mixture if its components have the same
speed through an adsorbing medium.
d) Simple filter paper can be used to demonstrate chromatography.
e) A mixture with the components having the same boiling point cannot be separated
by distillation.

STEP 3

6. Answer the following questions
a) What is mixture? Give any four examples.
b) What is distillation? Can you get pure alcohol from the mixture of alcohol and
water using simple distillation process? Explain the process.
c) What are miscible liquids? Give two examples.
d) Define fractional distillation. Explain the working principle of fractional
distillation in short with its diagram.
e) Define chromatography. What are the applications of chromatography? List any four.
f) Define paper chromatography and column chromatography.
g) Explain the separation of colours using filter paper.
h) Name the specific processes that are used to separate the following mixtures.
i) Mixture of salt and water
ii) Mixture of hydrocarbons
iii) Black ink of a marker
iv) Mixture of red, blue and black ink
i) Describe an experiment to separate the colours present in the extract of flowers.

7. Draw the following diagrams.
a) An apparatus setup for fractional distillation
b) An apparatus setup for column chromatography
c) An apparatus setup for paper chromatography

176 M etal anEdstiNmoatne-dmteetaachling periods Theory Practical
7 1
UNIT

13 Metal and Non-metal

Syllabus issued by CDC Gold

 Introduction to metals, non-metals and metalloids.

 Classification of elements into metals, non-metals and metalloids.

 Position of metals, non-metals and metalloids in periodic table

 Introduction, characteristics and uses of some metals, non-metals and metalloids (gold, silver,
copper, iron, aluminium, silicon and sulphur).

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 define metals, non-metals and metalloids with examples.
 classify elements into metals, non-metals and metalloids.
 state the position of metals, non-metals and metalloids in the periodic table.
 introduce and explain characteristics and uses of some metals, non-metals and metalloids (gold,

silver, copper, iron, aluminium, silicon and sulphur).

Key terms and terminologies of the unit

1. Metal : Metals are those electropositive elements which are malleable, ductile and good conductor
of heat and electricity.

2. Metalloids : Those elements which show properties of both metals and non-metals are called metalloids.

3. Non-metals : Non-metals are those electronegative elements which are non-malleable, non-ductile and
bad conductor of heat and electricity.

4. Malleability : The property of a metal by virtue of which it can be converted into a thin sheet by heating
and hammering is called malleability. So, metals are malleable.

5. Ductility : The property of a metal by virtue of which it can be converted into a thin and long wire by
heating and hammering is called ductility. So, metals are ductile.

6. Gold : Gold is a yellow shiny metal which does not react with acid, base, salt, air, water and
moisture in ordinary conditions. It is highly malleable and ductile in nature.

7. Silver : Silver is a white shiny metal. It is highly malleable, ductile and the best conductor of heat and
electricity.

8. Copper : Copper is a reddish-brown metal. It is highly malleable, ductile and good conductor of heat
and electricity.

9. Iron : Iron is a reactive, grey metal. It is malleable, ductile and conductor of heat and electricity.

10. Aluminium : Aluminium is a light bluish white metal which is highly malleable and ductile in nature.

Modern Concept Science and Environment – 8 177

11. Silicon : Silicon is a grey coloured metalloid which is mainly obtained from sand or silica and
used as a semi-conductor in electronic devices.

12. Sulphur : Sulphur is a yellow crystalline non-metallic solid which is insoluble in water and mostly
used to make gun powder, fire crackers, matchsticks, fungicides, etc.

13.1 Introduction

A total of 118 elements have been discovered so far. On the basis of physical and chemical
properties like electrical conductivity, malleability, ductility, boiling point, melting point,
these elements are divided into metals, non-metals and metalloids. Most of the metals are
hard, malleable, ductile, and good conductor of heat and electricity. For example, iron, silver,
copper, aluminium, gold, etc. Similarly, most of the non-metals are non-malleable, non-ductile,
and bad conductor of heat and electricity. For example, sulphur, phosphorus, chlorine, iodine,
etc. In between metals and non-metals, there are some elements which show properties of
both metals and non-metals. They are called metalloids. For example, silicon, germanium,
antimony, bismuth, etc.

13.2 Metals

Metals are usually solids (except mercury), strong and hard (except some soft metals like
lithium, sodium, etc.) at room temperature. They are malleable, ductile and good conductor
of heat and electricity. Metals are used to make ornaments, jewelleries, heavy machineries,
vehicles, utensils, simple to advanced tools and other equipments of daily use. Iron, copper,
zinc, aluminium, tin, gold, silver, etc. are the most commonly used metals.

Cup Jewellery Kettle Plate

Position metals in periodic table

In modern periodic table, metals are kept on the left side. The elements of groups IA, IIA, IIIA, IB
to IIB and VIII of the modern periodic table are metals. Metals are divided into following groups-

i) Alkali metals: Alkali metals are kept in IA group of the modern periodic table. This
group comprises lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs),
and francium (Fr). Sodium (Na) and potassium (K) are the most abundant alkali metals.
Alkali metals are soft and silvery white. Their melting points are low. They are highly
reactive metals. They react with water to give soluble hydroxides which are alkaline.

ii) Alkaline earth metals: Alkaline earth metals are kept in the group IIA of the modern
periodic table. This group includes beryllium (Be), magnesium (Mg), calcium (Ca),
strontium (Sr), barium (Ba), and radium (Ra). Calcium (Ca) and magnesium (Mg) are

178 Metal and Non-metal

both abundant in soil and rocks. The alkaline earth metals are relatively harder than

alkali metals. They have high melting points. They also react with water to form soluble

hydroxides which are alkaline. Memory Tips

iii) Transition metals: The transition metals lie between Lanthanides and actinides are

groups IIA and IIIA of the modern periodic table, i.e. taken out from the transition

from groups IIIB to IIB including the group VIII. elements and kept below the main

iv) Lanthanides and actinides: Lanthanides and actinides table. So, they are also called inner
are kept below the main body of the periodic table. transitional elements.

Periodic table showing metals, nonmetals and metalloids

13.3 Metalloids

Metalloids are the solid elements that have the characteristics between the metals and
non-metals. They behave as semiconductors of heat and electricity. So, they are used as
semiconductors in electronic devices. Silicon is an extensively used semi-conductor.

Position of metalloids in periodic table

In the periodic table, metalloids are located between metals and non-metals. They are kept
below non-metals on the groups IVA, VA and VIA. Examples of metalloids are boron (B),
silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po).

FACT WITH REASON

Why are metalloid used in electronic devices?
Metalloids are used in electronic devices excessively because they can work properly even at high
temperature.

Modern Concept Science and Environment – 8 179

13.4 Non-metals

Non-metals are those electronegative elements which are non-malleable, non-ductile and bad
conductor of heat and electricity. They exist in different states of matter at room temperature.
Solid non-metals are brittle in nature. They are poor conductor of heat and electricity (good
insulators). Non-metals generally have low melting points and boiling points.

Position of non-metals in periodic table

In the periodic table, non-metals are kept on the right side next to the rightmost group of inert
gases. They are also located above the metalloids in IVA, VA and VIA groups. Hydrogen (H),
carbon (C), nitrogen (N), phosphorus (P), oxygen (O), sulphur (S), selenium (Se), etc. are major
non-metals.

Differences between metals and non-metals.

SN Metals SN Non-metals

1 Metals usually occur in solid state 1 Non-metals occur in all three physical

except mercury. states viz. solid, liquid and gas.

2 They have high melting point and 2 They usually have low melting and low

high boiling point. boiling point.

3 They possess metallic lustre. 3 They do not possess metallic lustre.

4 They are malleable and ductile. 4 They are usually brittle in nature.

5 They are good conductor of heat and 5 They are bad conductor or insulators of

electricity. heat and electricity.

13.5 Some useful metals, metalloids and non-metals

We use different kinds of metals, non-metals and metalloids in our daily life. They make a
part of our life. The modern technology is totally based upon them. Among them some useful
metals, non-metals and metalloids are discussed in this unit.

i) Gold

Gold is a soft and yellow shiny metal. It is found in quartz veins Gold
and alluvial soil as a free metal or sometimes in a combined
state. It is widely distributed but a rare metal on the earth crust.
It is almost always associated with silver in variable amounts.
The naturally occurring gold-silver alloy is called electrum.

Properties of gold

i) Pure gold is the most malleable and ductile in nature.
ii) It has a yellow bright lustre.
iii) It is one of the softest metals (hardness, 2.5 to 3 in Mho scale).
iv) It is a good conductor of heat and electricity.
v) It melts at about 1064°C, boils at about 2856°C and has a specific gravity of 19.3.

180 Metal and Non-metal

vi) Gold is an extremely inactive metal. Also called a noble metal. Therefore, it does

not react with air, moisture, acid, alkali and Memory Tips

most solvents. Aquaregia is a mixture of three

vi) It gets dissolved in aquaregia to give gold parts of concentrated hydrochloric
chloride. acid and one part of concentrated

Uses of gold nitric acid.

i) Gold is widely used for making coins, idols, statues, medals, jewellery, etc.

ii) It is used to make gold leaf electroscope, in the arts of gilding and lettering.

iii) Radioisotopes of gold are used in biological research and in the treatment of
cancer.

iv) It is also required in the manufacturing of televisions, video cassette recorders,
cell phones and computers.

v) The cockpit windows of modern aircraft, windows of modern office buildings,
parts of space vehicles are coated with gold.

vi) Gold is highly resistant to bacteria. So, dentists use it to repair or replace damaged
or decayed teeth.

FACT WITH REASON

Gold is also used in dentistry, why?

Gold is also used in dentistry to make teeth because it is highly resistant to air, water, chemical and
bacteria.

Gold is used to make jewellery, why?

Gold has an attractive luster. It doesn’t react with air and water. Thus, gold does not get rusted as like
iron. Gold is the most malleable and ductile, making it possible to beat gold into very thin sheets and
to draw wires. So, gold is used to make jewellery.

ii) Silver Silver

Silver is a soft metal with white shiny lustre. It occurs
naturally in both pure state and in the form of ore (combined
form). The important ore of silver is argentite. Silver also
occurs as a constituent of lead, copper and zinc ores. About
half of the world production of silver is obtained as a by-
product in the processing of such ores.

Properties of silver Memory Tips

i) Pure silver has the highest electrical and Ore is a naturally occurring solid
thermal conductivity among all metals.
material from which a metal can

ii) After gold, silver is the most malleable and be extracted profitably.

ductile metal.

iii) Silver melts at about 962°C, boils at about 2162°C and has a specific gravity of 10.5.

iv) It does not react with oxygen or water at ordinary temperatures.

Modern Concept Science and Environment – 8 181

v) It does not react with dilute acids.
vi) It reacts with concentrated nitric acid and sulphuric acid.

Uses of silver

i) Silver is used for making jewelleries, coins, idols, statues, etc.
ii) It is used to coat smooth glass surfaces for mirrors.
iii) It is used in the circuit of electrical and electronic components.
iv) Compounds of silver are used in medicines.
v) It is used to fill cavities caused by tooth decay.
vi) The silver bromide, silver chloride, silver iodide, etc. are used in emulsions for

photographic plates, film, and paper.

iii) Copper

Copper is a reddish-brown metal. It is the 25th most abundant element
in the earth’s crust. It also occurs freely in nature but mostly found in
combined state. It is also found along with other metals such as gold,
silver, bismuth, lead, etc. The major ore of copper is chalcopyrite.

Properties of copper Copper
i) Copper is a reddish-brown metal.

ii) It is a good conductor of heat and electricity.

iii) It melts at about 1085°C, boils at about 2562°C and has a specific gravity of 8.95.

iv) It is malleable and ductile in nature.

v) It turns into black oxide on heating.

vi) It reacts with acid to form salt and hydrogen gas.

Uses of copper

i) Copper is used for making household cooking utensils.
ii) It is widely used for making electric wires in house wiring, lamp cords, electrical

devices such as generators, motors, controllers, signalling devices, electromagnets,
communications equipment, etc.
iii) It is used for making coins, medals, statues, etc.
iv) It is used for making alloys like brass, bronze, etc.
v) Copper compounds are used in medicines for making fungicides and germicides.
vi) It is used for electroplating.
vi) Copper compounds are used in chemical laboratories.

iv) Iron

Iron is one of the oldest metals discovered and used by mankind. It is
the most popular metal for making different things. Iron does not
occur in free state but  mainly occurs  in a combined state making

Iron

182 Metal and Non-metal

different ores. Haematite is a major ore of iron. Other important ores of iron are magnetite,
siderite, limonite, etc. It is also one of the most abundant metals in the earth’s crust. Very small
amount of iron occurs in natural water, plants’ body and even in human blood. Red blood
cells contain iron in haemoglobin which makes our blood red in colour.

Properties of iron Memory Tips

i) Pure iron is a greyish-white metal. Humans' blood looks red due to
the presence of iron containing
ii) It can be magnetized easily at ordinary compound called haemoglobin.
temperature but difficult when heated.

iii) Pure iron melts at about 1538°C, boils at about 2861°C and has a specific gravity of 7.87.

iv) It is a good conductor of heat and electricity.

v) It is malleable and ductile in nature.

vi) When a piece of iron is exposed to moist air for a long time, a layer of reddish-
brown substance is formed on its surface called rust. The process of formation of
rust is called rusting.

vii) Iron reacts with dilute acids to form a salt and hydrogen gas.

viii) It reacts with oxygen to form iron oxides.

ix) It is an active metal. It combines with the halogens, sulphur, phosphorus, carbon,
and silicon.

Use of iron

i) Iron is used for manufacturing machines, vehicles, arms and bullets, etc.
ii) Iron is used for making cooking utensils and household utensils.
iii) Iron rods and wires are used in construction of buildings, bridges, hydropower

stations, tunnels etc.
iv) Iron is used for manufacturing stainless steel which is widely used for making

various domestic, medical and industrial equipment and utensils.
v) It is used in the production of galvanized iron sheets and electromagnets.
vi) Iron compounds are used for medicinal purposes in the treatment of anaemia.

v) Aluminium Aluminium

Aluminium is the most abundant metal in the earth’s crust.
It is the third most plentiful chemical element on our planet
(after oxygen and silicon). It is the second most popular metal
for making things (after iron or steel). Aluminium never
found as a free metal but commonly found combined with
silica as silicates. But it cannot be extracted economically
from silicates. Bauxite is a hydrated oxide of aluminium. It is
a major ore of aluminium.

Modern Concept Science and Environment – 8 183

FACT WITH REASON

Aluminium is preferred for long distance high voltage transmission, why?
Aluminium is preferred for a long distance high voltage transmission because it is lighter in comparision
to copper.
Aluminium reacts with air but rusting does not occur in aluminium, why?
Aluminium reacts with air but rusting does not occur because once it reacts with air,a thin film of
aluminium oxide is formed over its surface. This thin film covers aluminium from its outer surface. As
a result it prevents further rusting.

Properties of aluminium

i) Aluminium is a silvery-white metal with light weight.

ii) It melts at 660°C, boils at 2519°C and has a specific gravity of 2.7.

iii) It is a reactive metal as it readily reacts with oxygen (air).

iv) It is highly malleable and ductile in nature.

v) It does not get rusted. Memory Tips

vi) Aluminium does not react with moisture at Aluminium  is present in more
normal temperatures. than 270 minerals. It is the most

vi) It is a good conductor of heat and electricity. abundant element on the earth's

vii) Oxides of aluminium show both acidic and crust after oxygen and silicon. It

basic properties. is also the most abundant metal

Uses of aluminium found naturally on the earth.

i) Aluminium is used in the construction of aircraft, trains, automobiles, etc.
ii) It is used for making cooking utensils and other household utensils.
iii) It is used for making electric wires and overhead electric cables.
iv) It is used for making coins and alloys.
v) It is used for making foils and food wrappers.
vi) It is used for making silver paints.
vii) Aluminium is widely used for making containers, flexible packages and cans.

FACT WITH REASON

Aluminium is widely used in aircrafts, trains and vehicles, why?

Aluminium is widely used for making parts of aircraft, trains and vehicles because it is a light metal
and does not get rusted. It is highly malleable and ductile.

vi) Silicon Silicon

Silicon is the most abundent element in the earth’s crust apart from
oxygen. About 28 % of the earth’s crust contains silicon. It does not
occur in Free State. It is found in the form of silicon dioxide and other

184 Metal and Non-metal

complex silicates. Sand (silicon dioxide) is a major source of silicon. The silicates (oxides of
silicon) are the chief constituents of clay, soil, and rocks. The mineral like quartz, cristobalite
and tridymite are the naturally occurring crystalline forms of silica. Silicon occurs in the rocks
in the form of feldspar, mica, zeolite, etc.

Properties of silicon

i) Silicon is a metalloid or semi-metal as it shows the properties of both metals and
non-metals.

ii) It is a grey coloured solid substance.
iii) It does not occur in free state in nature.
iv) It occurs in crystalline as well as amorphous forms.
v) Silicon melts at about 1414°C, boils at about 3265°C and has specific gravity 2.33.
vi) Silicon does not react with air, water and moisture at normal temperatures.
vii) Silicon does not react with nitric acid, hydrochloric acid, sulphuric acids, etc.
viii) It reacts with sodium hydroxide to form sodium silicate and hydrogen gas.

Uses of silicon

i) It is mostly used as a semiconductor in transistors and electronic equipment.
ii) Oxide of silicon is used in the manufacture of different glasses.
iii) It is used for making ceramic utensils.
iv) It is used for making polish and colours.
v) It is used in the steel industries as a constituent of silicon-steel alloys.
vi) Zeolite, a compound of silicon, is used as a water softening agent.
vii) A compound of silica called silica gel is used as a drying and decolorizing agent.
viii) Silicon monoxide is used as a coating to protect the outer surface of materials.

vii) Sulphur Sulphur

Sulphur is a tasteless, odourless, light yellow, non-metallic
element. It is found both in free stateas well as in combined
states. Free sulphur occurs due to the weathering of sulphur
containing rocks. In a combined state, sulphur is found as
sulphide of lead, zinc, copper, mercury, silver, etc. Sulphur
is found mixed with gypsum and pumice stone especially in
volcanic regions. It is present in the molecules of many organic
substances such as mustard, egg, hair, proteins and oil of
garlic, etc.

Properties of sulphur

i) Sulphur is a yellow crystalline solid.
ii) It is insoluble in water.
iii) It is tasteless and odourless element.

Modern Concept Science and Environment – 8 185

iv) The vapours of sulphur are poisonous for bacteria and fungi but not for human
beings and other animals.

v) It is a poor conductor of heat but bad conductor of electricity.
vi) It reacts with oxygen to give sulphur dioxide.
vii) It melts at 115.21°C and boils at 444.6°C.
viii) It combines with hydrogen and other metallic elements in presence of heat to

form sulphides.
ix) Sulphur slowly reacts with moisture and produces sulphuric acid.

Uses of sulphur

i) Sulphur is used in manufacturing of sulphur compounds, such as sulphuric acid,
sulphites, sulphates, sulphur dioxide, etc.

ii) It is used in manufacturing of sulpha drugs and many skin ointments.
iii) It is used in the production of matchsticks, fire crackers, dyes, gunpowder, etc.
iv) Sulphur is used for making fungicides, germicides and insecticides.
v) The sulphur salt, sodium thio-sulphate, commonly called hypo, is used in

photography for “fixing” negatives and prints.
vi) It is used in beauty parlours for hair-setting
vii) It is used for hardening of rubber by heating rubber with sulphur.

ACTIVITY 1

1. List the various metallic equipment which are using in your home. Identify the metals and mention
their name.

2. List the various non-metallic equipment which are using in your home. Identify the non-metals and
mention their name.

ANSWER WRITING SKILL

1. Mention the position of metals, non-metals and metalloids in the modern periodic table.
Ans: In modern periodic table metals are present in left side, non-metals are present in right side and

metalloids are present between metals and non-metals.
2. Define Malleability and ductility.
Ans: The property of metals to beaten into a thin sheet or any shape is called malleability. Similarly, the

property of metals due to which it can be changed into a thin and long wire is called ductility.
3. Which metal is present in human blood? Name the molecule in which this metal is present.
Ans: Iron is present in human blood. The molecule in which iron is present is called haemoglobin.
4. Why is gold used for making ornaments?
Ans: Gold is used for making ornaments because it is yellow shiny metal with highly malleable and

ductile in nature. It also does not react with air, water, acids, base and other chemicals in ordinary
conditions.

186 Metal and Non-metal

5. Copper is used for making cooking utensils, why?

Ans: Copper is used for making cooking utensils because it is a good conductor of heat. Copper is also
highly malleable and ductile, making it possible to give desing to cooking pots.

6. Iron does not occur in a free state in nature, why?

Ans: Iron is a reactive metal. It reacts with air, water, acid, base and other elements in normal conditions
to form various compounds. So, iron does not occur in a free state in nature.

7. Differentiate between metals and metalloids.

Ans: Differences between metals and metalloids are:

S.N. Metals S.N. Metalloids

1 Metals are malleable, ductile and 1 Metalloids show the properties of both

good conductor of heat and electricity. metals and non-metals.

2 Atoms of metals lose electrons to form 2 Atoms of metalloids share electrons

an electrovalent bond. Example:- to form a covalent bond. Example:-

iron, gold, copper, etc. silicon, arsenic, etc.

8. Write down any four uses of sulphur.

Ans: Four uses of sulphur are:
i) Sulphur is used in manufacturing of sulphur compounds, such as sulphuric acid, sulphites,
sulphates, sulphur dioxide, etc.
ii) It is used in manufacturing of sulpha drugs and many skin ointments.
iii) It is used in the production of matchsticks, fire crackers, dyes, gunpowder, etc.
iv) Sulphur is used for making fungicides, germicides and insecticides.

STEPS EXERCISE

STEP 1

1. Fill in the blanks with appropriate words.
a) Pure …….. has the highest malleability and ductility.
b) …… is a reddish brown layer formed on the surface of iron exposed in moist air.
c) …………… is used in softening hard water.
d) ………… wires are used to make high voltage transmission line.
e) ………… is a liquid metal.
f) Gun powder is made from …………….

2. Write True for the correct and False for the incorrect statements.
a) Mercury is a strong and solid metal.
b) Non-metals are bad conductors of heat and electricity.
c) Argentite is an ore of gold.
d) Aluminium is used to make body parts of aircrafts.
e) Sulphur is widely used as a semiconductor.
f) Rusting occurs rapidly on iron.

Modern Concept Science and Environment – 8 187

STEP 2

3. Answer the following questions in one word.
a) How many elements have been discovered so far?
b) Write down the name of non-metal which is insoluble in water and used for
making gun powder?
c) Which side of the modern periodic table contains metals?
d) Which metal is present in human blood?
e) Write the property of a metal that can be used to change it into an electric wire?

4. Write any two differences between:

a) Metal and non-metal b) Metal and metalloid

c) Alkali and alkaline earth metals d) Iron and aluminium

e) Gold and Silver

5. Give reasons.
a) Metalloids are used in electronic devices.
b) Aluminium reacts with air but rusting does not occur.
c) Aluminium is widely used in aircrafts, trains and vehicles.
d) Aluminium is preferred to copper for long distance high voltage transmission line.
e) Gold is also used in dentistry.
f) Gold is used to make jewelleries.

6. Identify each of the elements with following properties.
a) It is insoluble in water and used to make gun powder.
b) It has specific gravity of 2.6 and used to make parts of aircrafts.
c) It is a semimetal whose oxide is used to make glasses.
d) It is obtained from argentite ore and used to make coins and medals.
e) It is a yellow shiny metal and used to make jewelleries.

STEP 3

7. Answer the following questions
a) What are metals? Write four examples of metals.
b) Write any four properties of metals.
c) Define non-metals with any four properties and four examples.
d) What are metalloids? Write any two examples.
e) Write down any four properties of the following elements.
i) Silver ii) Copper iii) Aluminium iv) Gold
v) Iron vi) Silicon vii) Sulphur
f) Write down any four uses of the following elements.
i) Silver ii) Copper iii) Aluminium iv) Gold
v) Iron vi) Silicon vii) Sulphur

188 Ac i d , B a sEestaimnadteSdatletaching periods Theory Practical
4 2
UNIT

14 Acid, Base and Salt

Syllabus issued by CDC pH Colour chart

 Introduction to acid
 Properties and uses of acids
 Some acids used in our daily life and their sources
 Introduction to bases
 Properties and uses of bases
 Introduction to salt
 Properties and uses of salt
 Indicators, pH and pH scale

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 introduce acid, base and salt and explain their properties and uses.
 define indicators and identify acid, base and salt by using a litmus paper.
 prepare litmus paper by using petals of flowers.
 introduce pH and pH scale.

Key terms and terminologies of the unit

1. Acids : Acids are those chemical substances which give hydrogen ions when dissolved in
water.

2. Strong acids : Acids which undergo almost complete dissociation in aqueous solution and produce
high concentration of hydrogen ions are called strong acids.

3. Weak acids : Acids which undergo partial dissociation in aqueous solution and produce low
concentration of hydrogen ions are called weak acids.

4. Organic acids : Acids which are obtained from living organisms and contain carbon atoms are called
organic acids.

5. Inorganic acids : Acids which are obtained from minerals and do not have carbon atom are called
inorganic acids or mineral acids.

6. Bases : Metallic oxides and hydroxides are called bases.

7. Alkalis : The bases that dissolve in water and produce hydroxyl (OH-) ions are called alkalis.

8. Strong bases : Bases (alkalis) which give more amounts of hydroxyl ions in aqueous solution are called
strong bases.

9. Weak bases : Bases (alkalis) which give very less amount of hydroxyl ions in aqueous solution are
called weak bases.

Modern Concept Science and Environment – 8 189

10. Salt : Salt is a chemical substance which is formed by partial or complete replacement of
hydrogen ion of an acid molecule by a metal or ammonium radical.

11. Indicators : Indicators are those chemical substances which are used to indicate whether the
12. Ordinary substance is acidic or basic or neutral in nature.

: Ordinary indicators are obtained from the parts of plants and indicate only whether
the substance is acid, base or salt.

13. Universal indicator : A universal indicator is a special kind of indicator which is obtained from many simple
indicators and is used to measure the strength of acidity or alkalinity.

14. pH : The measure of hydrogen ion concentration present in a solution is called pH.
15. pH scale
: The standard scale which is used to measure the strength of acidic and basic solution
is called pH scale.

16. pH meter : pH meter is a scientific instrument which is used to measure the pH value of a given
solution directly in the number.

14.1 Introduction

We eat different types of foods, fruits and vegetables. Among them, some have sour taste like
orange, lemon, grape, apple; some have bitter taste like bitter gourd, edible soda; and some
have salty taste like table salt. In our surroundings, there are several kinds of compounds
which cannot be tasted due to their corrosive and poisonous nature. So, to study all those
edible and non-edible compounds, they are divided into three groups. They are acids, bases
and salts.

14.2 Acid

The word acid is derived from Latin word “acidus” which means sour in taste. Most of the acids,

which are edible or non-edible, are sour in taste. But it is dangerous to touch or taste acids in

laboratory. According to Swedish chemist Arrhenius, “Acids Memory Tips

give hydrogen ions when dissolved in water and conduct 1. Milkalsocontainsacidcalledlactic
electricity.” So, “acids are those chemical substances which acid which helps to form yoghurt.
give hydrogen ions when dissolved in water.”
2. Proteins are made up of amino
HCl H+ + Cl– acids.

HNO3 H+ + NO3– 3. Vitamin C is also an acid called
H2SO4 2H+ + SO4– – ascorbic acid.

14.3 Classification of Acids

Classification of acids on the basis of strength Memory Tips

a) Strong acids: Acids which undergo almost complete When the degree of dissociation is
more than 30%, it is considered a
dissociation in aqueous solution and produce high strong acid.
concentration of hydrogen ions are called strong
acids. Due to more hydrogen ion concentration, they

are good conductor of electricity and have low pH value. Examples: Hydrochloric acid

(HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3), etc.

190 Acid, Base and Salt

b) Weak acids: Acids which undergo a partial dissociation in A strong acid such A weak acid such
aqueous solution and produce low concentration of hydrogen as HCl dissociates as H2CO3 does not
completely into its ions. dissociate completely.
ions are called weak acids. Due to low concentration of

hydrogen ions, they do not conduct electricity easily and

have a high pH value. Examples: Acetic acid (CH3COOH),
Carbonic acid (H2CO3), Formic acid (HCOOH), Ascorbic acid
(C6H8O6), etc.

Classification of acids on the basis of chemical nature

a) Organic acids: Acids which are obtained from living organisms and contain carbon atom
are called organic acids. Examples: Acetic acid (CH3COOH), Formic acid (HCOOH),
Maleic acid (C4H4O4), Ascorbic acid (C6H8O6), Citric acid (C6H8O7), etc. These are weak
acids and produce less concentration of hydrogen ions in aqueous solution.

b) Inorganic acids (Mineral acids): Acids which are obtained from minerals and do not contain
carbon atom are called inorganic acids or mineral acids. Inorganic acids may be strong or
weak. Inorganic acids are commonly used in laboratories. Examples: Hydrochloric acid
(HCl), Nitric acid (HNO3), Sulphuric acid (H2SO4), Carbonic acid (H2CO3), etc.

FACT WITH REASON

Why is hydrogen sulphate called an acid?
Hydrogen sulphate is called an acid because it gives hydrogen ion when dissolved in water.

14.4 Properties of Acids

a) Physical properties of acids
i) Acids have a sour taste due to the presence of hydrogen ions (H+). Most of the
fruits are sour in taste. However, it is dangerous to touch and taste acids in
laboratory as they burn our skin, tongue, etc.

ii) Acids change blue litmus paper into red and methyl orange into red.

iii) Strong acids are corrosive in nature.

b) Chemical properties of acids
i) Acids react with bases/alkalis and produce salt and water.

Acid + Base Salt + Water

HCl + NaOH NaCl + H2O

H2SO4 + MgO MgSO4 + H2O

ii) Dilute acids react with active metals and produce salt and hydrogen gas.

Dilute acid + Metal Salt + Hydrogen

2HCl + Mg MgCl2 + H2↑
2HCl + Ca CaCl2 + H2↑
H2SO4 + Zn + H2↑
ZnSO4

Modern Concept Science and Environment – 8 191

3. Acids react with carbonates and bicarbonates and produce salt, water and carbon
dioxide.
4.
Carbonate + Acid Salt + Water + Carbon dioxide
FACT
MgCO3 + H2SO4 MgSO4 + H2O + CO2↑
CaCl2 + H2O + CO2↑
CaCO3 + 2HCl 2NaCl + H2O + CO2↑
Salt + Water + Carbon dioxide
Na2CO3 + 2HCl

Bicarbonate + Acid

NaHCO3 + HCl NaCl + H2O + CO2↑

Ca(HCO3)2 + 2HCl CaCl2 + H2O + CO2↑

Acids dissolve in water and give hydrogen ions.

HCl H+ + Cl–

H2SO4 2H+ + SO4– –
HNO3 H+ + NO3–

WITH REASON

HCl is a strong acid but CH3COOH is a weak acid, why?
HCl is a strong acid because it undergoes almost complete ionization to give a large amount of hydrogen
ion. But CH3COOH is a weak acid because it does not ionize completely in water.

Uses of acids

a) Sulphuric acid is used in industries for making drugs, detergents and chemical fertilizers.
It is widely used in laboratories and industries. Therefore, sulphuric acid is also called
the ‘king of chemicals’.

b) Hydrochloric acid is used in laboratories and in tanning and printing industries.
c) Nitric acid is used for making explosives, plastics and dyes.
d) Carbolic acid (Phenol) is used to kill germs.
e) Boric acid is used for washing eyes and wounds.
f) Acetic acid (vinegar) is used for preserving and flavouring foods.
g) Citric acid is used in medicines, as a source of vitamin C and flavouring drinks.
h) Carbonic acid is used in soft drinks and soda water.
i) Oxalic acid is used to remove ink-stain.
j) Tartaric Acid is added to baking soda (NaHCO3) for making baking powder. It

neutralises the bitterness produced by the baking soda.

Some acids of our daily use with their sources are as follows:

S.N. Name of acids Source

1. Citric acid Lemon, tomato
2. Lactic acid Milk, curd

192 Acid, Base and Salt

3. Ascorbic acid Sour fruits
4. Tartaric acid Grapes
5. Oxalic acid Chariamilo
6. Formic acid Red ant

14.5 Base

Metallic oxides and hydroxides are called bases. Most bases or metal oxides dissolve in water
and give hydroxyl ions. The bases that dissolve in water and produce hydroxyl (OH-) ions
are called alkalis. e.g. NaOH, KOH, Ca(OH)2, Mg(OH)2, etc. Some bases like PbO, BaO, HgO,
CuO, etc. do not dissolve in water. So these compounds are bases but not alkalis. Therefore,
all alkalis are bases but all bases are not alkalis.

FACT WITH REASON

Ferrous oxide is a base but not an alkali, why?
Ferrous oxide is a base because it is a metal oxide but it is not an alkali as it does not dissolve in water
to give hydroxide.

NaOH is an alkali, why?

NaOH is an alkali because it dissolves in water and produces hydroxyl ion.

All alkalis are bases but all bases are not alkalis, why?
All alkalis are bases because all alkalis are metal oxides or metal hydroxides which dissolve in water.
All bases are not alkalis because all bases do not dissolve in water.

Differences between bases and alkalis

SN Bases SN Alkalis

1 All metallic oxides are called bases. 1 Only water soluble metallic oxides or
bases are called alkalis.

2 All bases do not give hydroxyl ions 2 All alkalis give hydroxyl ions when

when dissolved in water. dissolved in water.

3 All bases are not alkalis. Examples: 3 All alkalis are bases. Examples: NaOH,
MgO, Na2O, HgO, Fe2O3, etc. KOH, Mg(OH)2, NH4OH, etc.

Strong bases and weak bases

Bases (alkalis) which give more amount of hydroxyl ions (OH⁻) in aqueous solution are
called strong bases. e.g. Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium
hydroxide [Ca(OH)2], etc. They undergo almost complete dissociation and have a high pH
value.

Bases (alkalis) which give very less amount of hydroxyl ions (OH⁻) in aqueous solution are
called weak bases. e.g. Ferric hydroxide [Fe(OH)3], Copric hydroxide [Cu(OH)2], etc. They
have very less degree of ionization and have a low pH value.