Science 8

but some elements have symbols from their Latin names. A Swedish chemist,
J. J. Berzelius introduced the system of symbol in 1914. Symbols of elements
may consist of one, two or three letters.
If the symbol of an element has a single letter, it is written in capital form.
%XW LI WKH V\PERO RI DQ HOHPHQW KDV WZR OHWWHUV WKH ÀUVW OHWWHU LV ZULWWHQ LQ
capital form and the second letter is written small. Similarly, if there are three
OHWWHUV LQ D V\PERO RQO\ ÀUVW OHWWHU LV ZULWWHQ FDSLWDO DQG WKH UHPDLQLQJ DUH
small.

6\PEROV RI HOHPHQWV IURP WKH ÀUVW OHWWHU RI WKHLU QDPH

Name of element Symbol
Hydrogen H
Sulphur S
Boron B
Phosphorus P
Carbon C
Vanadium V
Nitrogen N
Ytterium Y
Oxygen O
Iodine I
Fluorine F
Uranium U

6\PEROV RI HOHPHQWV IURP WZR OHWWHUV RI WKHLU QDPH

Name of element Symbol
Helium He
Argon Ar
Lithium Li
Calcium Ca
Beryllium Be
Cobalt Co
Neon Ne
Nickel Ni
Magnesium Mg
Barium Ba
Aluminium Al

147 Times' Crucial Science & Environment Book - 8

Bromine Br
Silicon Si
Zinc Zn
Chlorine Cl
Arsenic As

3. Symbols of elements from three letters of their name:

Name of element Symbol
Unnilquadrium Unq
Unnilpentium Unp
Unniloctium Uno

6\PEROV RI HOHPHQWV IURP WKHLU /DWLQ QDPH

English name of element Latin name Symbol
Sodium Natrium Na
Potassium Kalium K
Iron Ferrum Fe
Copper Cuprum Cu
Silver Argentum Ag
Mercury Hydrargyrum Hg
Gold Aurum Au
Lead Plumbum Pb
Tin Stannum Sn
Antimony Stibnum Sb
Tungsten Wolfram(German name) W

5. Symbols of elements from the names of scientists:

Name of element Name of scientist Symbol
Cm
Curium Madam Curie Es
No
Einsteinium Albert Einstein Fm

Nobelium Alfred Nobel

Fermium Enrico Fermi

The English alphabet C is quite fortunate regarding its use in the names
of elements. The names of twelve elements begin from the letter C. But
the alphabets J and Q are quite unfortunate because there is no element
whose name starts from J or Q.

Times' Crucial Science & Environment Book - 8 148

Sn. Name of element Symbol
1. Carbon C
2. Chlorine Cl
3. Calcium Ca
4. Chromium Cr
5. Cobalt Co
6. Cadmium Cd
7. Cesium Cs
8. Copper Cu
9. Cerium Ce
10. Curium Cm
11. Californium Cf

Valence shell and valence electrons

An atom may consist of several Nucleus Valence electron
shells. The shell nearest to the
nucleus is regarded as the innermost Innermost Valence
shell whereas that farthest from the Shell Shell
nucleus is regarded as the outermost
shell. The outermost shell of an sĂůĞŶĐĞ ƐŚĞůů Θ ǀĂůĞŶĐĞ ĞůĞĐƚƌŽŶ ŽĨ ĂŶ ĂƚŽŵ
atom is also known as valence shell.
The electrons present in the valence
shell are called valence electrons.

For example, an atom of sodium contains three shells. Hence, the third shell
is called valence shell and the electron present in this shell is called valence
electron. Sodium has one valence electron. Similarly, magnesium has two
valence electrons. The number of valence electrons plays an important role in
determining the valency of an element.

Valency

The number of valence electrons that determines the combining capacity of an
element is called valency. The valency of an element is represented in positive
whole numbers such as 1, 2, 3, 4, ...., etc. The inert gases do not combine with
other elements. Hence, their valency is taken as equal to zero (0).

In classical concept, the valency of an element is determined by taking hydrogen
as a standard reference. It is because hydrogen is the simplest element and its
valency is always one. The valency of an element is determined by counting
the number of hydrogen atoms which combine with one atom of that element.
For example:

149 Times' Crucial Science & Environment Book - 8

D 7KH YDOHQF\ RI ) LQ WKH FRPSRXQG K\GURJHQ ÁXRULGH +) LV EHFDXVH
atom of F combines with 1 atom of H.

b) The valency of O in the actoommpsooufnhdydwraotgeern(.H2O) is 2 because 1 atom of
oxygen combines with 2

Similarly, the valency of N in ammonia (NH3) is 3 and that of C in methane
(CH4) is 4.

In case an element does not combine with hydrogen, its valency is determined
by comparing it with oxygen and chlorine atoms whose valency with hydrogen
is known. For example, magnesium does not combine with hydrogen but it
combines with oxygen to form MgO. In MgO, one atom of Mg combines with
one atom of O. Since the valency of O is 2, the valency of Mg is also 2. The
valency of some elements is given below:

Monovalent Bivalent Trivalent Tetravalent Pentavalent +H[DYDOHQW
(Valency 1) (Valency 4) (Valency 5) (Valency 6)
(Valency 2) (Valency 3)
Hydrogen Carbon Nitrogen Sulphur
(H) Beryllium Nitrogen (C) (N) (S)
(Be) (N)
Lithium Silicon Phosphorus
(Li) Oxygen Phosphorus (Si) (P)
(O) (P)
Sodium Sulphur
(Na) Magnesium Chromium (S)
Silver (Mg) (Cr)
(Ag)
Calcium Aluminium
Chlorine (Ca) (Al)
(Cl)
Zinc
Bromine (Zn)
(Br)
Barium
Iodine (Ba)
(I)
Sulphur
(S)

Variable valency

Some elements show more than one kind of valency. This is called variable
valency. The elements show variable valencies because of the amount of reacting
substances and the use of physical conditions to carry out the reactions. The
variable valencies of some of the elements are given below:

S.N. Name of element Symbol Valency
1. Iron Fe 2, 3
2. Copper Cu 1, 2
3. Mercury Hg 1, 2

Times' Crucial Science & Environment Book - 8 150

4. Gold Au 1, 3
5. Lead Pb 2, 4
6. Tin Sn 2, 4
7. Manganese Mn 2, 4
8. Sulphur S 2, 4, 6

The name of compound having higher valency of metal ends with ’ic’ whereas
that with lower valency ends with ’ous’. For example:

Fe2O3 (Ferric oxide): Higher valency of iron
FeO (Ferrous oxide): Lower valency of iron

Radical

An atom or a group of atoms carrying positive or negative charge that behaves
as a single unit in a chemical reaction is called radical. The radicals are highly
reactive and unstable chemical species. They do not exist independently.
Hence, we cannot see the radicals but we can see their chemical effects.
7KH UDGLFDOV FDQ EH FODVVLÀHG LQWR WZR W\SHV RQ WKH EDVLV RI QDWXUH RI FKDUJH
carried by them. They are:

1. (OHFWURSRVLWLYH UDGLFDOV A radical that carries positive charge is called
electropositive radical. Generally, the electropositive radicals are called
basic or metallic radicals. For example, Na+, Mg+ +, Al+ + +, Ca+ +, etc.

7KH HOHFWURSRVLWLYH UDGLFDOV FDQ EH FODVVLÀHG LQWR WZR W\SHV 7KH\ DUH

D 6LPSOH HOHFWURSRVLWLYH UDGLFDOV These radicals consist of single
atom of an element, for example, Na+, Ca+ +, Cr+ + +, etc.

E &RPSRXQG HOHFWURSRVLWLYH UDGLFDOV These radicals contain two
or more atoms of different elements, for example, NH4+, H3O+, etc.

Some common electropositive radicals and their valencies are
JLYHQ LQ WKH IROORZLQJ WDEOH

S.N. Name of radical Symbol Valency
1. Sodium Na+ 1
2. Magnesium Mg+ + 2
3. Aluminium Al+ + + 3
4. Silicon Si+ + + + 4
5. Potassium K+ 1
6. Calcium Ca+ + 2
7. Ammonium NH4+ 1

151 Times' Crucial Science & Environment Book - 8

8. Zinc Zn+ + 2
9. Silver Ag+ 1
10. Ferrous Fe+ + 2
11. Ferric Fe+ + + 3
12. Cuprous Cu+ 1
13. Cupric Cu+ + 2
14. Aurous Au+ 1
15. Auric Au+ + + 3
16. Mercurous Hg+ 1
17. Mercuric Hg + + 2
18. Aluminium Al+ + + 3

(OHFWURQHJDWLYH UDGLFDOV A radical that carries negative charge is
called electronegative radical. Generally, the electronegative radicals
are acidic or non-metallic radicals, examples, F ï , Clï , Oï ï , N ï ï ï etc.

7KH HOHFWURQHJDWLYH UDGLFDOV FDQ DOVR EH FODVVLÀHG LQWR WZR W\SHV 7KH\ DUH

a) Simple electronegative radicals: These radicals consist of single
atom of an element, example, F ï , Oï ï, Nï ï ï, etc.

b) Compound electronegative radicals: These radicals contain two or
more atoms of different elements. For example, OHï, NO3ï, SO4ï ï,
CO3ï ï, etc.

Some common electronegative radicals and their valencies are
JLYHQ LQ WKH IROORZLQJ WDEOH

S.N. Name of radical Symbol Valency
1. Hydroxide 1
2. Nitrate OH– 1
3. Nitrite 1
4. Nitride NO3– 3
5. Sulphate NO2– 2
6. Sulphite N– – – 2
7. Sulphide 2
8. Bisulphate SO4– – 1
9. Carbonate SO3– – 2
10. Bicarbonate S– – 1

HSO4–
CO3– –
HCO3–

Times' Crucial Science & Environment Book - 8 152

11. Phosphate PO4– – – 3
12. Cyanide CN– 1
13. Chlorate 1
14. Chloride ClO3– 1
Cl–

The charges of the radicals are not shown for the ease of reading.

Octet and octet rule

The stable arrangement of eight electrons in the valence shell of an atom is
called an octet. The elements neon (Ne), argon (Ar), krypton (Kr), xenon (Xe)
and radon 5Q KDYH RFWHW L H WKH\ KDYH HLJKW HOHFWURQV LQ WKHLU YDOHQFH VKHOO
The elements having octet do not react with other elements to form compound.

The elements which do not have octet try to attain it by gaining or losing
electrons from their valence shell. This tendency is called octet rule.

Hence, the tendency of an element to attain eight electrons in its valence shell
by gaining, losing, or sharing electrons is called octet rule.

Duplet and duplet rule

The stable arrangement of two electrons in the valence K-shell of an element
is called duplet. Helium is the only element which has duplet. The elements
such as hydrogen, lithium, beryllium and boron try to attain duplet gaining
or losing electrons. This tendency is called duplet rule. Hence, the tendency
of an element to attain two electrons in its valence shell by losing or gaining
electrons is called duplet rule.

Molecular formula

A molecule is formed by the combination of two or more atoms of the same
RU GLIIHUHQW HOHPHQW LQ GHÀQLWH SURSRUWLRQ E\ ZHLJKW $ PROHFXOH FDQ EH
represented in terms of short formula called molecular formula. The symbolic
representation of a molecule of a substance which shows the actual number of
atoms present in it is called molecular formula.
A molecule can be monoatomic, diatomic, triatomic or polyatomic. A molecule
which is made of single atom is called monoatomic molecule. The inert gases
have monoatomic molecules. It means that the atoms of these elements act as
molecules because they are stable and exist freely in nature.
A molecule which is made of two atoms is called diatomic molecule. For
NewxhaaCemrle,paMlesg,HOH2,2S,eOOtc42a,isrNea2,dpFioa2lt,yoaCmtlo2i,cmBciocrm2maponolduecnIu2dlaemr.eoldeicautloems.iHc e2Olemiseanttrsi.aStoimmiilcamrlyo,leHcuClle,

153 Times' Crucial Science & Environment Book - 8

Note

1. H is the symbol of hydrogen. It represents an atom of hydrogen but H2
represents a molecule of hydrogen.

2. Co is the symbol of element cobalt but CO is the molecular formula of
a compound called carbon monoxide.

Information obtained from molecular formula

1. A molecular formula represents one molecule of the substance.
2. It shows the actual number of atoms of each element present in a

molecule.
3. It shows the valency of the combining atoms.
4. It helps to calculate the molecular weight of the substance.

How to write molecular formula?

Writing the molecular formula of elements is easy because there are similar
kinds of atoms in an element. But we should be careful while writing molecular
formula of compounds.
Following steps should be followed while writing the molecular formula of
compounds:
6WHS Write down the symbols of the combining atoms or radicals separately.

For example: if the compound is ammonia, then,
NH
6WHS Write down the valency of each atom or radical on its top.
31
NH
6WHS 5HZULWH WKH V\PEROV DQG H[FKDQJH WKHLU YDOHQFLHV
31
NH
= NH3 (It is the molecular formula of ammonia)
If one of the valencies is multiple of another, the common valency is
removed, e.g.
24 2 1
CO
= CO2

Times' Crucial Science & Environment Book - 8 154

The compound radicals are enclosed by brackets if necessary, e.g.
12

NH4 SO4
= (NH4)2SO4

Some more examples are given below:

1. Water (Hydrogen + Oxygen)
12
HO
= H2O

2. Methane (Carbon + Hydrogen)
41
CH
= CH4

3. Limestone or Marble (Calcium carbonate)
12 2 1
Ca CO3
= CaCO3

4. Hydrochloric acid (Hydrogen + Chlorine)
11
H Cl
= HCl

The molecular formulae of some common compounds are given in the following
table:

S. N. Name of the compound Chemical name Molecular formula

1 Water Hydrogen oxide H2O
NH3
2 Ammonia Ammonia CH4
CO
3 Methane Methane CO2
HCl
4 Carbon monoxide Carbon monoxide H2SO4
5 Carbon dioxide Carbon dioxide
6 Hydrochloric acid Hydrogen chloride
7 Sulphuric acid Hydrogen sulphate

155 Times' Crucial Science & Environment Book - 8

8 Nitric acid Hydrogen nitrate HNO3
9 Common salt Sodium chloride NaCl

10 Caustic soda Sodium hydroxide NaOH

11 Caustic potash Potassium hydroxide KOH

12 Marble, limestone or chalk Calcium carbonate CaCO3
CaO
13 Quick lime Calcium oxide

14 Slaked lime or lime water Calcium hydroxide Ca(OH)2

15 Phosphoric acid Hydrogen phosphate H3PO4

16 Nitric oxide Nitric oxide NO

17 Laughing gas Nitrous oxide N2O
18 Blue vitriol
19 Washing soda Copper sulphate CuSO4.5H2O
20 Baking soda
21 Gypsum Sodium carbonate Na2CO3.10H2O
22 Epsom salt
Sodium bicarbonate NaHCO3

Calcium sulphate CaSO4.2H2O

Magnesium sulphate MgSO4.7H2O

Chemical reaction

A process in which one or more substances undergo change in molecular level
to form new substances is called chemical reaction. A chemical reaction is
represented in chemical equation. A chemical equation consists of two parts-
UHDFWDQWV DQG SURGXFWV VHSDUDWHG E\ DQ DUURZ ń

The chemical substances which undergo change to form new substances are called
reactants. Similarly, the new substances, which are formed as a result of chemical
change are called products. For example, if hydrogen is burnt with oxygen, it
produces water. This change can be represented in chemical equation as:

+\GURJHQ 2[\JHQ ń :DWHU

5HDFWDQWV Product

Chemical reactions can be expressed in two ways. They are:

1. Word equation

2. Chemical equation

1. Word equation

An equation in which the full names of reactant and product molecules are
written to represent a chemical change is called word equation. For example,
if calcium carbonate is heated, calcium oxide and carbon dioxide are formed.
This chemical change can be represented in word equation as:

&DOFLXP FDUERQDWH ń &DOFLXP R[LGH &DUERQ GLR[LGH

Times' Crucial Science & Environment Book - 8 156

Similarly,

1LWURJHQ +\GURJHQ ń $PPRQLD

:DWHU &DUERQ GLR[LGH ń &DUERQLF DFLG

&KHPLFDO HTXDWLRQ RU IRUPXOD HTXDWLRQ

An equation in which the reactants and products are expressed in terms
of symbols and formulae is called chemical equation. It is also known as
formula equation. A chemical equation may be unbalanced or balanced.
In an unbalanced equation, the number of atoms of one or more elements
is not the same on reactant and product side. Such equation is also
called skeleton equation. This equation can be balanced by multiplying
reactant or product molecules by positive whole numbers. The equation,
thus obtained, is called balanced chemical equation. For example, if
nitrogen is heated with hydrogen, ammonia is produced. This change
can be represented as:

:RUG HTXDWLRQ 1LWURJHQ +\GURJHQ ń $PPRQLD

Skeleton equation: N2 + H2 ń 1+3
Balanced chemical equation: N2 + 3H2 ń 1+3
The chemical equation in which total number of atoms of each element in
reactant and product molecules are equal is called balanced chemical equation.

Balancing the chemical equations

Following rules will be helpful to balance the chemical equations:

1. Write down the word equation of the given chemical change.

2. Write the formula equation using symbols or molecular formula of each
reactant and product.

3. Count the number of atoms of different elements on each side of the
chemical equation.

Following points will be helpful while counting the number of atoms:

a. The number written on the right lower corner of an atom indicates
its total number in a molecule. For example, in CH4, there are 1C
and 4H atoms.

b. If the atoms are enclosed by brackets, the number applies to all
atoms inside the bracket. For example, in Al2 (SO4)3, there are 2Al,
3S and 12O atoms.

F 7KH QXPEHU RU FRHIÀFLHQW ZULWWHQ LQ IURQW RI D PROHFXOH EHORQJV
to all the atoms of a molecule. For example, 2CH4 means 2 C atoms
and 2 × 4 H (=8H) atoms.

157 Times' Crucial Science & Environment Book - 8

d. Try to balance the number of atoms by multiplying the reactants
and products by simple whole numbers. Try to balance the odd
QXPEHU RI DWRPV ÀUVW

e. Count the number of atoms of each kind. If their number is the
same on both sides, the equation is balanced.

6RPH H[DPSOHV RI EDODQFHG FKHPLFDO HTXDWLRQV

1. Hydrogen combines with chlorine gas to form hydrochloric acid.

:RUG HTXDWLRQ +\GURJHQ &KORULQH ń +\GURFKORULF DFLG

Skeleton equation: H2 + Cl2 ń +&O
Balanced chemical equation: H2 + Cl2 ń +&O
2. Magnesium burns in the atmosphere of nitrogen to form magnesium nitride.

:RUG HTXDWLRQ 0DJQHVLXP 1LWURJHQ ń 0DJQHVLXP QLWULGH
Skeleton equation: Mg + N2 ¨ Mg3N2
Balanced chemical equation: 3Mg + N2 ń 0J3N2

3. When potassium chlorate is heated, it decomposes to give potassium
chloride and oxygen gas.
Word equation: Potassium chlorate ¨ Potassium chloride + Oxygen

Skeleton equation: KClO3 ń .&O 22
Balanced equation: 2KClO3 ń .&O 22
4. When sodium hydroxide is mixed with hydrochloric acid, salt and water
are produced:

:RUG HTXDWLRQ 6RGLXP K\GUR[LGH +\GURFKORULF DFLG ń &RPPRQ VDOW
+ water

6NHOHWRQ HTXDWLRQ 1D2+ +&O ń 1D&O +2O
The equation is balanced itself.

5. When aluminium metal is placed in hydrochloric acid, aluminium
chloride and hydrogen are produced.

:RUG HTXDWLRQ $OXPLQLXP +\GURFKORULF DFLG ń $OXPLQLXP FKORULGH
+ Hydrogen

6NHOHWRQ HTXDWLRQ $O +&O ń $O&O3 + H2
%DODQFHG HTXDWLRQ $O +&O ń $O&O3 + 3H2

Times' Crucial Science & Environment Book - 8 158

abbreviated : short

ƌĞƉƌĞƐĞŶƚĂƟŽŶ ͗ ĚĞƐĐƌŝƉƟŽŶ ŽĨ ƐŽŵĞƚŚŝŶŐ ŝŶ Ă ƉĂƌƟĐƵůĂƌ ǁĂLJ

variable : more than one

1. The use of symbols, formulae and structures to represent the molecules or chemical
reactions is called language of chemistry.

2. The short and abbreviated form for the full name of an element is called symbol.

3. The number of electrons gained, lost or shared by a combining atom in a chemical
reaction is called valency.

4. An atom or group of atoms carrying positive or negative charge which behaves as a
single unit in a chemical reaction is called radical.

5. The symbolic representation of a molecule of a substance which shows the actual
number of atoms present in it is called molecular formula.

6. A process in which one or more substances undergo change in molecular level to
form new substances is called chemical reaction.

7. The chemical reaction can be represented in two types of equations-word equation
and chemical or formula equation.

Exercise

A. Answer these questions in very short.
1. Write down the symbol of elements chromium and curium.
2. What is a valence shell?
3. What do you mean by valence electrons?
'HÀQH WKH WHUPV RFWHW UXOH
5. What is duplet?
6. What is the valency of Fe in Fe2O3?
7. Give two examples of compound electropositive radicals.

% :ULWH GRZQ WKH V\PERO DQG YDOHQF\ RI WKH IROORZLQJ

1. Magnesium 2. Sodium 3. Carbon

4. Zinc 5. Gold 6. Silver

7. Ammonium ion 8. Carbonate ion 9. Sulphate ion

10. Phosphate ion 11. Hydroxide ion 12. Chlorate ion

13. Barium 14. Nitrate ion 15. Chlorine

159 Times' Crucial Science & Environment Book - 8

& $QVZHU WKHVH TXHVWLRQV
1. What is a symbol? Give examples.
2. Write down the Latin name and symbol of sodium, silver, mercury
and lead.
3. What do you mean by valency? Explain with examples.
4. What are radicals? Explain two types of radicals.
5. What is a chemical equation? Give some examples.
6. Write differences between reactants and products.
7. What information do you obtain from molecular formula?
8. What is the difference between Co and CO?
9. What is meant by 2H and H2?
'HÀQH FKHPLFDO UHDFWLRQ

D. Write down the molecular formula of the following compounds
E\ FULVV FURVV PHWKRG

1. Water 2. Ammonia 3. Methane

4. Ferric chloride 5. Common salt 6. Hydrochloric acid

7. Magnesium oxide 8. Barium chloride 9. Auric chloride

10. Carbon dioxide 11. Copper sulphate 12. Marble or limestone

13. Sulphuric acid 14. Nitric acid 15. Washing soda

16. Edible soda 17. Mercuric chloride 18. Ammonium sulphate

( &RPSOHWH WKH IROORZLQJ WDEOH

2[LGH +\GUR[LGH Chloride Sulphate Carbonate
Na2O
Sodium
Potassium KOH
Calcium
Magnesium CaCl2

Iron MgSO4 FeCO3
Al2(SO4)3
Aluminium
Cu(OH)2
Copper

F. Write down the names of the following compounds.

1. Ca(NO3)2 2. AgNO3 3. Ca(HCO3)2
4. PbO2 5. H2CO3 6. CO

Times' Crucial Science & Environment Book - 8 160

7. CuSO4 8. Ca3(PO4)2 9. (NH4)2SO4
10. Fe2(SO4)3 11. NH3 12. MgCl2

G. Translate the following word equations into balanced chemical
equations.

+\GURJHQ 2[\JHQ ń :DWHU

&DUERQ 2[\JHQ ń &DUERQ GLR[LGH

1LWURJHQ +\GURJHQ ń $PPRQLD

&DUERQ +\GURJHQ ń 0HWKDQH

6XOSKXU 2[\JHQ ń 6XOSKXU GLR[LGH

=LQF 6XOSKXULF DFLG ń =LQF VXOSKDWH +\GURJHQ

6RGLXP :DWHU ń 6RGLXP K\GUR[LGH +\GURJHQ

6RGLXP K\GUR[LGH +\GURFKORULF DFLG ń 6RGLXP FKORULGH :DWHU

H. Balance the following skeleton equations.

1. Fe + Cl2 ń )H&O3
2. NaOH + H2SO4 ń 1D2SO4 + H2O
3. KClO3 ń .&O 22
4. CaCO3 +&O ń &D&O2 + H2O + CO2
5. Cu(NO3)2 ń &X2 122 + O2

What compounds are being used at your home? Observe the different
FRPSRXQGV DQG WU\ WR ÀQG WKHLU FRPPRQ QDPHV FKHPLFDO QDPHV DQG FKHPLFDO
formulae.

161 Times' Crucial Science & Environment Book - 8

13CHAPTER Separation of
Mixtures

Mikhail Tswett
He also known by the name Mikhail Tsvet. He is known

for the discovery of adsorption chromatography.

Estimated Periods : 7
Objectives: At the end of the chapter, the students will be able to:

XVH GLIIHUHQW PHWKRGV IRU WKH VHSDUDWLRQ RI PL[WXUH
H[SODLQ GLVWLOODWLRQ DQG FKURPDWRJUDSK\ WHFKQLTXHV IRU WKH VHSDUDWLRQ RI PL[WXUHV

Can you make a mixture of different substances?
Can you separate the components of any mixture?
How do you separate rice from husk?
Can you name some methods of separation of mixture? Discuss.

Introduction

A mixture is an impure substance which is formed by the physical combination
of two or more substances in any proportion by weight. A substance which
forms a part of a mixture is called component of the mixture.
We use several substances in the form of mixture in our daily life. But,
sometimes, we need pure substances. So, we need to separate the mixture into
the pure forms of its components. The process of separating a mixture into its
individual components is called separation of mixture. The separation of the
components of a mixture depends on the principle that different substances
have different properties.
There are several methods for the separation of a mixture. You have studied
the following methods of separation of mixture in your previous grades:

Sedimentation and decantation

The process of settling down of heavy insoluble particles at the bottom of the
container is called sedimentation. This process makes insoluble solid particles
lie at the bottom while the clear liquid remain as upper layer. The process of

Times' Crucial Science & Environment Book - 8 162

pouring the clear liquid into another vessel without disturbing the sediments
is called decantation. These processes are used to separate heterogeneous
mixtures such as muddy water, sand and water, etc.

)LOWUDWLRQ

The process of removing insoluble solid particles from a liquid by the use of
ÀOWHU SDSHU LV FDOOHG ÀOWUDWLRQ Sand and dust particles are separated from
ZDWHU E\ WKH SURFHVV RI ÀOWUDWLRQ

Evaporation

The process of separating a mixture by evaporating the solvent from a solution
due to heating is called evaporation. This process is used to obtain salt from
sea water.

Distillation

The process of evaporation of a liquid component of a mixture and its
condensation into liquid again is called distillation. This method is used to
separate the mixture of water and alcohol. It is also used if we have to obtain
both salt and water separately from a salt solution.

Sublimation

The process of converting a solid substance directly into vapour and
condensing the vapour into solid again is called sublimation. This process is
used to separate the mixture of camphor and sand, iodine and common salt,
ammonium chloride and sand, etc.

Magnetic separation

The process of separating magnetic substances from non-magnetic substances
by the use of a magnet is called magnetic separation. For example, iron, cobalt,
etc are separated from their mixture with other substances by this method.
In this chapter, we shall study some more methods of separation of mixture.
These methods are:
1. Distillation
2. Fractional distillation
3. Centrifuging
4. Chromatography
1. Distillation

The process of evaporating the liquid component of a mixture and
condensing it into liquid in another vessel is called distillation. A liquid

163 Times' Crucial Science & Environment Book - 8

VXEVWDQFH FDQ EH SXULÀHG RU D PL[WXUH FDQ EH VHSDUDWHG E\ GLVWLOODWLRQ
For example, pure water can be separated from its solution with common
salt through simple distillation.

Thermometer

Cork
Stand

Water out
Water in

ŝƐƟůůĂƟŽŶ ŇĂƐŬ Receiver
Wire gauze
t
Tripod stand
Burner ŝƐƟůůĂƚĞ
(Pure water)

ŝƐƟůůĂƟŽŶ

In a simple distillation, the solution to be separated is taken in a round
ERWWRPHG ÁDVN 7KH ÁDVN LV ÀWWHG LQ SURSHU SRVLWLRQ RYHU D EXUQHU ZLWK
WKH KHOS RI D VWDQG 7KH PRXWK RI WKH ÁDVN LV FORVHG ZLWK D FRUN KDYLQJ
two holes. A thermometer is inserted through one hole while the upper
end of the condenser tube is inserted through the other hole. The middle
part of the condenser tube has a water cooling system. It has two opening
WXEHV &ROG ZDWHU ÁRZV LQWR WKH WXEH WKURXJK ORZHU RSHQLQJ ZKHUHDV WKH
hot water leaves the tube through upper opening.

:KHQ WKH VROXWLRQ SUHVHQW LQ WKH URXQG ERWWRPHG ÁDVN LV KHDWHG ZDWHU
gets converted into vapour. Thus formed vapour goes into the condenser
tube where it is cooled down to liquid state by cold water. The water
condensed in this way is pure and is collected in the receiver.

ĐƟǀŝƚLJ ϭϯ͘ϭ To separate salt and water from salt solution by simple distillation

0DWHULDOV UHTXLUHG
5RXQG %RWWRPHG ÁDVN VWDQG WULSRG VWDQG EXUQHU WKHUPRPHWHU ZLUH JDXJH
condenser, measuring cylinder, etc.
3URFHGXUH
1. Dissolve some common salt in water and pour it into a round bottomed

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3XW WKH ÁDVN RQ WKH ZLUH JDXJH RYHU WKH WULSRG VWDQG DQG À[ LW LQ SRVLWLRQ

with the clamp of the stand.
3. Insert a thermometer through one of the holes of the cork and connect

WKH ÁDVN ZLWK WKH FRQGHQVHU WKURXJK DQRWKHU KROH
4. Connect the side-hole near lower end of the condenser to the tap of cold

Times' Crucial Science & Environment Book - 8 164

ZDWHU ZLWK D ÁH[LEOH SLSH &RQQHFW DQRWKHU SLSH WR WKH RSHQLQJ QHDU
upper end of the condenser for the outlet of heated water.
3ODFH D FRQLFDO ÁDVN EHORZ WKH IUHH HQG RI WKH FRQGHQVHU WR UHFHLYH WKH
condensed pure water.
+HDW WKH ÁDVN ZLWK EXUQHU WR ERLO WKH PL[WXUH DQG REVHUYH
2EVHUYDWLRQ
+HDWLQJ WXUQV WKH ZDWHU RI WKH ÁDVN LQWR YDSRXU DQG WKH SURFHVV RI
condensation changes the vapour back to liquid. The condensed liquid
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XQWLO GU\ VDOW LV OHIW LQ WKH ÁDVN
&RQFOXVLRQ
A solution of common salt and water can be separated by simple distillation.

)UDFWLRQDO GLVWLOODWLRQ

It is the advanced form of distillation. The process of separating the
mixture of two miscible liquids by using a fractional column is called
fractional distillation. It is used to separate the components of liquid
mixture differing slightly in their boiling points. When the liquid-mixture
LV KHDWHG WKH OLTXLG KDYLQJ ORZHU ERLOLQJ SRLQW WXUQV LQWR YDSRXU ÀUVW
and gets separated. For example, a mixture of water and alcohol can be
separated by fractional distillation.

&ƌĂĐƟŽŶĂů ĚŝƐƟůůĂƟŽŶ

Alcohol boils at 78°C whereas water boils at 100°C at sea level conditions.
When the mixture of water and alcohol is distilled, the alcohol in the
mixture starts boiling at 78°C and gets converted into gaseous alcohol.
On the other hand, water remains in liquid state because of its high
boiling point. The gaseous alcohol then moves through the condenser
tube and condenses back to pure alcohol in liquid state. The separated
pure alcohol is collected in the receiver. When whole of the alcohol is
separated, water is then distilled and separated.

165 Times' Crucial Science & Environment Book - 8

The fractional distillation is used in the processing of crude oil in to
different products. Petrol is separated from the crude oil by fractional
distillation.
Crystallization
The solid substances are of two types on the basis of their structure.
They are crystalline solids and amorphous solids. The solids in which
the atoms or molecules are arranged in a regular geometric pattern are
called crystalline solids. Sodium chloride, copper sulphate, sugar, alum,
phenol, etc are crystalline solids. On the other hand, the solids in which
the atoms or molecules are not arranged in regular geometric pattern are
called amorphous solids. For example, plastic, wood, sand, soil, etc are
amorphous solids.
The crystalline solids are made up of crystals. A crystal is a solid
substance bounded by plane faces that meet at sharp edges to form a
regular geometric shape. There are different kinds of crystals on the
basis of their shape and size. They are: cubic, tetragonal, orthorhombic,
monoclinic, triclinic, hexagonal, etc.

Copper sulphate Sugar Common salt Alum

Crystallization is one of the common methods of separation of a mixture.
It is used to separate the soluble solids from their solution. In this
method, the given solution is heated to evaporate the excess of solvent
so that the solution becomes saturated at higher temperature. The hot
saturated solution is then allowed to cool down without disturbing it for
about half an hour. During this time, the dissolved solute liberates out
from the solution and gets deposited as beautiful crystals at the bottom
of the vessel. These crystals can be obtained by removing the liquid
from the vessel. Thus, the process of separation of a mixture by forming
crystals while cooling the hot saturated solution of the solute is called
crystallization. This process can be used to separate copper sulphate,
sugar, alum, etc from their solution.

ĐƟǀŝƚLJ ϭϯ͘Ϯ To separate copper sulphate crystals from its solution.

0DWHULDOV UHTXLUHG

Times' Crucial Science & Environment Book - 8 166

A procelain basin, a tripod stand, wire gauge, burner, a glass rod, copper
sulphate solution, etc.
3URFHGXUH
1. Pour the solution of copper sulphate into the procelain basin.
2. Place the basin on wire gauge over the tripod stand and heat it.
3. The evaporation of solution removes the

excess of water and the solution becomes
saturated.
4. Dip a glass rod into the boiling solution
and take it out. Wait for a while. If tiny
crystals are seen on the glass rod, the
solution is ready for crystallization.
5. Now, remove the burner and place the
procelain basin in a cool place for 20-30 minutes. (If you need faster
FRROLQJ ÁRDW WKH EDVLQ RQ WKH ZDWHU WURXJK EXW WKH IDVWHU FRROLQJ
produces small crystals).
Observation
Attractive blue crystals of copper sulphate settle at the bottom of the basin.
<RX FDQ WDNH WKHVH FU\VWDOV RXW ZLWK WKH KHOS RI VSDWXOD DQG SODFH RQ D ÀOWHU
paper. The liquid left on the procelain basin after removing the crystals is
called mother liquor.
Conclusion
Copper sulphate is separated from its solution by the process of crystallization.

3. Centrifuging

The process of separating very Cover

ÀQH VXVSHQGHG VROLG RU VHPL
solid particles from a mixture by
rotating it at high speed is known
Test tube holder

as centrifuging. This process is Test tube
done with the help of an instrument
called centrifuge. A centrifuge
is a device which can rotate in a Rotor

horizontal circle about the central
axis. Centrifuging can be used to
separate the components of blood, Centrifuge Switch

urine, chemical mixtures, etc. So, it is used extensively in the labs of
hospitals and science research laboratories.

In centrifuging, the mixture (to be separated) is poured in the test tubes.
The test tubes are placed in the centrifuge. The centrifuge is then made

167 Times' Crucial Science & Environment Book - 8

to rotate at very high speed either by hand (manual centrifuge) or by
electricity (electric centrifuge). When the machine is stopped, it will
be found that the heavier particles settle at the bottom of the test tube
and the clear liquid forms the upper level. These two layers are then
separated by the process of decantation.
The process of centrifuging is also used in dairy industries to separate
cream from milk, in sugar industries to separate molasses from mother
liquor of sugar, etc.
Application of centrifuging
The process of centrifuging is used to separate:
a) cream from milk in dairies.
b) blood cells from plasma in hospitals.
c) cream from curd at home.
d) molasses from crude sugar in sugar industries.
e) water from wet clothes in a washing machine.
4. Chromatography
The word chromatography has been derived from two Greek words
Kroma and graphy. Here, kroma means colour and graphy means
writing. This process was used initially to separate coloured substances
SUHVHQW LQ D PL[WXUH ,W ZDV GHYHORSHG E\ D 5XVVLDQ ERWDQLVW Mikhail
Tswett in 1906 AD to separate the coloured pigments of the plants. At
present, it is used as the most versatile technique for the separation,
SXULÀFDWLRQ LGHQWLÀFDWLRQ DQG FKDUDFWHUL]DWLRQ RI ERWK FRORXUOHVV DQG
coloured components of a mixture. It involves the use of an absorbing
surface.
Thus, chromatography is a technique of separating the components of a
mixture by passing it through a sheet or column of an absorbing material.
This technique is based on the principle that the different components
of a mixture are absorbed at different regions as they move along the
column at different speeds.
There are several techniques of chromatography. Some of them are as
follows:
1. Paper chromatography
2. Thin layer chromatography (TLC)
3. Gas-liquid partition chromatography (GLC)
4. High performance liquid chromatography (HPLC)

Times' Crucial Science & Environment Book - 8 168

Paper chromatography

7KH SURFHVV RI FKURPDWRJUDSK\ LQ ZKLFK D ÀOWHU SDSHU RU FKURPDWRJUDSK\
paper is used to separate the coloured substances present in a mixture is
called paper chromatography.

If two or more different substances are soluble in the same solvent,
their solubilities are different. This difference in solubility can be used
to separate the mixture of these substances. When the mixture of such
VXEVWDQFHV LV SODFHG QHDU RQH HQG RI D ÀOWHU RU FKURPDWRJUDSK\ SDSHU
and the paper is placed in contact with water, the more soluble substance
moves faster and gets separated from the other substance. But it should
be noted that the mixture should not touch the water directly. For
example, a mixture of red and blue inks can be separated by this method.

ĐƟǀŝƚLJ ϭϯ͘ϯ To separate the different colours of ink by using paper

chromatogrphy.
0DWHULDOV UHTXLUHG

:DWHU LQ D EHDNHU VRPH SLHFHV RI ÀOWHU SDSHU PL[WXUH RI UHG DQG EODFN LQN
etc.

3URFHGXUH

&XW D KROH LQ WKH FHQWUH RI D FLUFXODU ÀOWHU ^ƉƌĞĂĚŝŶŐ ŽĨ ŝŶŬ
SDSHU DQG LQVHUW D UROO RI DQRWKHU ÀOWHU
paper in the hole. Filter paper
ZŽůů ŽĨ ĮůƚĞƌ ƉĂƉĞƌ

2. Put a drop of ink near the hole of the
FLUFXODU ÀOWHU SDSHU

,PPHUVH WKH UROO RI ÀOWHU SDSHU SDUWLDOO\ Water

in the water of the beaker. Make sure that WĂƉĞƌ ŚƌŽŵĂƚŽŐƌĂƉŚŐLJ
WKH FLUFXODU ÀOWHU SDSHU GRHVQ·W WRXFK WKH
water of beaker directly.

4. Leave the apparatus undisturbed for about an hour and then observe.

2EVHUYDWLRQ

The different colours present in the ink separate into the rings of different
FRORXUV LQ WKH FLUFXODU ÀOWHU SDSHU

&RQFOXVLRQ

The different colours move with different speeds along with water and are
DEVRUEHG DW GLIIHUHQW UHJLRQV RI WKH ÀOWHU SDSHU

([SODQDWLRQ

7KH ZDWHU IURP WKH EHDNHU ULVHV XS LQ WKH UROO RI ÀOWHU SDSHU DQG VSUHDGV
DFURVV WKH FLUFXODU ÀOWHU SDSHU 7KH GLIIHUHQW FRPSRQHQWV RI WKH LQN VSUHDG DW
different rates along with moving water and get separated from each other.

169 Times' Crucial Science & Environment Book - 8

Adsorption chromatography

A technique of chromatography in which the colours of Mixture
dissolved substances are separated in the form of bands
while moving down a column of an adsorbent is called
adsorption chromatography. This technique is also called
column chromatography. Different substances such as
Glass tube

alumina, chalk, cellulose powder, silica gel, magnesium
oxide, etc are used as adsorbent.
Adsorbent

Adsorption chromatography is based on the principle that
a certain adsorbent can adsorb different substances to
different extents. In this technique, a solution containing
the substances of different colours to be separated is
SRXUHG LQWR D YHUWLFDO JODVV WXEH SDFNHG ZLWK ÀQHO\ GLYLGHG Solvent

adsorbent. While moving down the column, the colours of different substances
present in the solution are separated in the form of colour bands.

Application of chromatography

The technique of chromatography is used:

a) to separate a mixture of different coloured substances.

b) to separate small amounts of different products of a chemical reaction.

c) to detect and identify drugs present in blood.

d) to purify and analyze different organic compounds.

Chromatography is very useful technique of separation of mixtures. Besides
WKH DERYH PHQWLRQHG DSSOLFDWLRQV WKHUH DUH VHYHUDO RWKHU ÀHOGV LQ ZKLFK
different forms of chromatography are in wide range of use.

insoluble : a substance which doesnot get dissolved

heterogeneous ͗ ǀĂƌŝĞĚ ŝŶ ĐŽŵƉŽƐŝƟŽŶ

miscible : liquids which get mixed

ĨƌĂĐƟŽŶĂů : part by part

1. A mixture is an impure substance which is formed by the physical combination of
two or more substances in any proportion by weight.

2. The separation of the components of a mixture depends on the principle that
”different substances have different properties.”

Times' Crucial Science & Environment Book - 8 170

3. Crystallization is a technique of separation of a mixture in which a solute is
separated in the form of crystals by undisturbed cooling of its hot saturated
solution.

4. Chromatography is a technique of separating the components of a mixture by
passing it through a sheet or column of an adsorbing material.

5. Chromatography is based on the principle that the different components of
a mixture are adsorbed at different regions as they move along the column at
different speeds.

6. Paper chromatography and adsorption chromatography are the common techniques
of chromatography.

Exercise

A. Answer these questions in very short.
1. Name the method which is used to separate salt from sea water.
2. What is sublimation? Give some examples of sublimable substances.
3. What is distillation?
'HÀQH ÀOWUDWLRQ
5. What is the principle of chromatography?
6. Write the principle on which the separation of mixture is based.

% :ULWH GRZQ WKH PHWKRG WR VHSDUDWH WKH PL[WXUH RI

1. Alcohol and water 2. Iodine and salt

3. Salt and water 4. Sugar and water

5HG DQG EOXH LQN &UHDP IURP FXUG

&RSSHU VXOSKDWH DQG ZDWHU ,URQ ÀOOLQJV DQG VDQG

9. Molasses from crude sugar 10. Solid substances from urine

& :ULWH GRZQ WKH VWHSV IRU WKH VHSDUDWLRQ RI WKH IROORZLQJ PL[WXUHV
,URQ ÀOOLQJV VDQG FRPPRQ VDOW
2. Iron dust + sand + common salt + camphor

' $QVZHU WKHVH TXHVWLRQV
1. What is a mixture? Give examples of homogeneous and
heterogeneous mixture.

171 Times' Crucial Science & Environment Book - 8

2. What is meant by separation of mixture? On what principle does it
depend?

3. What is centrifuging? Explain the structure and working of a
centrifuge.

4. What is crystallization? How can you obtain big crystals?
5. Draw the crystal structures of common salt and copper sulphate.
6. What is chromatography? Write down its application.
7. Write down short notes on paper chromatography.

( 'LDJUDPPDWLF TXHVWLRQV
1. Identify the following crystals:

2. Draw a labelled diagram of:

i. Crystallization ii. Paper chromatography

Study the procedue of carrying out paper chromatography. Then separate the
components of ink using this method.

Times' Crucial Science & Environment Book - 8 172

14CHAPTER Metals and
Non-Metals

John Dalton

-RKQ 'DOWRQ ZDV DQ (QJOLVK FKHPLVW SK\VLFLVW DQG PHWHRURORJLVW
He is best known for his ioneering work in the development of
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VRPHWLPHV UHIHUUHG WR DV 'DOWRQLVP LQ KLV KRQRXU

Estimated Periods : 9
Objectives: At the end of the chapter, the students will be able to:

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H[SODLQ WKH SURSHUWLHV DQG XWLOLWLHV RI JROG VLOYHU LURQ FRSSHU DOXPLQLXP DQG FDUERQ

What kinds of elements are the metals?
Can you give some examples of metals?
Can you define non-metals?
How are metalloids different from metals and non-metals? Discuss.

Introduction

7KH HOHPHQWV FDQ EH FODVVLÀHG LQWR PHWDOV QRQ PHWDOV DQG PHWDOORLGV RQ WKH
basis of electrical and thermal conductivities. Metals are good conductors of
heat and electricity, non-metals are bad conductors whereas the metalloids
are poor conductors.
Position of metals, non-metals and metalloids in the periodic table
An outline diagram of modern periodic table is given below:

Metals
Non-Metals
dƌĂŶƐŝƟŽŶ DĞƚĂůƐ Metalloids
and

Metals

WŽƐŝƟŽŶ ŽĨ ŵĞƚĂůƐ͕ ŶŽŶͲŵĞƚĂůƐ ĂŶĚ ŵĞƚĂůůŽŝĚƐ ŝŶ ƚŚĞ ƉĞƌŝŽĚŝĐ ƚĂďůĞ

173 Times' Crucial Science & Environment Book - 8

There is separate and distinct position for metals, non-metals and metalloids
in the periodic table. Metals are on the left side of modern periodic table. All
elements of group 1 (except hydrogen) are metals. All the elements of group 2
and 13 are metals. The metals of group 1 and 2 are highly reactive. Sodium,
potassium, magnesium, calcium, etc are some reactive metals.
Non-metals are on the right side of modern periodic table. The upper elements
of group 14, 15 and 16 are non metals whereas all elements of group 17 are non
metals. The last group, i.e. Group Zero is the group of inert gases. These are
also the non-metals. Carbon, nitrogen, oxygen, sulphur, phosphorus, chlorine,
etc are non-metals. The metalloids lie at the border of metals and non-metals.
Their properties are intermediate between those of metals and non-metals.
Silicon, germanium, arsenic, selenium, etc. are some metalloids.

Metals

Metals are the elements which conduct heat and electricity and are malleable,
ductile and lustrous. The metals are generally the hard solids. For example,
iron, aluminium, copper, silver, platinum, gold, etc are metals. Some of the
metals like lithium, sodium, etc. are so soft that they can be scratched with
nails and can be cut easily with a knife. Similarly, mercury is a liquid metal.
Properties of metals
1. A metal is a good conductor of heat and electricity.
2. A metal can be beaten into very thin sheets, i.e. metals are malleable.
$ PHWDO FDQ EH GUDZQ LQWR YHU\ ÀQH ZLUH L H. metals are ductile.
4. Metals produce a distinct metallic sound when hammered, i.e. metals

are sonorous.
5. Metals shine brightly when freshly cut, i.e. metals are lustrous.
6. Metals are generally solid, hard and strong.
7. Generally, the metals have high melting and boiling points.
8. Chemically, metals tend to form positive ions, i.e. metals are

electropositive.

Non-metals

Non-metals are the elements which do not conduct heat and electricity and
are brittle and non-lustrous. They are not malleable and ductile. Non-metals
are generally found in nature as gases or soft and brittle solids. For example:
*DVHV K\GURJHQ KHOLXP QLWURJHQ R[\JHQ ÁXRULQH FKORULQH DUJRQ HWF
/LTXLG bromine

Times' Crucial Science & Environment Book - 8 174

6ROLGV carbon, sulphur, phosphorus, iodine, etc.
Graphite is a form of carbon. It is a non-metal but is a good conductor of
electricity and possesses metallic lustre. Similarly, iodine is also a non-metal
which possesses metallic lustre.
Properties of non-metals
1. Non-metals are generally brittle, i.e. they break down into pieces when

hammered. Hence, non-metals cannot be beaten into thin sheets or
drawn into wires. Thus, non-metals are not malleable and ductile.
2. Non-metals are the bad conductors of heat and electricity. Exception:
Graphite is a good conductor of electricity and is used in making
electrodes.
3. Non-metals generally do not shine and hence are non-lustrous and dull.
Exception: Graphite and iodine are lustrous.
4. Non-metals are generally gases or soft and brittle solids. Bromine is a
liquid non-metal.
5. Non-metals are non-sonorous, i.e. they produce dull sound when
hammered.
6. The non-metals are light as compared to metals.
7. Non-metals have low melting and boiling points.

Metalloids

The elements which have the properties intermediate between metals and
non-metals are called metalloids. They are also called semi-metals.
The elements beryllium, silicon, germanium, arsenic, selenium and antimony
possess the characteristics of both metals and non-metals, and thus are called
metalloids. Like metals, metalloids posses metallic lustre, form alloys and are
hard solids. On the other hand, they are not malleable and ductile similar to
that of non-metals.
1. Metalloids are poor conductors of heat and electricity. They conduct heat

and electricity better than non-metals but do not conduct electricity as
better as metals. Thus, this property is intermediate between the metals
and non-metals.
2. Generally they are brittle.
3. Generally, they show poor malleability and ductility.
4. They possess metallic lustre.
5. They can form alloys.

175 Times' Crucial Science & Environment Book - 8

Comparison of metals, non-metals and metalloids

Metals Non-metals Metalloids
1. Metals are good
1. Non-metals are the 1. Metalloids are the poor
conductors of heat bad conductors of heat conductors of heat and
and electricity. and electricity. electricity.
2. They are malleable
and ductile. 2. Non-metals are brittle. 2. They show poor
malleability and
3. They are lustrous. ductility.

4. Metals are 3. They are generally 3. They are lustrous.
generally solid, dull.
hard and strong.
4. Non-metals are mainly 4. Metalloids are generally
5. Metals have high gases. The solids are hard solids.
melting and boiling soft and brittle.
points.
5. Non-metals have low 5. M.P and b.p of metalloids
6. Example: Iron, melting and boiling are lower than those of
copper, silver, gold, points. metals and higher than
lead, aluminium, those of non-metals.
etc.
6. Example: Hydrogen, 6. Example: Silicon,
oxygen, carbon, arsenic, germanium,
phosphorus, sulphur, etc.
chlorine, etc.

The metals and non-metals differ from each other in chemical properties also.
The metals react with oxygen to form basic oxides.

2Mg + O2 ¨ 2MgO

4Na + O2 2Na2O

But the non-metals react with oxygen to form acidic oxides.

C + O2 CO2
S + O2 SO2

Gold

Gold is an attractively yellow coloured precious
metal. It belongs to period 6 and group 11 of the
periodic table. Gold is the most malleable and
ductile metal. It doesn't corrode easily and has
great metallic lusture. So, it is also known as
king of all metals.

Gold is also known as coinage metal. Its symbol Au is derived from its Latin
name ‘Aurum’.

Times' Crucial Science & Environment Book - 8 176

Symbol : Au

Atomic No. : 79
Atomic weight : 197.2
Valency : 1 and 3

(OHFWURQLF FRQÀJXUDWLRQ

Shell KL M N O P
No. of electrons 2 8 18 32 18 1

Occurrence

Gold is found in nature in free as well as in combined states. Most often, it is
found in free state because it does not react with other elements easily. It is
usually found with quartz in rocks or in alluvial sands.

Properties
1. It is a yellow coloured shining metal.
2. It is a good conductor of heat and electricity.
,W LV D KHDY\ PHWDO ZLWK VSHFLÀF JUDYLW\
4. Its melting point is 1063°C and boiling point is 2610°C.
5. It forms an amalgam with mercury.

Uses
1. Gold is used extensively in making jewellery, coins and medals.
2. It is used in making statues.
3. It is used in making gold leaf electroscope.
4. It is used for gold plating. Articles made up of cheap metals can be plated

with gold so that they are cheap but look like gold.
,W LV XVHG IRU ÀOOLQJ WHHWK

6. Compounds of gold are used in laboratory, medicines and photography.

Silver

Silver is a lustrous white metal. It is known as
coinage metal for its use to make coins. It has
been known to mankind from ancient times.
Silver belongs to period 5 and group11 of the
periodic table. The symbol Ag is derived from
its Latin name ‘Argentum’. The introduction of

177 Times' Crucial Science & Environment Book - 8

silver can be summarized as:

Symbol : Ag
Atomic No. : 47
Atomic weight : 108
Valency :1

(OHFWURQLF FRQÀJXUDWLRQ

Shell KLM N O
No. of electrons 2 8 18 18 1

Occurrence

Silver is very less reactive metal. It is found in free as well as in combined
states in nature. In combined forms, it occurs in the form of sulphide and
chloride.

Properties
1. It is a white shining metal.
2. It is the best conductor of electricity and heat.
3. Its melting point is 960°C and boiling point is 1950°C.
,WV VSHFLÀF JUDYLW\ LV

Uses
1. It is used to make ornaments, coins and decorative articles.
,W LV XVHG IRU VLOYHULQJ PLUURU DQG ÀOOLQJ WKH WHHWK
3. It is used in silver plating.
4. Its compound, silver bromide is used in photography.
5. It is used to prepare silver compounds such as silver nitrate.
6. It is used in medicines. Colloidal silver is used in ophthalmic treatment.
7. Silver wire is used in electrical instruments.

Copper

Copper is a lustrous reddish brown metal. It is also
a coinage metal. It belongs to group 11 and period 4
of the periodic table. Its symbol Cu is derived from
its Latin name ‘Cuprum’.

Symbol : Cu

Times' Crucial Science & Environment Book - 8 178

Atomic No. : 29

Atomic weight : 63.5

Valency : 1 and 2

(OHFWURQLF FRQÀJXUDWLRQ

Shell KL M N O P
No. of electrons 2 8 18 1 - -

Occurrence

Copper is found in free as well as in the form of compounds in nature. It is
found in small amounts in free state. The main compounds of copper found in
nature are oxides and sulphides.

Properties
1. Copper is a reddish brown metal with an attractive lustre.
2. It is a good conductor of heat and electricity.
3. Its melting point is 1083°C and boiling point is 2582°C.
,WV VSHFLÀF JUDYLW\ LV
5. It readily forms alloys with many other metals.

Uses
1. Copper is used extensively in making electrical wires and cables.
2. It is used for making coins, jewellery and decorative articles.
3. It is used in electroplating.
4. It is used to make alloys like brass, bronze, etc.
5. It is used to make different types of household utensils such as cooking

utensils, boilers, kettles, etc.
6. Its salts are used in agriculture as insecticides and pesticides.

Iron

Iron is a silvery white lustrous metal.
It is a transition element. It belongs
to group 8 of the periodic table. Its
symbol Fe is derived from its Latin
name ‘Ferrum’.

Symbol : Fe

Atomic No. : 26

179 Times' Crucial Science & Environment Book - 8

Atomic weight : 56

Valency : 2 and 3

(OHFWURQLF FRQÀJXUDWLRQ

Shell K LMN
No. of electrons 2 8 14 2

Occurrence
Iron is the second most abundant metal occurring in the earth’s crust. It is
rarely found in free state in nature. It is found in combined states mostly
in the form of oxides and carbonates. Iron is also found in the body of living
organisms in the form of haemoglobin.

Properties
1. Ordinary iron is impure, grey and crystalline. But pure iron is silvery

white and lustrous metal.
2. It is a good conductor of heat and electricity.
,WV VSHFLÀF JUDYLW\ LV
4. Its melting point is 1540°C and boiling point is 2450°C.

5. It is a magnetic substance. It becomes a strong magnet when magnetized
RU SODFHG LQ D PDJQHWLF ÀHOG

Uses
1. Iron is the most useful metal in construction. It is used for making rods,

pipes, bolts, chains, railway track, vehicles, etc.
2. It is used to make cutting tools, agricultural tools and weapons.
3. It is used in the manufacture of steel.
4. It is used as catalyst in different chemical reactions.
5. Iron is an important constituent of human blood.
,W LV XVHG WR SUHSDUH PHGLFLQHV 'HÀFLHQF\ RI LURQ LQ WKH KXPDQ ERG\ LV

treated by using iron tablets.

Aluminium

Aluminium is the bluish white lustrous
metal. It belongs to period 3 and group 13 of
the periodic table.

Symbol : Al
Atomic No. : 13

Times' Crucial Science & Environment Book - 8 180

Atomic weight : 27

Valency :3

(OHFWURQLF FRQÀJXUDWLRQ

Shell K LM
No. of electrons 283

Occurrence

Aluminium is the most abundant metal in the earth’s crust. It is not found in
free state. In combined states, it is found in the form of different compounds
VXFK DV R[LGHV ÁXRULGHV VXOSKDWHV HWF $PRQJ WoKfHVaHl u mbaiunxiuitme ,(Afrlo2Om3.2wHh2iOch)
is the important naturally occurring compound
aluminium is extracted.

Properties
1. It is a bluish white lustrous metal.
,W LV YHU\ OLJKW PHWDO DQG LWV VSHFLÀF JUDYLW\ LV RQO\
3. It is a good conductor of heat and electricity.
4. Its melting point is 660°C and boiling point is 1800°C.

Uses
1. It is used to make the body parts of aeroplanes, vehicles, etc.
2. It is used to make household utensils.
3. Aluminium foil is used for wrapping foods, cigarettes, pharmaceutical

products, etc.
4. It is used to make electric wires.
5. It is used for making coins.
6. Aluminium is used for making alloys.

Silicon

Silicon is a shining metalloid. It shows the characters
of both metals and non-metals. It exists as crystalline
as well as amorphous solid.

Symbol : Si

Atomic number : 14

Atomic weight : 28

Valency :4

181 Times' Crucial Science & Environment Book - 8

Occurrence

Silicon is found in abundant amount in combined state in nature. Silicon is
mostly found in the form of sHanowdeovresri,liictais(SnioOt 2f)o.uInt disianlspourfoeufnordmininsonilatausrteh. e
silicates of different metals.

Properties

1. Silicon is a grey shining metalloid. It is a grey solid.

2. It exists as crystalline or amorphous solid.

3. The crystalline silicon is a poor conductor of electricity but the amorphous
silicon is a bad conductor of electricity.

4. It does not react with acid.

5. It is not affected by air and water.

Uses
1. It is used to manufacture glass.

2. It is used as a semi-conductor in electronic devices.

3. It is used extensively in ceramics.

4. It is used to manufacture colours and paints.

5. Silicon containing rocks and sandstone are used to construct house,
statue, building, etc.

Sulphur

Sulphur is a straw-yellow coloured shining non-metal. It may be crystalline or
amorphous.

Symbol :S

Atomic number : 16

Atomic weight : 32

Valency : 2, 4 and 6

Occurrence

Sulphur is found in free as well as combined form in nature. It is found in free
state in volcanic areas. In combined form, it exists as sulphide. Sulphur is also
found in onion, mustard oil, garlic, etc.

Properties
1. Sulphur is a straw-yellow coloured shining non-metal.

2. Sulphur may be available in crystalline or amorphous form.

Times' Crucial Science & Environment Book - 8 182

3. It is a bad conductor of heat and electricity.
4. It burns explosively in air to form sulphur dioxide.
5. It is insoluble in water.
6. It does not react with cold dilute acids.

Uses
1. It is used to manufacture sulphuric acid (H2SO4).
2. It is used to make medicines.
3. It is an important component of gun powder.
,W LV XVHG WR PDNH ÀUH FUDFNHUV H[SORVLYHV HWF
5. It is used as igniter in matchsticks.
6. It is used to manufacture insecticides.

utensils : tools or devices or containers

coinage ͗ ĐŽŝŶƐ ĐŽůůĞĐƟǀĞůLJ

ƉŚĂƌŵĂĐĞƵƟĐĂů : related to medicines

jewellery : personal ornaments

ďƌŝƩůĞ : hard but liable to break

1. Gold is a yellow coloured precious metal.
2. Gold is a heavy metal with specific gravity 19.3.
3. Gold is used extensively in making jewellery, coins and medals.
4. Silver is a lustrous white metal.
5. Silver is used to make ornaments, coins and decorative articles.
6. Copper is a lustrous reddish-brown metal.
7. Iron is the second most abundant metal occurring in the earth’s crust.
8. Aluminium is the most abundant metal in the earth’s crust.
9. The property of an element to exist in different physical forms which differ

slightly in physical and chemical properties is called allotropy.
10. Diamond and graphite are the crystalline allotropes of carbon.

183 Times' Crucial Science & Environment Book - 8

Exercise

A. Answer these questions in very short.
1. Write down the Latin name and symbol of gold.
2. Where is gold found naturally?
3. Write down the atomic number and atomic weight of silver.
4. Which is the most abundant metal in the earth’s crust?
5. Name the coinage metals.
6. Which is the hardest substance in the world?

% :ULWH GRZQ WKH SK\VLFDO SURSHUWLHV RI

1. Gold 2. Silver 3. Iron 4. Aluminium

& :ULWH GRZQ WKH XVHV RI

1. Copper 2. Silver 3. Silicon

4. Sulphur 5. Iron

' $QVZHU WKHVH TXHVWLRQV
1. Explain the position of metals, non-metals and metalloids in the
periodic table.
2. Explain the occurrence and uses of iron.
3. Explain the occurrence of aluminium in nature.
4. Explain the occurrence of gold in nature.

Observe the different articles or things used at your home. Identify whether
any metals or non-metals are used in these things. Write down the uses of
these articles or things. Also write the properties of metals and non-metals
involved in these articles or things

Times' Crucial Science & Environment Book - 8 184

15CHAPTER Acid, Base
and Salt

Gilbert Newton Lewis

Gilbert Newton Lewis was an American physical chemist known for
the discovery of the covalent bond and his concept of electron

pairs; his Lewis dot structures and the concept of acids and bases.

Estimated Periods : 6
Objectives: At the end of the chapter, the students will be able to:

JLYH WKH JHQHUDO LQWURGXFWLRQ RI DFLG EDVH DQG VDOW
H[SODLQ WKH SURSHUWLHV RI DFLG EDVH DQG VDOW
LQWURGXFH DQG H[SODLQ WKH XVHV RI LQGLFDWRUV VXFK DV OLWPXV SDSHU DQG S+ VFDOH

Can you name some sour tasting substances or foods?
Why are they sour?
What are salts? Do all the salts have salty taste? Discuss.

Introduction

We use several compounds in our daily life. Some of these compounds are
edible. The edible compounds may have sour, bitter, salty or sweet taste. The
non-edible substances should not be tasted because they may be harmful to
our health. So, we cannot distinguish different types of substances on the basis
of their taste only. We have to study their other properties.

Acid

The term acid is derived from the Latin word acidus which means sour taste.
Previously, the sour tasting substances were regarded as acids. The sour taste
is one of the characteristics of acid but all acids cannot be tasted because
WKH\ DUH GDQJHURXVO\ FRUURVLYH +HQFH DQ DFLG LV GHÀQHG RQ WKH EDVLV RI LWV
chemical nature.

An acid is a substance that gives hydrogen ions or protons (H+) when dissolved
in water. For example, hydrochloric acid (HCl), sulphuric adciisdso(lHve2SdOin4),wnaitterirc,
tahceidse(HacNidOs3)p,raocdeutcice aHc+idio(nC.HF3oCrOeOxaHm),peletc, are acids. When

HCl H+ + Clï

H2SO4 2H+ + SO4ï ï

185 Times' Crucial Science & Environment Book - 8

HNO3 H+ + NO3ï

CH3COOH H+ + CH3COOï

Some athciedysasruecchalalesdHmCinl,eHra2Sl aOc4i,dHs.NThOe3s, eetacciadrseaprreeaplasorekdnforwomn atshienomrginaenriaclascaidnsd.
hence

Some of the acids occur naturally in some fruits, vegetables and foodstuffs. All
these acids contain carbon and hydrogen as their essential constituents. The
acids which are obtained from fruits, vegetables or other foodstuffs are called
organic acids. The organic acids are generally edible and are sour in taste.
Some of the organic acids and their sources are given below:

SN Name of organic acid Sources

1. Acetic acid Sour pickles, vinegar, etc.

2. Citric acid Lemon, orange, tangerine, manderin, tomato, etc.

3. Lactic acid Milk, curd, etc

4. Tartaric acid Grapes, pomelo, etc.

5. Ascorbic acid (Vitamin C) Amala (Emblica), Harro, Barro, etc.

6. Oxalic acid Oxalis plant (Chari amilo)

The ants and bees also contain organic acid as their poison. The sting of an ant
or bee is painful because they inject formic acid (methanoic acid) in our body
while stinging. The molecular formula of formic acid is HCOOH.

Differences between organic and inorganic acid:

Organic acids Inorganic acids

1. The acids which are obtained from 1. The acids which are obtained from
plant or animal source are called minerals present in the earth’s crust
organic acids. are called inorganic acid.

2. They are weak acids. 2. Most of these acids are strong.

3. Generally, they are non-corrosive 3. They are strongly corrosive and
and edible. generally non-edible.

4. Example: acetic acid, citric acid, 4. Examples: Hydrochloric acid,
lactic acid, etc. sulphuric acid, nitric acid, etc.

Properties of acid

The properties of acids can be studied under the following headings:

Physical properties

1. The acids have sour taste. But all acids may not have sour taste. It is
dangerous to taste some mineral acids such as HCl, H2SO4, etc because
they are highly corrosive.

Times' Crucial Science & Environment Book - 8 186

2. They change the colour of blue litmus paper into red, methyl orange to
red and phenolphthalein to colourless.

3. The acids are soluble in water.

Chemical properties

1. Acids give H+ ions when dissolved in water.

+&O ń ++ + Clï

2. Acids react with metals to form salt and hydrogen gas. Due to the
reaction, the metals dissolve in dilute solutions of acids. Example:

=LQF 6XOSKXULF DFLG ń =LQF VXOSKDWH +\GURJHQ

Zn + H2SO4 ń =Q624 + H2Ń
&DOFLXP +\GURFKORULF DFLG ń &DOFLXP FKORULGH +\GURJHQ

&D +&O ń &D&O2 + H2Ń
3. Acids react with bases to form salt and water.

+\GURFKORULF DFLG 6RGLXP K\GUR[LGH ń 6RGLXP FKORULGH :DWHU

+a&ciOd 1Db2as+e ń 1sDa&ltO +2wOater

4. Acids react with carbonates and bicarbonates of metal to produce carbon
dioxide gas.

+\GURFKORULF DFLG FDOFLXP FDUERQDWH ń &DOFLXP FKORULGH &DUERQ
dioxide + Water

2HCl + CaCO3 ń&D&O &22 + H2O
Similarly,

2HClM+agMnegs(iHumCObi3c)a2r bńon 0ateJ&O2 + 2H2O + 2CO2

Uses of acids

1. Mineral acids are used as reagents in laboratories.
2. Organic acids are used as nutrients.
3. Hydrochloric acid is used for cleaning sinks and boilers. It is also used in

textile industries as bleaching agent.
4. Nitric acid is used in the manufacture of chemical fertilizers, plastics

and explosives.
5. Sulphuric acid is used in the manufacture of drugs, detergents, chemical

fertilizers, etc.
6. Boric acid solution is used for washing eyes and wounds.

187 Times' Crucial Science & Environment Book - 8

7. Ascorbic acid is an important nutrient which prevents scurvy in human
beings.

8. Carbonic acid (CO2 dissolved in water) is used in soft drinks.

Base

The oxides and hydroxides of metals are called bases. For example, sodium
i(onFxeiwd2Oeat3)e(,Nrcawo2phOpi)le,er calcium oxide (CaO), aluminium oxide t(hAel2bOa3s),esfearrreicsooluxibdlee
oxide (CuO), etc are some bases. Some of
many others are insoluble. The water soluble bases react with
water to form their hydroxides called alkalis.

The alkalis are the bases which dissolve in water to give hydroxyl (OHï) ions.
For example, NaOH, KOH, Ca(OH)2, etc.

NaOH H2O Na+ + OHï(hydroxyl ion)

KOH H2O K+ + OHï

Some common alkalis are:

SN Common name of alkali Chemical name Molecular formula

1. Caustic soda Sodium hydroxide NaOH

2. Caustic potash Potassium hydroxide KOH

3. Slaked lime Calcium hydroxide Ca(OH)2
4. Aqueous ammonia Ammonium hydroxide NH4OH
5. Milk of magnesia Magnesium hydroxide Mg(OH)2

As stated earlier, all oxides and hydroxides of metals are bases. But only water
soluble metal oxides and hydroxides are regarded as alkalis. Since most of
the metallic oxides and hydroxides are insoluble in water, the alkalis are the
small parts of bases. Hence, all alkalis are bases but all bases are not alkalis.

Differences between bases and alkalis

Bases Alkalis

1. All oxides and hydroxides of metals 1. The water soluble oxides and
are bases. hydroxides of metals are alkalis.

2. Most bases do not dissolve in water 2. All alkalis give hydroxyl (OHï) ions
and hence do not give (OHï) ions. when dissolved in water.

3. All bases are not alkalis. It means 3. All alkalis are bases.
that base is a bigger group of
compounds.

4. FEex2aOm3,pAlels2O: C3,aPOb,ON,aH2Og,ON, aCOuHO,, KOH, 4. Examples: CaO, Na2O, NaOH, KOH,
etc. etc.

Times' Crucial Science & Environment Book - 8 188

Properties of base

Physical properties

1. Bases are bitter in taste. But strong alkalis are corrosive and poisonous.
So, it is unwise to taste them.

2. They are soapy to touch.

3. Bases change the red colour of litmus paper into blue, methyl orange to
yellow and phenolphthalein to pink.

4. The bases may or may not be soluble in water.

Chemical properties

:DWHU VROXEOH EDVHV JLYH RII K\GUR[\O LRQV 2+ï ZKHQ GLVVROYHG LQ
water.

1D2+ ń1D+ + OHï

Ca (OH)2 ń &D++ + 2OHï
2. A base (or an alkali) reacts with acid to form salt and water. The reaction

between an acid and a base to form salt and water is called neutralization
reaction.

+\GURFKORULF DFLG 6RGLXP K\GUR[LGH ń 6RGLXP FKORULGH :DWHU

+&O 1D2+ ń 1D&O +2O
acid base salt water

3. Bases react with carbon dioxide to produce metallic carbonate and water.
Example:

6RGLXP K\GUR[LGH &DUERQ GLR[LGH ń 6RGLXP FDUERQDWH :DWHU

2NaOH + CO2 ń 1D2CO3 + H2O

Uses of bases

1. Caustic soda (sodium hydroxide) is used in the manufacture of paper,
textiles, detergents and soaps.

2. Potassium hydroxide found in ash is used for cleaning utensils, washing
clothes, etc.

3. Mixture of magnesium hydroxide and aluminium hydroxide are used as
antacid. An antacid is a medicine to treat excess acidity in the stomach.

4. Ammonium hydroxide is a very important lab reagent. It is also used in
the manufacture of chemical fertilizers.

5. Calcium hydroxide is used in agriculture to reduce the acidity of soil.

189 Times' Crucial Science & Environment Book - 8

Differences between acid and base

Acid Base

1. An acid gives off hydrogen ions (H+) 1. Water soluble bases give off hydroxyl
when dissolved in water. (OHï) ions when dissolved in water.

2. An acid has sour taste. 2. A base has bitter taste.

3. It turns blue litmus into red. 3. It turns red litmus into blue.

4. It corrodes the metals and their oxides, 4. It may not corrode them.
carbonates, etc.

5. Examples: HCl, H2SO4, HNO3, 5. Examples: MgO, K2O, CaO, CuO,
CH3COOH, etc. Fe2O3, MnO2, etc.

Salt

A salt is a neutral substance which is formed by the displacement of hydrogen
of acid by metal or ammonium radical. A salt is also formed when an acid
reacts with a base. For example,

+\GURFKORULF DFLG 6RGLXP K\GUR[LGH ń 6RGLXP &KORULGH :DWHU

+&O 1D2+ ń 1D&O +2O
acid base salt water

Similarly,

6RGLXP K\GUR[LGH 6XOSKXULF DFLG ń 6RGLXP VXOSKDWH :DWHU

2NaOH + H2SO4 ń 1D2SO4 + 2H2O
base acid salt water

7\SHV RI VDOWV

7KH VDOWV FDQ EH FODVVLÀHG LQWR WKUHH W\SHV RQ WKH EDVLV RI WKHLU QDWXUH 7KH\ DUH

1. Normal (neutral) salt
A salt which is produced by the complete displacement of hydrogen
atom from an acid by a metal is called normal salt. It is prepared by the
reaction between strong acid and strong base or weak acid and weak
base. For example,

SN Name of salt Molecular formula
1. Sodium chloride (Common salt) NaCl
2. Potassium chloride KCl
3. Sodium sulphate Na2SO4
4. Potassium sulphate K2SO4

Times' Crucial Science & Environment Book - 8 190

$FLGLF VDOW

The salt formed by the partial displacement of hydrogen atom of an acid
by a metal is called acidic salt. Such salt is acidic in nature. The acidic
salt is prepared by the reaction between strong acid and weak base. For
example, ammonium chloride (NH4Cl), sodium bisulphate (NaHSO4), etc.

oAofflhtChyuodSurOgogh4eicnnopowfpaHetr2eSsruO. l4pbhyatCeu(,CituiSsOa4c)idisicfoinrmneadtubrye.thIteicsodmupelteottehreephlyadcreomlyesnist

CuSO4 + 2H2O H2SO4 + Cu(OH)2

Ccoum(OplHet)2eliys very weak iaolnksalainbdutshHo2wSOs 4itiss a strong tahceids.oSluo,tiHon2S. OT4hiuosn,itzhees
to give H+ effect in
solution becomes acidic.

3. Basic salt

A salt formed by the partial displacement of hydroxyl group of a base by
an acid radical is called basic salt. It is prepared by the reaction between
strong base and weak acid. A basic salt is alkaline in nature. For example,
sodium acetate (CH3COONa), sodium carbonate (Na2CO3), etc.

The aqueous solution of Na2CO3 is alkaline due to hydrolysis reaction.

Na2CO3 + 2H2O H2CO3 + 2NaOH

A chemical reaction in which a compound reacts with water to form new
substances is called hydrolysis.

Properties of salt

1. A few salts are salty in taste while most of the salts have bitter
taste. Some salts are tasteless.

2. Generally, they have high melting and boiling points.

3. They are generally soluble in water. But, some salts such as AgCl,
PbCl2, CaCO3, etc are insoluble in water.

4. Salts conduct electricity in their aqueous solution state.

5. Most of the salts are neutral to indicators.

6. Some salts crystallize out from their saturated aqueous solution by
absorbing some water molecules. Thus absorbed water is called water
of crystallization. The salts which contain water of crystallization
are called hydrated salts. For example, copper sulphate crystallizes
RXW IURP LWV VDWXUDWHG VROXWLRQ DEVRUELQJ ÀYH PROHFXOHV RI ZDWHU
and is written as CuSO4. 5H2O.

191 Times' Crucial Science & Environment Book - 8

Uses of salts
1. Common salt is used in our food as a seasoning and a preservative.

2. Common salt is also used to manufacture sodium metal, chlorine
gas, washing soda, etc.

3. Washing soda is used to manufacture glasses, soap, detergent, etc.

4. Baking soda is used in baking powder.

5. Silver bromide is used in photography.

6. Ppuotraifsyhinagluwmat[eKr.2SO4.Al2 (SO4)3.12H2O] is used as antiseptic and for

7. Copper sulphate (CuSO4) is used as germicide and fungicide.
8. Gypsum (CaSO4.2H2O) is used as an important ingredient of cement.

Indicators

We may identify acids or bases by their tastes
because acids taste sour and bases taste
bitter. But it is unwise to identify acids and
bases by tasting or touching them because
they can be seriously harmful. Hence, acids,
EDVHV DQG VDOWV DUH LGHQWLÀHG ZLWK WKH KHOS RI
some special chemicals called indicators. litmus paper litmus paper dipped in
ƐŽůƵƟŽŶ

A chemical substance which indicates the acidic, basic or neutral nature of
substances is called indicator. Litmus paper, methyl orange, phenolphthalein,
red cabbage juice, etc are some examples of indicators. The indicators can
change their own colour when they come in contact with other chemicals.

/LWPXV

Litmus paper is the most common indicator used in laboratory. Litmus is
H[WUDFWHG LQ WKH IRUP RI VROXWLRQ IURP SODQWV VXFK DV OLFKHQ UHG URVH ÁRZHU RU
EHHW URRW $ OLWPXV SDSHU LV SUHSDUHG E\ VRDNLQJ ÀOWHU SDSHU LQ OLWPXV VROXWLRQ
and then drying. It is available in two colours-red and blue. A blue litmus
paper turns red if it comes in contact with an acid whereas a red litmus paper
turns blue if it comes in contact with a base. But a red litmus remains red
in acid solution and a blue litmus remains blue in basic solution. Litmus is
available in solution also.

Colour of indicators

The following table shows the colour change of indicators in acid, base and salt
solutions.

Times' Crucial Science & Environment Book - 8 192

Indicator Colour in acidic Colour in Colour in salt
solution basic solution solution

Blue Litmus paper 5HG No change No change

5HG OLWPXV SDSHU No change Blue No change

Methyl orange 5HG Yellow No change

Phenolphthalein No change Pink No change
(Colourless solution) 5HPDLQV FRORXUOHVV

Universal indicator

Ordinary indicators like litmus, methyl orange, Phenolphthalein, etc can
identify whether a given substance is acidic, basic or neutral. But they cannot
show the strength of an acid or a base. Another type of indicator named
XQLYHUVDO LQGLFDWRU LV XVHG IRU ÀQGLQJ WKH VWUHQJWK RI DQ DFLG RU D EDVH 7KXV
a universal indicator is a mixture of ordinary indicators which is used to show
the strength of an acid or base.

ĐƟǀŝƚLJ ϭϱ͘ϭ To study the colour change of indicators in different solutions.

0DWHULDOV UHTXLUHG

Some test tubes, solutions of HCl, NaOH and NaCl in three separate beakers,
litmus paper, methyl orange and phenolphthalein indicators, etc.

3URFHGXUH
1. Take about 2-3 ml of HCl,

NaOH and NaCl solutions
in three separate test tubes.
2. Insert a small piece of blue
litmus paper in each of
these test tubes and observe the change in colour. Note down the colour
change in the table given below.
3. Now, remove the solutions from the test tubes and clean them with water.
5HSHDW WKH VDPH DFWLYLW\ E\ XVLQJ UHG OLWPXV SDSHU PHWK\O RUDQJH DQG
phenolphthalein separately.

2EVHUYDWLRQ

The colour change in indicators in different solutions will be observed as follows:

Indicator Colour in Colour in Colour in
HCl solution
NaOH solution NaCl solution

Blue Litmus paper 5HG No change No change

5HG OLWPXV SDSHU No change Blue No change

Methyl orange 5HG Yellow No change

Phenolphthalein No change Pink No change
(Colourless solution) 5HPDLQV FRORXUOHVV

193 Times' Crucial Science & Environment Book - 8

&RQFOXVLRQ
An indicator changes into a particular colour in acid, base or salt solution.
When the medium is changed, the indicator changes its colour. With the help
of colour change, we identify whether the given solution is acidic, basic or
neutral (i.e. salt solution).

pH scale

The strength of an acidic or basic solution is measured by pH scale. The pH scale
has the range of pH value from 1 to 14. Acidic solution has pH value less than 7
whereas the basic solution has pH value more than 7. A neutral substance has pH
value equal to 7. pH means potential of hydrogen ions in solution.

pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Colour
Orange red

Orange
Yellow
Green
Bluish green
Blue
Bluish violet
5HG Violet

Increase in acidity Neutral Increase in basicity

pH meter is also used to measure the pH value of a substance. The pH meter
is an instrument which can directly measure the pH value of a substance.

The pH value of some of the substances is given below:

SN. Substance pH
1. Lemon juice 2.5
2. Vinegar 3
3. Apple 3
4. Butter 6
5. Water 7
6. Salt solution 7
7. Sugar solution 7
8. Ethanol 7
9. Kerosene 7
10. Soda 8.5
11. Milk of magnesia 9
12. Toothpaste 9

+RZ WR ÀQG S+ YDOXH RI D VXEVWDQFH"
The substance whose pH value is to be measured is kept in a beaker and is

Times' Crucial Science & Environment Book - 8 194

mixed with distilled water. The mixture should be stirred well. Then a few
drops of universal indicator are added to the solution. The change in the colour
of the solution is observed. The colour of the solution is then matched with the
pH scale colour chart and the pH value of the substance is calculated.

corrosive ͗ Ă ĐŚĞŵŝĐĂů ǁŚŝĐŚ ĐĂƐƵĞƐ ĚĂŵĂŐĞ ƚŽ ůŝǀŝŶŐ ƟƐƐƵĞ

phenolphthalein ͗ Ă ĐŚĞŵŝĐĂů ǁŚŝĐŚ ŝƐ ƵƐĞĚ ƚŽ ŝĚĞŶƟĨLJ ĂĐŝĚ͕ ďĂƐĞ Žƌ ƐĂůƚ

hydrolysis ͗ ƌĞĂĐƟŽŶ ǁŝƚŚ ǁĂƚĞƌ

litmus paper ͗ Ă ƉĂƉĞƌ ǁŚŝĐŚ ŝƐ ƵƐĞĚ ƚŽ ŝĚĞŶƟĨLJ ĂĐŝĚ͕ ďĂƐĞ Žƌ ƐĂůƚ

methyl orange ͗ Ă ĐŚĞŵŝĐĂů ǁŚŝĐŚ ŝƐ ƵƐĞĚ ƚŽ ŝĚĞŶƟĨLJ ĂĐŝĚ͕ ďĂƐĞ Žƌ ƐĂůƚ

1. An acid is a substance that gives hydrogen ions or protons (H+) when dissolved in
water.

2. Acids can be categorized into two types- organic acids and mineral or inorganic
acids.

3. The oxides and hydroxides of metals are called bases.

4. The alkalis are the bases which give hydroxyl (OH–) ion when dissolved in water.

5. A salt is a neutral compound which is formed by the replacement of hydrogen of
acid by metal or ammonium radical.

6. An indicator is used to know whether the given compound is acid, base or salt.

7. A chemical substance which indicates the acidic, basic or neutral nature of other
substances is called indicator.

8. Litmus paper, methyl orange, phenolphthalein, red cabbage juice, etc are some
examples of indicators.

Exercise

A. Answer these questions in very short.
1. Why is it dangerous to taste acid?
2. Give two examples of organic acids.
3. Write down the sources of citric acid.
4. Name the acid that is injected by bees and ants while stinging.
5. What is the use of sulphuric acid?

195 Times' Crucial Science & Environment Book - 8

6. Write down the molecular formula of acetic acid.
7. What is the agricultural use of calcium hydroxide?
8. Give an example of acidic salt.
9. What is meant by water of crystallization?
10. What is the colour of phenolphthalein in an alkali solution?

B. Give reasons.

1. All alkalis are bases but all bases are not alkalis.

2. Lemon juice is sour in taste.

3. Aqueous solution of copper sulphate is acidic.

4. Ferric hydroxide [Fe(OH)3] is not an alkali although it is a metallic
hydroxide.

5. Aqueous solution of sodium carbonate is alkaline.

6. An acid cannot be stored in a metal container.

C. Write down differences between.

1. Acid and base 2. Base and salt

3. Base and alkali 4. Inorganic acid and organic acid

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1. Caustic soda 2. Caustic potash 3. Gypsum

4. Hydrochloric acid 5. Aluminium hydroxide 6. Sulphuric acid

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1. Acidic salt 2. Basic salt

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1. Acetic acid 2. Tartaric acid

3. Lactic acid 4. Ascorbic acid

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1. What is an acid? What are its physical properties?
2. Write down the chemical properties of acid.
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4. Write down the chemical properties of a base.

Times' Crucial Science & Environment Book - 8 196