Modern Graded Science 8

Molecule

The smallest particles of the substances whether elements or compounds which are
capable of independent existence are called molecules. They are made up of two or
more single or many types of atoms which are chemically bonded together.

A molecule of an element is Do you know?
composed of similar type of atom/

atoms chemically bonded together. Plasma is often called the fourth state of matter

For example, a molecule of oxygen which is electrically charged; does not hold its

is made up of two atoms of oxygen shape, has huge amount of energy and can be

as O = O. It is written as O2. Here, found on the earth in flames and lightning.

O2 represents a molecule of oxygen.

But an iron molecule consists of a single iron atom Fe.

A molecule of a compound is composed of two or more atoms of different elements
bonded together. For example, a molecule of sodium chloride is made up of one atom of
sodium and one atom of chloride. It is represented by NaCl.

Atom

An atom is the smallest particle of an element which takes part in a chemical reaction
without division. It may or may not be capable of independent existence. Atoms of the
same elements are identical in all respects e.g. hydrogen atom, nitrogen atom, etc. But
the atoms of different elements have different kinds of atoms. For example, an atom of
sodium is different from the atom of magnesium.

Structure of the atom nucleus shells
electrons
According to scientists, an atom is a sphere with a diameter
of approximately 10-10 metres. In the 19th century, it was
found that there are other smaller particles in an atom. They
are electrons, protons and neutrons. These particles are
called sub-atomic particles or fundamental particles.

Among the three sub-atomic particles of an atom, protons

and neutrons are located at the centre of an atom called a protons neutron

nucleus. Electrons revolve around the nucleus in a fixed Fig 11.1 structure of the beryllium atom
path called a orbit or shell. The atomic model or electrons

distribution around the nucleus is very similar to the arrangement of the solar system.

The nucleus represents the sun and the electrons represent the planets. The modern

atomic model is based on this model. The main features of the structure of the atom are

as given below:

a. The central portion of any atom is the nucleus.

b. The nucleus contains protons, which are positively charged particles and the
neutrons, which are charge less particles.

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The negatively charged particles are called electrons. They revolve around the nucleus
with a high speed in certain closed circular paths called shells.

The structure of an atom with the position of protons, neutrons and electrons of beryllium
is shown in the figure above.

A short description of sub-atomic particles

a. Electron

An electron is a very minute particle and it has a unit of negative electric charge. Its
mass is only 1/1837th of the mass of a hydrogen atom. It is represented by e-. It always
revolves round the nucleus of an atom in its fixed shell. The negative charge of an
electron exactly balances the positive charge of the proton.

b. Proton

A proton is the particle that has a unit of positive electric charge. It has the mass equal
to the mass of an atom of hydrogen. It is represented by p+. It is located in the nucleus.

c. Neutron

A neutron is the particle which is situated at the nucleus of an atom. It is an electrically
neutral particle. It is denoted by nº. Its mass is about equal to the mass of one proton. All
elements except hydrogen have neutrons. It helps to find the atomic weight of the elements.

Atomic mass unit (amu)

The mass of an atom is extremely small. It cannot be measured in gram or milligram.
Because of this reason, the mass of an atom (proton, neutron and electron) is measured
in atomic mass unit. The mass of one hydrogen atom is 1 amu. One gram is equal to
6× 1023 amu.

1 g = 6 × 1023 amu

The mass of a proton is nearly equal to that of one hydrogen atom. So, the mass of

proton is about 1 amu. The mass of one neutron is equal to that of mass of one proton.

But the mass of one electron is equal to about 1 th the mass of proton. We can say
1837

that, the mass of 1837 electrons is equal to the mass of one proton.

Thus, 1p+ = 1n0 = 1837e– = 1 amu.

Electric charge

Protons and electrons are charged particles. The electric charge is measured in coulomb.
Briefly, it is written as C.

1 coulomb = 6.25 × 1018 e.s.u. = charge of 6.25 × 1018 electrons

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The most common characteristics of the three sub-atomic particles are as follows:

S.No. Name of the sub Location Relative mass [ in atomic Charge [ in electrostatic
atomic particles mass unit (amu)] unit (esu)]

1. Electron (e-) Shell 1/1837th of H -1

2. Proton (p+) Nucleus 1H +1

3. Neutron (n0) Nucleus 1H 0

The atoms of different elements have different numbers of protons and electrons. The
number of protons in the nucleus is equal to the number of electrons in different shells
of a neutral atom. Due to the presence of protons, the nucleus behaves to be positively
charged. An atom of each element is electrically neutral because naturally the number of
protons is equal to the number of electrons.

Importance of atom

a. The molecules of an element are formed by the combination of atoms. The
molecules form matter. Thus, atom is the basic unit of matter.

b. Compounds are formed by the chemical combination of the atoms of different
elements in a definite proportion.

c. The atoms of elements take part in a chemical reaction to form new
substances.

Atomic number

The number of positively charged particles or protons present in the nucleus of the atom
of an element is called its atomic number. It is denoted by Z. In an electrically neutral
atom, the number of protons is equal to the number of electrons. Hence, the atomic
number can be defined as the number of electrons present in a neutral atom.

Every element has its own atomic number. Therefore, the atomic number of an element
distinguishes it from the atoms of other elements.

Mathematically,

Atomic number = Number of protons = Number of electrons

i.e. Z = p+ = e–

For example, the atomic number of oxygen is 8. It means that there are 8 protons or/and
8 electrons in the atom of oxygen.

The atomic number of an element determines the properties of the element.

Atomic mass

An atom consists of electrons, protons and neutrons. The mass of electron is extremely
small and it is taken as a negligible mass compared to the mass of a proton or neutron.

Matter 149

Protons and neutrons are located in the nucleus. Practically, the atomic mass is
centralized in the nucleus. Thus, the atomic mass of an element is defined as the sum of
the number of protons and neutrons present in the nucleus of an atom of that element.
It is also called atomic weight. It has no unit. It is denoted by A. Usually, atomic mass of
an element is constant.

Mathematically,

Atomic mass = Number of protons + Number of neutrons.
i.e. A = p+ + n°

When the atomic mass of an element is given, we can find the number of protons,
electrons and neutrons.

... Atomic number = No. of protons = No. of electrons

We also have,

Atomic mass = No. of protons + No. of neutrons
... Number of neutrons = Atomic mass - Atomic number.

i.e. n° = A – Z

The arrangement of electrons around the nucleus and the presence of the number of
protons determine the properties of an atom. The nucleus determines the mass of an
atom, while the number of electrons determines the size of the atom of the element.

Activity 11.1

Sketch atomic structure and then write the of the following elements as given in the example.

(i) Ca (ii) Na (iii) Mg (iv) Ar (v) F and (vi) O

E.g. Phosphorus (P)

Atomic structure p+ nº e- Z A Electronic configuration
15 16 15 15 31 K LMN
285×

p+ 15
no 16

Representation of atomic symbol

The atomic symbol of a neutral atom say 'Y' may be written as “ZYA". Where Y is the
atomic symbol of the element, Z is the atomic number and A is the atomic mass. For
example, the atomic symbol of sodium is 11Na23. In it, the number at the right top of
the atomic symbol of sodium is the mass number; the left bottom number is the atomic
number and Na is the atomic symbol of sodium. The symbols also represent the nucleus
of the element.

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Molecular weight

The sum of the atomic weights of all the atoms of the same or different elements present
in a molecule is called molecular weight. Molecular weight can be calculated by adding
the atomic weights of the elements present in a molecule. For example, an oxygen
molecule has two oxygen atoms and the atomic weight of oxygen is 16.

Then, the molecular weight of oxygen (O2) = 2 • 16 = 32 amu.
Similarly, the molecular weight of carbon dioxide (CO2) = 1 • 12 + 2 • 16

Here the molecular weight of carbon = 12 + 32 = 44 amu.
(C) is 12 and that of oxygen (O) is 16

Electronic configuration of an atom

We have already learnt that protons and neutrons are located in the nucleus. Electrons
revolve around the nucleus in their orbits or shells. The path of electrons around the
nucleus is called an orbit or shell and the number of shells makes energy level. The
shells range from one to seven. The shell near the nucleus is called lower energy level
and the shell that is away from the nucleus is called higher energy level. The systematic
distribution of electrons outside the nucleus in different energy levels or shells of an
atom is called electronic configuration. The following points should be noted to write the
electronic configuration of an element.

a. The number of electrons present in an atom of the element, which can be known
by its atomic number.

b. The maximum number of electrons that can be accommodated in different shells
of an atom which can be known by 2n2 rule or Bohr-Burry scheme.

To explain the arrangement of electrons in different shells, Bohr and Bury
(1921 AD) proposed Bohr-Burry scheme for the distribution of electrons in different shells
as given below:

1. The maximum number of electrons in each shell is determined by the 2n2 formula.
[where 'n' is the number of the orbit.] The number of orbit closest to the nucleus is
represented by K and beyond the nucleus L, M, N in order.

For example,

For the K- shell, the number of electrons will be 2n2 = 2×(1)2 = 2×1 = 2 (where n = 1)

For the L- shell, the number of electrons will be 2n2 = 2×(2)2 = 2×4 = 8 (where n = 2)

For the M- shell, the number of electrons will be 2n2 = 2×(3)2 = 2×9 = 18 (where n = 3)

For the N- shell, the number of electrons will be 2n2 = 2×(4)2 = 2×16 = 32 (where n = 4)

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But this rule is not applicable to O, P and Q shells of an atom. The number of
electrons that can be accommodated by O, P and Q are 32, 18 and 8 respectively.

2. The outermost shell cannot have more than 8 electrons and the second outermost
shell cannot have more than 18.

Max e–

8 shells
18
maximum number of electrons are 32
accommodated in different shells 32
18
according to 2n2 rule 8
2

7 6 5 4 3 2 1 nucleus K L M N O P Q shell number

Fig 11.2 arrangement of electrons in different shells of an atom

3 It is not necessary to follow only 2n2 to distribute electrons in different shells but a
new shell commences when any second outermost shell attains 8 electrons.

4. The energy level increases as we move from the first shell to the last shell. So, the
electrons are filled from the lower shell to the higher shells.

Using the above rules, the atomic structure of p+ 2 p+ 2
helium (He) can be given as below: no 2 no 2

Number of protons (p+) = 2

Number of electrons (e–) = 2 Fig 11.3 (a) (b)
Number of neutrons (n0) = 2

The nucleus of a helium atom contains 2 protons and 2 neutrons. It has the K-shell only.
In the K-shell, 2 electrons are revolving around the nucleus in their own path. These
two electrons lie in the area of the K-shell but their paths are different as in figure (b). To
make it easier to understand, only one circle is made for a shell and two electrons are
arranged in it.

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The first 20 elements with their symbol, atomic number, number of protons, number
of electrons, number of neutrons, atomic mass and electronic configuration are
given below:

S.No. Elements Symbol At. Protons Electrons Neutrons At. No. of electron in different orbits
num mass K LMN

1 Hydrogen H11 1 0 11 - - -

2 Helium He 2 2 2 2 42 - - -

3 Lithium Li 3 3 3 4 72 1- -

4 Beryllium Be 4 4 4 5 92 2- -

5 Boron B55 5 6 11 2 3- -

6 Carbon C66 6 6 12 2 4- -

7 Nitrogen N77 7 7 14 2 5- -

8 Oxygen O8 8 8 8 16 2 6- -

9 Fluorine F99 9 10 19 2 7- -

10 Neon Ne 10 10 10 10 20 2 8- -

11 Sodium Na 11 11 11 12 23 2 81 -

12 Magnesium Mg 12 12 12 12 24 2 82 -

13 Aluminium Al 13 13 13 14 27 2 83 -

14 Silicon Si 14 14 14 14 28 2 84 -

15 Phosphorus P 15 15 15 16 31 2 85 -

16 Sulphur S 16 16 16 16 32 2 86 -

17 Chlorine CI 17 17 17 18 35 2 87 -

18 Argon Ar 18 18 18 22 40 2 88 -

19 Potassium K 19 19 19 20 39 2 881

20 Calcium Ca 20 20 20 20 40 2 882

Atomic structure of boron

Atomic number = 5 = [ e– = p+] 5 p+
Atomic weight = 11 amu = [p+ + no] 6 no

Valency = 3

If the number of electron in the last orbit is 1 to 4 , the valency Fig 11.4 boron
will be equal to the number of electrons in the last orbit [except
He; it has valency 0]. If the number of electrons in the last orbit Shells K LM
is 5 to 8, the valency will be equal to 8-valence electron.
E. configuration 2 3 –

Group number in the periodic table = IIIA [number of e- in the last orbit]

Period number in the periodic table = 2 [ equal to the shell number]

Electronic configuration = 2, 3

Matter 153

Atomic structure of some other elements

11 p+ 18 p+ 20 p+
12 no 22 no 20 no

Sodium Argon Calcium

Shells K LMN Shells K LMN Shells K LMN
88x
E. configuration 2 8 1 x E. configuration 2 E. configuration 2 8 8 2

Activity 11.2 Fig 11.5

Sketch the atomic structure and write the informations as shown in the example.

(i) Chlorine (Cl) (ii) Sulphur (S) (iii) Boron (B)

(iv) Neon (Ne) (v) Potassium (K) (vi) Argon (Ar)

Example, Magnesium (Mg) Atomic number (Z) = 12

Atomic mass (A) = 24 amu

Electronic configuration = K L M

p+ 12 2 8 2

no 12

Valence electron = 2

Valency = 2

Group no. in periodic table = IIA

Atomic structure of magnesium Period no. in periodic table = 3rd

Radicals

A radical is an atom or a group of atoms carrying a positive or negative charge that acts
as a single unit in the chemical reaction. Radicals are highly reactive and unstable. So,
we cannot see them but we can see their effects. Radicals are classified into two types
depending upon the nature of changes carried by them. They are electropositive and
electronegative radicals.

a. Electropositive radicals

An atom or a group of atoms carrying a positive charge on them are called electropositive
radicals. They are also called basic radicals or metallic radicals e.g. Ca++, Mg++, Ba++,
Al+++, NH4+, etc.

b. Electronegative radicals

An atom or a group of atoms carrying a negative charge are called electronegative
O- -, NO3-,
radicals. They are also called acidic radicals or non-metallic radicals e.g.
CO3- -, HCO3- , etc.

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Radicals do not exist independently. They are present in a combined form with other
atoms or radicals. They have their own symbols and valencies. On the basis of their
construction, radicals are also classified into simple radical and compound radical.
A simple radical is that radical which contains only one atom e.g. Na+, O- -, C++++,
N- - -, Mg++, H+, etc. On the other hand, a compound radical consists of a group of
atoms e.g. O2- - (peroxide) CO3- - (carbonate), NO3- (nitrate), etc.

Valency

The combining capacity of an atom or a radical with other atoms or radicals to form a
molecule is called its valency. Numbers such as 0,1, 2, 3, 4, 5 and 6 represent valency.
All the elements have their own valency. Inert elements have valency '0' (zero) thus, they
cannot form compounds but other elements can do.

The valency of an element or a radical is a measure of its combining capacity. To measure
the valency of an element or a radical, hydrogen can be taken as the standard element.
This is because hydrogen is the simplest element and its valency is 1. The valency
of other elements or radicals is determined by the number of hydrogen atoms which
combines with or displaces one atom of the element or radical. The common examples
to find out the valency of elements or radicals are:

a. The valency of chlorine (Cl) in hydrogen chloride HCl is 1 because 1 atom of
Cl combines with 1 atom of H.

b. The valency of oxygen O in water H2O is 2 because 1 atom of O combines
with two atoms of H.

c. The valency of nitrogen N in ammonia NH3 is 3 because 1 atom of N
combines with 3 atoms of H.

d. The valency of carbon C in methane CH4 is 4 because 1 atom of C combines
with 4 atoms of H.

Hydrogen does not combine with all other elements. In such a condition, the valency of
an element or a radical is determined by comparing it with that of chlorine or oxygen
whose valency with hydrogen is known. For example, 1 atom of Na combines with 1
atom of Cl in sodium chloride, thus, the valency of sodium in sodium chloride (NaCl) is 1.

Similarly, the valency of magnesium Mg in magnesium oxide MgO is 2 because 1 atom
of oxygen combines with 1 atom of magnesium. Now, the valency of the element that
combines with oxygen can be found out by taking the valency of oxygen to be 2.

Some elements, sometimes, show variable valencies because of experimental conditions
and the amount of reacting substances used to form a compound. Usually, the names of
elements with a lower valency ends with suffix -'ous' and with the higher valency ends with
suffix - 'ic'. For example, iron having its valency 2 is ferrous and that having its valency 3
is ferric. Symbols and valencies of some basic and acid radicals are given below:

Matter 155

Symbol and valency of some basic radicals

S.No. Radical Symbol Valency S.No. Radical Symbol Valency
1. Hydrogen H 1 12. Zinc Zn 2
2. Sodium Na 1 13. Magnesium Mg 2
3. Potassium K 1 14. Ferrous (Iron) Fe 2
4. 15. Ferric (Iron) Fe 3
5. Silver Ag 1 16. Manganous Mn 2
6. Ammonium NH4 1
Cu 1 (Manganese) Mn 3
7. Cuprous 17. Manganic
(Copper) Cu 2 Pb 2
8. (Manganese)
Cupric Hg 1 18. Plumbous Pb 4
9. (Copper) Al 3
Mercurous Hg 2 (lead) C 4
10. (Mercury) 19. Plumbic (lead) Si 4
11. Mercuric Ca 2
(Mercury) Ba 2 20. Aluminium
Calcium
21. Carbon
Barium
22. Silicon

Symbol and valency of some acid radicals

S.No. Radical Symbol Valency S.No. Radical Symbol Valency
2
1. Chloride Cl 1 11. Oxide O 2
2
2. Bromide Br 1 12. Peroxide O2 2
SiO3 2
3. Iodide I 1 13. Silicate CO3 2
2
4. Chlorate CIO3 1 14 Carbonate S 2

5. Nitrite NO2 1 15. Sulphide SO3 3
SO4
6. Nitrate NO3 1 16. Sulphite CrO4 3

7. Hydroxide OH 1 17. Sulphate N

8. Bicarbonate HCO3 1 18. Chromate PO4
9. Bisulphate HSO4 1 19. Nitride
10. Permanganate MnO4 1 20. Phosphate

Molecular formulae of elements

The molecular formula of an element is defined as the symbolic representation which
shows the actual number of atoms in one molecule of the element. For example, a
molecule of nitrogen contains two nitrogen atoms, thus its molecular formula is N2. Here,
the number of nitrogen below 2 stands for the number of nitrogen atoms.

Similarly, the molecules of hydrogen, oxygen and chlorine are represented as H2, O2 and
Cl2 respectively. They are diatomic elements. It means their molecule consists of 2 atoms.

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The symbols and molecular formulae of other elements like helium (He), neon (Ne),
krypton (Kr), iron (Fe), silver (Ag), etc. are just the same because they are mono-atomic
molecules, which have a single atom in their molecule.

Note: O2 represents one molecule of oxygen whereas 2O represents 2 atoms of oxygen.

Molecular formula of a compound

The molecular formula of a compound is defined as the symbolic representation which
shows the actual number of atoms of different elements present in one molecule of the
compound. For example, one molecule of ammonia contains one atom of nitrogen and
three atoms of hydrogen. Thus, the molecular formula of ammonia is NH3. Similarly, one
molecule of water is made up of two atoms of hydrogen and one atom of oxygen. Thus,
its molecular formula is H2O.

Information given by a molecular formula

A molecular formula gives the following information:
a. A molecular formula represents one molecule of the substance.
b. It gives the actual number of atoms of each element present in one molecule.
c. It shows the combining capacity of its component elements.
d. It helps to know about the molecular weight of the substance.

Steps of writing molecular formulae of compounds

To write a molecular formula, the following steps are usually adopted if we only know the
symbols and valencies of the radicals.

Step 1: First, the name of the compound is written.

e.g. Aluminium sulphate

Step 2: The symbol of basic and acidic radicals of the compound present in the
molecule is written side by side.

e.g. Al SO4
Step 3: The valency of each radical is written on the right upper corner

(superscript) of the symbol.
e.g. Al3 SO42
The valency of one radical is transferred to another radical and it is written in the right lower
corner (subscript). If necessary, L.C.M. of valencies is taken to get a simple whole number.
e.g. Al2(SO4)3
Step 4: If a compound radical takes part in the molecular formula, the radical

is enclosed in brackets and the valency obtained after the exchange is
written on the right lower side of the bracket at the bottom of the formula.

e.g. Al2(SO4­ )3 [ Molecular formula of aluminium carbonate].

Matter 157

Some other examples of molecular formulae

1. Calcium nitrate

Ca2 NO31 Ca1NO32 Ca(NO3)2

Molecular formula of calcium nitrate

2. Carbon dioxide

C4 O2 C2O4 CO2

Molecular formula of carbon dioxide

3. Silicon oxide

Si4 O2 Si2O4 SiO2

Molecular formula of silicon oxide

4. Aluminium oxide

Al3 O2 Al2O3 Al2O3

Molecular formula of aluminium oxide

5. Sodium peroxide

Na1 O22 Na2O2 1 Na2O2

Molecular formula of sodium peroxide

Word equation

We have already studied that changes may be physical or chemical. The process
of changing the chemical properties of any substance is called chemical reaction.
A chemical reaction can be represented by writing the full names of reactants and
products putting an arrow between them which is called a word equation. The arrow (→)
pointing from reactants to products shows the direction of chemical change.

The chemical substances which take part in the chemical reactions are called reactants
and the new substances which are formed from a chemical change are called products.
For example, sodium hydroxide reacts with hydrochloric acid to produce sodium chloride
and water. In this reaction, sodium hydroxide and hydrochloric acid are reactants and
sodium chloride and water are products.

Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water

Reactants Products

Similarly, when carbon burns in oxygen it produces carbon dioxide. The word equation for this
reaction is written as follows.

Carbon + Oxygen heat Carbon dioxide heat

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Chemical equation

A chemical equation may be defined as the symbolic representation of a chemical
reaction. If the chemical equation is written in terms of symbols and molecular formulae,
then it is called a formula equation. A formula equation may be unbalanced or balanced.
In the unbalanced equation, the number of atoms in reactants and products is not equal.
It is also called a skeleton equation. This equation can be balanced by comparing the
number of atoms. The equation in each side is called a balanced equation only when the
number of atoms of the all elements are equal in reactants and products.

To understand the above fact, the following chemical equation is taken as an example.
When a mixture of hydrogen and chlorine is placed in the sunlight, they react together and
form hydrogen chloride.

Hydrogen + Chlorine Hydrogen chloride

Reactants Product

H2 + Cl2 HCl (Formula equation/skeleton equation)

H2 + Cl2 2HCl (Balanced equation)

In this chemical equation, the number of hydrogen and chlorine are equal in the reactants

and the product. Thus, this equation is called a balanced equation.

Some clues for balancing chemical equations

The following clues should be kept in mind to balance a chemical equation:

a. First of all write the word equation clearly.

b. The molecular formula of each reactant and product is written correctly. The
written expression is called a skeletal unbalanced equation.

c. While counting the number of atoms in reactants and products, the following
points should be noted:

i. The number, which is written on the right lower corner of an atom, is for
that atom only. For example, AlCl3 has 1Al and 3Cl.

ii. The number, which is written on the right lower corner of a bracket is
for all those atoms enclosed in the bracket. For example, Ca(OH)2 has
1Ca, 2O and 2H.

iii. The number of coefficient is for all the number of atoms of the molecule.
For example, 2Ca(OH)­2 has 2Ca, (2 x 2) O = 4 O and (2 x 2) H = 4 H.

d. The atoms used at few places in an equation are balanced first, whereas the
hydrogen and oxygen atoms of the equation are balanced at the end.

Matter 159

Some examples of chemical equations

1. Magnesium + Nitrogen → Magnesium nitride [Word equation]

Mg + N2 Mg3N2 [Skeleton equation]
3Mg + N2
Mg3N2 [Balanced equation]

2. Magnesium oxide + Water Magnesium hydroxide [Word equation]

MgO + H2O ∆ Mg(OH)2 [Balanced equation]
Potassium chlorate
3. Potassium chloride + Oxygen [Word equation]

KClO3 ∆ KCl + O2 [Skeleton equation]
2KClO3 ∆ 2KCl + 3O2­ [Balanced equation]

4. Iron + Copper sulphate Copper + Iron sulphate [Word equation]

Fe + CuSO4 Cu + FeSO4 [Balanced equation
5. Aluminium + Oxygen ∆ Aluminium oxide [Word equation]

Al + O2 Al2O3 [Skeleton equation]
4Al + 2 O2 2Al2O3 [Balanced equation]

Some substances with their common names and molecular formulae

S.No. Substances Molecular S. Substances Molecular
formulae No. formulae
C
1. Coal, Charcoal H2O 18. Nitrogen trioxide N2O3
2. Water, water vapour NH3 19. Nitrogen pentoxide N2O5
3. Ammonia CO2 20. Phosphorus pentoxide P2O5
4. Carbon dioxide CO 21. Hydrochloric acid HCl
5. Carbon monoxide CH4
6. Methane Na2CO3 22. Hydrobromic acid HBr
7. Washing soda NaHCO3
8. Baking soda NaOH 3. Hydriodic acid HI
9. Caustic soda
10. Caustic potash KOH 24. Nitric acid HNO3
11. Blue vitriol
12. Common salt CuSO4 25. Sulphuric acid H2SO4
13. Manganese dioxide NaCl
14. Sulphur dioxide 26. Carbonic acid H2CO3
15. Nitrogen dioxide MnO2
16. Nitrous oxide SO2 27. Phosphoric acid H3PO4
17. Nitric oxide NO2
N2O 28. Rust Fe2O3.2H2O
NO
29. Gypsum salt CaSO4.H2O

30. Epsom salt MgSO4

31. Chalk or limestone or marble CaCO3

32. Quick lime CaO

33. Slaked lime or lime water Ca(OH)2

34. Silica SiO2

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Classification of elements : Periodic table

After the discovery of many elements, it was very difficult to study the properties of each
element and its compounds separately. To solve this difficulty, chemists have classified
the elements in different groups on different bases. The grouping of elements according to
their similar characteristics is called the classification of elements. For the same purpose,
periodic table was given. The classification of the elements into several groups and periods
in the periodic table on the basis of their properties has made their study easier. Hence, the
periodic table is the systematic arrangement or classification of all the elements on the basis
of their similar properties in tabular form. Many attempts were made by several chemists to
classify elements. None of them proved to be satisfactory. The most important concept of
classification of elements was given by the Russian scientist Dmitri Mendeleev in 1969 AD.
He classified all the known elements in the order of increasing atomic mass. He found that
the elements with similar properties appear at regular intervals.

This led Mendeleev to state his periodic law which was known as Mendeleev's periodic
law. This law states that "the physical and chemical properties of the elements are the
periodic functions of their atomic masses". Periodic functions mean after a regular
intervals, the properties of the elements are repeated although the atomic mass goes on
increasing. Mendeleev observed that the elements with similar properties fall one by one
in the same column. This results in the formation of a table which is called Mendeleev's
periodic table. The periodic table is divided horizontally into seven rows called periods
and vertically into eight columns called groups.

The elements with similar properties are occupied in the same group while the elements
of the same period show the gradual change in their properties from left to right in the
periodic table. Only 63 elements had been discovered when Mendeleev gave the periodic
table. He left some gaps in his periodic table for unidentified elements.

The zero group consisting inert gas elements like helium, neon and argon were not
discovered at the time of Mendeleev. So, these elements were not mentioned in the
Mendeleev's periodic table.

A portion of Mendeleev's original periodic table

Group Group Group Group Group Group Group Group
I II III IV V VI VII VIII

Period 1 H He

Period 2 Li Be B C N O F Ne

Period 3 Na Mg Al Si P S Cl Ar

Period 4 K Ca 1* Ti V Cr Mn Fe Cu
Cu Zn 2* 3* As Se Br Ni

Period 5 Rb Sr Y Zr Nb Mo 4* Ru Rh
Ag Cd I Pd
In Sn Sb Te

Period 6 Cs Ba La Hf Ta W Re Os Ir
Au Hg Th Pb Bi Po At Pt

Name given by Mendeleev
1* Eka – Aluminium, 2* Eka – Boron, 3* Eka – Silicon, 4* Eka - Manganese

Matter 161

The defects of Mendeleev's periodic table are as given below:

a. Position of hydrogen: The position of hydrogen is not properly defined. It is
because hydrogen resembles both group-I and group- VII elements i.e. it
forms both positive and negative ions during a chemical reaction.

b. Grouping of chemically dissimilar elements: Certain chemically dissimilar
elements are placed in the same group. For example, copper, silver and gold
are placed in group-I with the metals like lithium, sodium and potassium.
Their properties are quite different.

c. Fault of the periodic law: Certain elements of higher atomic mass are placed
before the elements of lower atomic mass. These do not obey the principle.
For example, argon (atomic mass = 39.9) is placed before potassium (atomic
mass = 39.1) in the Mendeleev's periodic table.

d. Position of metals and non-metals: Metals and non-metals are not placed separately.

Modern periodic table

Defects of Mendeleev's periodic table led to the conclusion that atomic mass cannot
be a basis for the classification of elements. To overcome the defects brought by
Mendeleev's periodic law, Henery Mosely in 1913 AD did an experiment and found that
the fundamental properties of an element are based on the atomic number not on their
atomic mass. This is quite convincing because the chemical properties of each element
depends on the number of electrons present in its atom. And the number of electrons
in the atom is the atomic number. Based on this observation, the modern periodic law
states that "the physical and chemical properties of elements are periodic functions of
their atomic numbers". The classification of the elements based on the modern periodic
law is known as the modern periodic table.

Based on the modern periodic law, the first twenty elements are arranged in the periodic
table as given below:

Group Group Group Group Group Group Group Group

I II III IV V VI VII VIII

Period 1 H1 He2

Period 2 Li3 Be4 B5 C6 N7 O8 F9 Ne10

Period 3 Na11 Mg12 Al13 Si14 P15 S16 Cl17 Ar18

Period 4 K19 Ca20

The modern periodic table has helped to solve the defects seen in Mendeleev's periodic
table. They are as follows:

a. Hydrogen is placed in group-I in the periodic table due to its least atomic number.

b. The wrong position of some elements like argon and potassium due to their
atomic weight has been solved by arranging the elements in the order of
increasing atomic number.

c. The group is divided into sub-group A and sub-group B. It makes the study of
elements specific, easier and systematic.

162 Modern Graded Science and Environment Book 8

Periods in the periodic table

The horizontal rows of the modern periodic table are called periods. There are seven periods
numbered as 1, 2, 3, 4, 5, 6 and 7. The number of periods is equal to the number of shells of
an atom of the elements in which electrons are filled as the atomic number increases. The
elements in a period have the same number of shells but their chemical properties are quite
different because the number of electrons is different. Each period has a fixed number of
elements. The periods are divided into short periods and long periods.

The 1st period consists of two elements, and the 2nd and the 3rd periods contain 8
elements each. They are called short periods. The rest periods are called long periods
as they contain 18 or more elements. The 4th and 5th periods contain 18 elements each
but the 6th and the 7th periods have 32 elements.

Groups in the periodic table

The vertical columns of the periodic table are called groups. There are altogether 18
vertical columns. These are named as group I to VII (sub-group I A to sub-group VII A,
sub-group I B to sub-group VII B), group VIII (3 columns) and group zero (0).

All the elements like H, Li, Na and K of the group I A have only one electron in the
outermost shell or valence shell. During a chemical reaction, the atom of each element
loses one electron to other atoms and gains a unit positive charge. Therefore, they are
considered as very reactive metals. The elements of this group are light in weight with
low melting points. They all react with water easily to produce hydrogen and a soluble
base. Thus, these elements are called alkali metals.

Elements of group II A like beryllium (Be), magnesium (Mg), calcium (Ca), etc. have
two valence electrons. They lose two valence electrons to others and carry two positive
charges. So, they are also reactive metals. They are called alkaline earth metals.

On the extreme right side of the table, inert elements are kept which fall in the Group
0. These elements are He, Ne, Ar, Kr, Xe and Rn. They all have eight electrons except
helium (He) in their outermost shell, therefore, they are also called octet but helium is
duplet. They are non-reactive elements, so none of these elements form any known
chemical compound. They are also known as inert gases or noble elements.

The elements of group VII are fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). They
all have seven electrons in the valence shell. Thus, they are very reactive non-metals as
they can easily gain one electron from other and possess a unit negative charge. All the
elements of this group react with water to form acid; they are also called halogens, as
they can form salt by reacting with metals.

It is interesting to note that there is a similarity in the properties of elements in each group.
For instance, hydrogen reacts with oxygen to produce hydrogen oxide or water while
hydrogen reacts with sulphur to form hydrogen sulphide. Sodium reacts with water to form
sodium hydroxide whereas potassium also reacts with water to form potassium hydroxide.

Matter 163

Period s-block elements Periodic table p-block elements

164 Modern Graded Science and Environment Book 8 Group Representative Elements Noble Gas 0
Representative Elements
n=1 Relative Atomic Mass (1)
Group IUPAC Group CAS

n= 2 Atomic Number
Symbol

n= 3 Element Name

Transition Elements / d-block elements

n= 4

n= 5

n= 6

n= 7 Inner Transition Elements / f-block elements

Series of Lanthanide
Elements (57-71)

Series of Actinide
(89-103)

Importance of modern periodic table

a. It gives a clear and distinct classification of all known elements.
b. It helps for the prediction of new elements.
c. It gives the correct position of elements.

d. It also helps to describe the simple properties of elements.

THINGS TO KNOW

1. The smallest particle of matter that is capable of independent existence is called a
molecule.

2. A mixture is a substance formed by mixing two or more elements or compounds in
any proportions by mass.

3. An element is a substance which cannot be split up into two or more simpler
substances.

4. A compound is a substance produced by the union of two or more elements in a
definite proportion by weight.

5. An atom is the smallest particle of an element which can take part in a chemical reaction.
6. Protons, neutrons and electrons are three sub-atomic particles.
7. An atom is electrically neutral because the number of protons is equal to the

number of electrons.
8. The path of electrons around the nucleus is called an orbit or shell and the number

of orbits makes energy level. The arrangement and distribution of electrons in
different shells is called electronic configuration.
9. Electropositive radicals are cations or basic radicals and electronegative radicals
are anions or acid radicals.
10. The radical made of a single atom is called a simple radical and that made of two
or more atoms is called a compound radical.
11. The combining capacity of an element or radical is called valency of that element
or radical.
12. Valency may be variable in some cases. The elements with lower valency end with
'ous' and those with higher valency end with 'ic'.
13. The symbolic representation of a molecule is called a molecular formula.
14. Molecular formulae are written using symbols and valencies.
15. The representation of a chemical reaction in the form of a chemical reaction is
called a chemical equation.
16. Reactants are those substances which take part in chemical change while products
are those new substances which are formed as a result of the chemical change.
17. The representation of a chemical change by writing the full names of the reactants
and products is called a word equation.
18. Mendeleev's periodic law states that physical and chemical properties of elements
are periodic functions of their atomic masses.

Matter 165

19. The modern periodic law states that the physical and chemical properties of
elements are periodic functions of their atomic numbers.

20. There are seven periods and nine groups in the periodic table. Groups are vertical
columns and periods are horizontal rows of the periodic table.

THINGS TO DO

1. To make the atomic model of nitrogen
a. Make small bead structures of mud. Colour the beads into different colours
to represent proton, neutron and electron.
b. Take one cardboard and in the centre of it make a circle by using thick wire
or thread and stick with the help of super glue. This represents the nucleus.
c. Stick 7 protons and 7 neutrons inside the nucleus which are made of mud.
d. Make two shells or orbits in the concentric circles around the nucleus with the
help of thick wire or thick thread by sticking it with glue.
e. Stick the 2 electrons made of mud in the first circle and 5 in the last circle.

2. Sketch a modern periodic table in a sheet of chart paper and hang it in your classroom.
3. Make a chart showing symbols and valencies of basic and acid radicals. Paste in the

front of your study table. Study them in free time. Learning the table by heart is a must
for the better understanding of chemistry.

TEST YOURSELF

1. Fill in the blanks.
a. The water is a ............. .
b. Proton, electron and neutron are ............ particles.
c. The group VIIA elements are called ............. .
d. The combining capacity of an element or radical is called ........... .
e. The atomic weight of potassium is ............ .

2. Match the following:

chlorine inert element

silicon duplet

sodium halogen

argon metalloid

helium alkali metal

3. Tick () for correct answer (MCQs).

a. Sodium belongs to ................. group of periodic table.

i. IIA ii. IB iii. IA iv. zero

b. The proton and electron are measured in:

i. amu ii. g iii. mg iv. coulomb

166 Modern Graded Science and Environment Book 8

c. The molecular weight of ammonia (NH3) is:

i. 17 amu ii. 18 amu iii. 14 amu iv. 21 amu

d. Scientists have discovered ........... elements so far.

i. 92 ii. 26 iii. 118 iv. 63

e. Basic radicals are usually called ............. radicals.

i. acidic ii. electronegative iii. electropositive iv. all of the above

f. The smallest particle of a compound is called

i. molecule ii. atom iii. element iv. electron

g. ............. is chargeless particle.

i. electron ii. neutron iii. proton iv. nucleus

4. Define the following terms and give an example of each.

a. element b. atomic weight c. periodic table

5. Identify whether the following are elements, compounds and mixture.

a. rusting of iron b. zinc c. common salt

d. gold e. smoke f. water

6. Distinguish between:

a. atomic number and atomic weight b. mixture and compound

c. group and period d. short period and long period

7. Write a short note on:

a. molecular weight b. structure of an atom c. Mendeleev's periodic table

8. Answer these questions.

a. What is the smallest particle which possesses the properties of a compound called?

b. What are the fundamental particles of an atom?

c. Compare the properties of protons, neutrons and electrons.

d. Define symbol and valency?
e. What does 2n2 denote? Explain it with the help of two examples.

f. What is octet? Why is it inactive?

g. What is meant by reactive elements?

h. What are the defects of Mendeleev's periodic table?

i. State the modern periodic law.

j. Write the features of the modern periodic table.

k. What are the demerits of Mendeleev's periodic table?

l. What are the uses of the periodic table?

m. Define reactant and product. Clear your answer with the help of examples.

n. Define radical? What are electropositive radicals? Write with examples.

o. The molecular formula of water is H2O. What does it express?

Matter 167

9. Give reasons.

a. Neon is an inert gas.

b. The valency of oxygen is two.

c. The elements of group IA are called alkali metals.

d. Chlorine is a very reactive non-metals.

e. Naturally, an atom is neutral in charge.

f. The nucleus of an atom is positively charged.

g. Modern periodic table is better than Mendeleev's periodic table.

h. Elements of the same group have the same chemical properties.

10. Write the symbols and valencies of the following radicals and mention
whether they are basic radical or acid radical.

a. Ammonium b. Silicate c. Plumbic d. Carbon

e. Zinc f. Aluminium g. Chlorate h. Peroxide

i. Silver j. Barium k. Nitrite l. Phosphate

11. Write the molecular formula of the following substances in the given pattern.
e.g. Calcium nitrate = Ca2NO31 = Ca(NO3)2

a. Ferrous oxide b. Ferric oxide c. Silicon chloride

d. Silicon oxide e. Barium nitrate f. Sodium peroxide

g. Calcium bicarbonate h. Silver nitride i. Aluminium carbonate

j. Calcium hydroxide

12. Write the molecular formulae of the following compounds directly as given in

the example.

e.g. Calcium nitrate = Ca(NO3)2

a. Magnesium oxide b. Sodium oxide c. Ammonium sulphate

d. Magnesium hydroxide e. Cuprous carbonate f. Zinc peroxide

g. Potassium chlorate h. Sulphuric acid i. Calcium carbonate

14. Translate the following word equations in the form of balanced formula equations.

a. Aluminium + Hydrochloric acid Aluminium chloride + Hydrogen

b. Iron + Oxygen Ferric oxide (Rust)

c. Calcium hydroxide + Carbon dioxide Calcium carbonate + Water

d. Magnesium + Nitrogen Magnesium nitride

e. Sulphuric acid + Potassium hydroxide Potassium sulphate + Water

f. Sulphuric acid + Zinc Zinc sulphate + Hydrogen

g. Calcium + Nitrogen Calcium nitride

h. Calcium + Water Calcium hydroxide + Hydrogen

i. Hydrogen + Oxygen Water

j. Carbon + Oxygen Carbon monoxide

k. Sulphur + Oxygen Sulphur dioxide

168 Modern Graded Science and Environment Book 8

l. Calcium bicarbonate Calcium carbonate + Water + Carbon dioxide

m. Carbon dioxide + Water Carbonic acid

n. Ammonia + Water Ammonium hydroxide

o. Ammonia + Sulphuric acid Ammonium sulphate + Hydrogen
15. Diagrammatic Questions:

a. m. Write the name of the elements of the given nucleus of the atoms.

1 p+ 17 p+ 5 p+ 11 p+ 19 p+
0 no 18 no 6 no 12 no 20 no

b. Which period of the periodic table do the elements given below belong to?

What are the full names of Na, Al and Cl? Why are sodium and chlorine more

active elements?

Elements Na Mg Al Si P S Cl

Electronic configuration 2,8, 1 2, 8, 2 2, 8, 3 2, 8, 4 2, 8, 5 2, 8, 6 2, 8, 7

c. Answer the questions given below with the help of the atomic structure of the
elements shown in the diagram.

p+ 10 p+ 12 p+ 16
no 10 no 12 no 16

A B C

i. Which group of the periodic table do elements A, B, and C occupy?

ii. Which one of them is an inert element?

iii. Which period of the periodic table does element B fall in?

iv. Why are elements B and element C more reactive?

v. Write the names of the elements: A, B and C.

d. Draw the atomic structures of the first twelve elements in the periodic table and

write the information which can be obtained by seeing the atomic structure.

GLOSSARY

Ion : electrically charged particles or atoms

Neutral : neither positive nor negative

Negligible : something with very little importance or size and worth

Aggregate : a total number or amount made up of smaller amount

that are collected together

Inert : inactive /noble

Aqueous (aq) : describing of a solution in water

Expression : things that people say

Matter 169

12Lesson MIXTURE

Total Estimated Pds: 5 [Th. 4 + Pr. 1]

On completion of this lesson, the students will be able to:

introduce steam distillation and fractional distillation.
demonstrate steam distillation and fractional distillation.
describe and demonstrate the process of chromatography.

We have already studied about pure and impure substances. Impure substances are
made up of two or more than two pure substances mixed together in any proportion.
They do not undergo a chemical change but simply lie together and retain their identity.
Therefore, such impure substances are called mixture. For example, salt solution, mud in
muddy water, copper sulphate in copper sulphate solution, mixture of water and alcohol,
etc. The mass formed when two or more than two substances are mixed together in any
proportion is called mixture.

The components of a mixture can be separated to get pure substances. The separation
of the components of a mixture depends on the principle that "different substances have
different properties.” The process by which the components of a mixture are separated
from each other is called separation of mixture. We have studied about many methods of
separation of mixture in junior classes. Now, we have to read about fractional distillation,
steam distillation and chromatography.

Distillation

The extraction of the volatile components of a mixture by the condensation and collection
of the vapors that are produced as the mixture is heated is called distillation. Pure water
is collected from salty water of ocean and different components of petroleum are
separated from crude oil mined from the earth with the help of this method.

Activity 12.1

To separate water from salt solution

Materials required

a distillation flask, a thermometer, salt solution,a tripod stand, a wire gauze,a spirit lamp or
bunsen burner,a receiver flask, a condenser and some cold water

170 Modern Graded Science and Environment Book 8

Procedure round bottom thermometer
flask
1. Take the salt solution cock
in a round bottom flask retort stand hot water
which fills less than
one third of it.

2. Fit the flask with a condenser
condenser and a
thermometer. The bulb salt cold water receiver flask
of the thermometer solution
should not touch the
solution.

3. Adjust a receiver burner

flask at the other end

of the condenser. distilled water
Also manage the flow Fig 12.1 distillation
of cold water in the

condenser as shown in the diagram.

4. Now fit the apparatus as shown in the diagram and heat the solution using a
bunsen burner.

5. Boil the solution at 100 ºC and collect the water in the receiver as distilled
water; the residue in the RB flask will be salt.

Observation

The water boils and evaporates in the form of water vapour. The vapour condenses into

liquid water while passing thorough the condenser. The water collects in the receiver and

the salt remains in the round bottom flask.

Conclusion petroluem gases

This method is a simple distillation process bubble caps petrol
in which the mixture of salt and water is paraffin
separated.

Potable water is obtained from sea water crude oil vapour diesel oil
with the help of this method. This method lubricating oils
is also called steam distillation. If we liquid crude oil heavy fuel oils
have to separate two or more liquids from heat tums to a vapour
a solution, a special type of distillation is bitumen
used. It is called as fractional distillation.
Fig 13.2
Fractional distillation

Fractional distillation is the separation of a mixture of two or more liquids of close boiling range into
its component parts by heating them to a temperature at which several component evaporates.

Mixture 171

It is a special type of distillation from which petroleum gas, petrol (gasoline), kerosene, diesel,
fuel oil and residual crude like lubricating oil, paraffin and asphalt are separated. This process
is based on the fact that "different components of the crude oil have different boiling points".

For it, crude oil is heated at 400ºC and it is passed through a long fractionating column
from near its bottom. Between the bottom to the top, the temperature varies from 400ºC
to 40ºC. The long fractionating column is divided into several compartments by means
of shelves with holes in them.

The holes are covered with bubble caps. These caps allow the light to pass the volatile
components up but less volatile components condense and flow into the trays below. Each
shelf is connected with an overflow pipe, which helps to collect the various fractions separately.

Activity 12.2 Do you know?

To separate alcohol and water from Colloids are the solutions with bigger particles
the mixture of alcohol and water than the component particles of solution. Milk
is an emulsified colloid.
Materials required

a retort stand, a burner tripod, a thermometer, a fractional column, some water and some
mixture of alcohol and water

Theory: Fractional distillation is a process used to separate the mixture of two or more
liquids of different boiling points. The liquid mixture is heated to the temperature at which
one of the liquid boils and evaporates. The vapour is allowed to cool which condenses back
to pure liquid. This process is continued until all the liquid is separated from the mixture.

Procedure

1. Take a mixture of water and alcohol to retort stand thermometer

be separated in a round bottom flask. hot water out
condenser
2. Fitted all the apparatus as shown in the fractionating
column
figure.

3. Heat the mixture of water and alcohol distillation cold water in
flask mixture receiver
using burner. flask
tripod stand
burner pcoumreploiqnueidnt
Observation

When the temperature in the thermometer is Fig 12.3 fractional distillation

recorded 78.5°C, the alcohol evaporates continuously on heating and water remains

inside the round bottom flask. This is due to the fact that water boils at 100°C. Some

vapour of water in the column condenses back to a liquid at temperature below 100°C.

The vapour of the alcohol condenses into liquid alcohol and collected into the receiver

on passing through the condenser.

172 Modern Graded Science and Environment Book 8

Conclusion
Alcohol is separated from the mixture of water and alcohol as they have different boiling points.

Uses of fractional distillation

1. Fractional distillation is used in industries for separating the components of
liquid mixture.

2. It is used to separate nitrogen and oxygen gases from liquid air.
3. It is used to separate different petroleum products from crude oil.

Activity 12.3

To separate different colours from the mixture of green, blue, black and red ink by
adsorption chromatography

Materials required

a retort stand, a glass tube, some chalk powder, some silica gel, some mixture of green,
blue, black and red ink and conical flasks

Procedure retort stand
glass tube
1. Take a small glass tube of about 10 cm long chalk powder
and heat its one end to make the lumen
narrow down. conical flask

2. Fill the two-third part of the glass tube with Fig 12.4
chalk powder or aluminium oxide.

3. Adjust the glass tube in vertical position as
shown in the figure.

4. Pour the mixture of ink into the glass tube.

5. Collect the ink of different colours one by
one into different conical flasks.

Observation

After some time, a particular coloured ink starts to drop from the bottom end of the
glass tube and collects into a conical flask. After a while, another coloured ink begins to
drop. This is also collected into another conical flask. This process is continued to other
coloured ink also.

Conclusion

The different coloured ink is separated from the mixture of ink by adsorption
chromatography.

Mixture 173

Chromatography

The Russian botanist, Mikhail Tswett invented chromatography in the beginning of the 19th
century by which pigments (colours) are separated. Chromatography means colour writing.
This process was developed originally to separate coloured substances present in a mixture.
Nowadays, chromatography is applied to separate and identify colourless as well as
coloured substances. Chromatography is a technique that is used for separating the
components of a mixture by passing it through a column or sheet of an adsorbing material.
The principle of this process depends upon the fact that "the different components of a
mixture are adsorbed into different extents and move down the column at different rates."

To understand the fact, let's put a drop of ink on a filter paper or on a piece of chalk. We
will notice that the ink on the chalk or on the filter paper is dark at the centre and light at
the edge. Can you say, why? This is due to different rates of movement of the ink by the
moving water which causes separation. There are various techniques of chromatography
as mentioned below:

1. Paper chromatography 2. Adsorption chromatography

1. Paper chromatography

It is a technique in which a filter paper or chromatography paper is taken as an adsorbent
material for analyzing mixtures by chromatography.

Activity 12.4

To separate the different colours of ink by using paper chromatography
Materials required

a few pieces of filter paper, a petri dish, some water and black ink

Procedure

filter paper role of filter paper

hole filter paper with spot of
black ink
drop of
black ink water

Fig 12.5

1. Make a hole in the centre of a filter paper and insert a role of another filter
paper (wick) in the hole.

2. Put a drop of black ink near the hole as shown in the figure.

3. Adjust the filter paper above a petri dish with water in such a way that the roll
of the filter paper is immersed partially in it.

4. Leave it for 1 to 2 hours and then observe it.

174 Modern Graded Science and Environment Book 8

Observation

The rings of different colours present in the ink are seen on the filter paper at different
distances from the centre.

The water rises up in the roll of the filter paper, spreads across the filter paper and carries
the colours with it. Some colours stick more firmly to the fibres of the filter paper than the
others. The colouring matter that sticks most firmly remains nearest to the hole and the
colouring matter that sticks least goes farthest. The different rates of movement of the
colouring matter by the moving water cause separation. This process of separation is
called paper chromatography.

Activity 12.5

To separate different colours from a mixture of blue and red ink
Materials required

a strip of filter paper, some red and blue ink, a vessel and a stick

Procedure a strip of filter stick
paper
1. Take a mixture of blue and red
red ink. a spot of ink blue
water
2. Adjust a strip of filter paper Fig 12.6
in such a way that the filter
paper must not touch the
vessel as shown in the
figure.

3. Leave it for 1 to 2 hours
and then observe it.

Observation
We will see the rings of blue and red colours on the filter paper.

Conclusion
This activity helps to separate different colours from a mixture of ink.
2. Adsorption chromatography

It is based on the principle that "a certain adsorbent can adsorb different substances to different
extents." The solution containing two or more than two dissolved substances to be separated
is allowed to pour into a vertical glass tube packed with finely divided adsorbent like alumina or
silicate gel or magnesium oxide or chalk powder. When the solution passes down through the
adsorbent, various colours of dissolved substances of a solution are separated in the form of bands.

Mixture 175

This way, substances having different colours can be solvent
coloured zone
separated using adsorption chromatography. Adsorption

chromatography is called column chromatography. adsorbent

Application of Chromatography coloured zone

1. It is used to separate coloured substances glass wool
present in natural pigments.

2. It is used to detect and identify drugs

present in blood. solvent

3. It is used to separate the components of Fig 12.7 adsorption chromatography
artificial colours.

THINGS TO KNOW

1. The process of separation of liquid from a solution by heating it at one place and
then by cooling the steam at other place is called distillation.

2. By steam distillation method, water can be separated from a salt solution.

3. Different components of petroleum can be separated by the process of fractional
distillation.

4. Chromatography is a method of separation of colours/pigments from a solution.

5. Chromatography is of many types like paper chromatography, adsorbent
chromatography and gas liquid partition chromatography.

6. Chromatography is based on the principle that speed of different-sized particles in
the same medium is different.

7. Types of mixtures and methods to separate the components of these mixtures are
different.

8. Fractional distillation is applicable to separate the mixture of liquids of close boiling
point range.

9. Types of mixtures and methods to separate the components of these mixtures:

a. Homogenous mixtures

Types of mixture Methods of separation of mixture
i. Solid + Liquid Evaporation (Salt from salt and water)
ii. Liquid + Liquid Crystallization (Salt from salt and water)
Centrifuging (Cream from milk)
Steam distillation (Salt + Water)
Fractional distillation (components of petroleum from crude oil)

176 Modern Graded Science and Environment Book 8

b. Heterogenous mixtures

Types of mixture Methods of separation of mixture
i. Solid + Solid Winnowing (Sand + Rice)
Sieving (Wheat grain from its floors + Rice)
Magnetic separation (Iron dust + Sand)
Sublimation (Sand + camphor)

ii. Solid + Liquid Sedimentation (Sandy water)
iii. Liquid + Liquid Decantation (Sand+mud+water)
Filtration (Sandy water)

Using separating funnel (Kerosene + water)

THINGS TO DO

Take some petals of a rose flower and grind them. Extract the solution and use it in paper
chromatography and note the different colour bands in the paper.

TEST YOURSELF

1. Fill in the blanks.

a. Silica gel is used as ............ .

b. The ............. distillation is used to obtain petroleum products from crude oil.

c. Chromatography is used to separate ........... .

d. Liebig's condenser is used in .......... .

e. Plant pigments can be separated by ............. .
2. Tick () the correct answer (MCQs).

a. The pure water is obtained from ocean water by:

i. chromatography ii. distillation

iii. sedimentation iv. filtration

b. The boiling point of alcohol is:

i. 100°C ii. 78.5° C iii. 875°C iv. 80°C

c. ............ is an example of adsorbent.

i. chalk powder ii. ink iii. chlorophyll iv. alcohol

d. The mixture of sand and rice is separated by:

i. sieving ii. winnowing iii. sublimation iv. decantation

e. Water can be separated from salt solution by:

i. fractional distillation ii. filtration

iii. steam distillation iv. sieving

Mixture 177

3. Define the following terms.

a. steam distillation b. fractional distillation c. chromatography

4. Write the steps to separate the following mixtures.

a. water from salt solution b. colours from a mixture

c. diesel, petrol, kerosene from crude oil

5. Distinguish between:

a. steam distillation and fractional distillation

b. paper chromatography and absorption chromatography

7. Answer the following questions.

a. What is distillation? How can you obtain pure water from ocean water?

b. What is chromatography? Write in short about adsorbent chromatography.

c. Write the procedure of paper chromatography.

d. What is steam distillation? How is it done?

e. Which method is used to separate the following mixtures?

i. Water and alcohol from a solution ii. Salt from salt solution

iii. Red and blue inks from a solution

f. If two different types of particles have the same speed in a medium, is
chromatography possible? Write with a reason.

g. Write the advantage of using chromatography.

h. Which method is most used to get pure water from sea water?

i. Write any two uses of fractional distillation.

j. The two liquids of the mixture having the same boiling point cannot be
separated by distillation. Explain.

6. Diagrammatic Questions:

a. Draw a labelled diagram of:

i. steam distillation ii. fractional distillation

b. Sketch a diagram for paper chromatography.

GLOSSARY

Adsorbent : a substance on the surface of which a surface is adsorbed

Adsorption : the formation of a layer of gas liquid, or solid on the surface
of a solid or of a liquid

Potable : able to drink

Crude oil : the raw mineral oil which is just taken out from a mine

Furnace : baking chamber

178 Modern Graded Science and Environment Book 8