ICSE Selina Class 8 Chemistry Chapter 5 – Language of Chemistry Solutions

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Some important points covered in the chapter are given below:

  • A particular element or an atom of an element is depicted by a chemical symbol.
  • The method by which an atom or radical combine with other chemical species easily is represented by its valency.
  • A group of atoms which behaves like a single unit and shows valency is known as radical.
  • The process of transformation of one set of chemical substances to another is called a chemical reaction.
  • A chemical equation expresses a chemical reaction by representing the reactants, products and their physical states symbolically.

Download ICSE Class 8 Chemistry Selina Solutions PDF for Chapter 5 – Language of Chemistry

ACTIVITY 1

Write the names and symbols of the first twenty elements that you have studied in class VI & VII.

Answer:

Name of the elements

Symbol

Valency

1. Hydrogen

2. Helium

3. Lithium

4. Beryllium

5. Boron

6. Carbon

7. Nitrogen

8. Oxygen

9. Fluorine

10. Neon

11. Sodium

12. Magnesium

13. Aluminium

14. Silicon

15. Phosphorus

16. Sulphur

17. Chlorine

18. Argon

19. Potassium

20. Calcium

H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar

K

Ca

1

0

1

2

3

4

3

2

1

0

1

2

3

4

3

2

1

0

1

2

ACTIVITY 2

Write the molecular formulae of:

1. Copper oxide

Answer: CuO

2. Iron (III) chloride

Answer: FeCl3

3. Sodium hydroxide

Answer: NaOH

4. Iron (II) sulphide

Answer: FeS

5. Lead (II) oxide

Answer: PbO

6. Hydrogen nitrate (nitric acid)

Answer: HNO3

7. Hydrogen sulphate (sulphuric acid)

Answer: H2SO4

8. Calcium hydroxide

Answer: Ca(OH)2

9. Magnesium carbonate

Answer: MgCO3

10. Ammonium carbonate

Answer: (NH4)2CO3

ACTIVITY 3

Write the molecular formula for each of the following compounds:

1. Sulphur trioxide

2. Iron (II) sulphide

3. Ammonia

Find the number and names of elements present in them and calculate their molecular masses.

Answer:

1. Sulphur trioxide

1. The molecular formula of sulphur trioxide is SO3.

2. The elements present in it are sulphur dioxide and oxygen.

3. One molecule of sulphur trioxide has one atom of sulphur and three atoms of oxygen.

4. The molecular mass of sulphur trioxide SO3

=32+3×16

=32+48=80 amu

2. Iron (II) sulphide

1. The molecular formula of iron (II) sulphide is FeS.

2. The elements present in it are iron and sulphur.

3. One molecule of iron (lI) sulphide has one atom of iron and one atom of sulphur.

4. The molecular mass of iron (II) sulphide (FeS)

= 55.5+32

=87.5 amu

3. Ammonia

1. The molecular formula of ammonia is NH3.

2. The elements present in it are nitrogen and hydrogen.

3. One molecule of ammonia has one atom of nitrogen and three atoms of hydrogen.

4. The molecular mass of ammonia(NH3)

=14+3×1

=14+3

=17 amu

Question 1

Define:

(a) Radical

(b) Valency

(c) Molecular formula

Solution:

(a) Radical: An atom of an element or a group of atoms of different elements that behaves as a single with a positive or negative charge on it is called radical.

(b) Valency: The number of electrons donated or accepted by the valence shell of an atom during chemical combination is called as valency.

(c) Molecular formula: Molecular formula is a symbolic representation of a molecule. It represents the number of atoms of each element present. These atoms combine in the whole to form the molecule.

Question 2.

Give the symbols and valencies of following radicals:

(a) Hydroxide

(b) Chloride

(c) Carbonate

(d) Ammonium

(e) Nitrate

Solution:

Element

Symbol

Valencies

(a) Hydroxide

(b) Chloride

(c) Carbonate

(d) Ammonium

(e) Nitrate

OH

Cl

CO23-

NH4+

NO3

1

1

2

1

1

Question 3.

Write the molecular formula for the oxide and sulphide of the following elements.

(a) Sodium

Solution:

(a) Sodium oxide Na2O

Sodium sulphide Na2S

(b) Calcium

Solution:

(b) Calcium oxide CaO

Calcium sulphide CaS

(c) Hydrogen

Solution:

(c) Hydrogen oxide H2O

Hydrogen sulphide H2S

Question 4.

Write the molecular formulae for the following compounds and name the elements present.

(a) Baking soda

Solution:

(a) Baking soda- NaHCO3

Elements present in Baking soda are sodium, hydrogen, oxygen and carbon.

(b) Common salt

Solution:

(b) Common salt – NaCl

Elements present in common salt are: Sodium and chlorine.

(c) Sulphuric acid

Solution:

(c) Sulphuric acid – H2SO4

Elements present in Sulphuric acid are Hydrogen, sulphur and oxygen.

(d) Nitric acid

Solution:

(d) Nitric acid – HNO3

Elements present in Nitric acid are: Hydrogen, nitrogen and oxygen.

Question 5.

The valency of aluminium is 3. Write the valency of other radicals present in the following compounds.

(a) Aluminium chloride

Solution:

(a) The molecular formula of Aluminium chloride- AlCl3

Valency of Al is 3.

Another radical present is chloride (Cl) and its valency is 1.

(b) Aluminium oxide

Solution:

(b) Molecular formula of Aluminium oxide – Al2O3

Valency of Al is 3

Another radical present is oxide (O2-) and its valency is 2.

(c) Aluminium nitride

Solution:

(c) The molecular formula of Aluminium nitride –(AlN)

Valency of aluminium is 3

Another radical present is Nitride (N3-) and its valency is 3

(d) Aluminium sulphate

Solution:

(d) The molecular formula of aluminium sulphate – (Al2SO4)3

Valency of aluminium is 3

Another radical present is sulphate (SO4-2) and its valency is 2.

Question 6.

What is variable valency? Give two examples of elements showing variable valency.

Solution:

Some elements have multiple valencies that means they show variable valency.

Ferrous is written as Iron (II), and Ferric is written as Iron (III).

Metal

Radicals

Valency

Iron

Ferrous [iron (II)]

Ferric [iron (III)]

2

3

Copper

Cuprous [copper (I)]

Cupric [copper (II)]

1

2

Question 7

(a) What is the chemical equation?

Solution:

(a) Chemical Equation- The symbolic representation of a chemical reaction using the symbols and the formulae of the substances involved in the reaction is called as a chemical equation.

(b) Why is it necessary to balance a chemical equation?

Solution:

(b) The chemical equation needs to be balanced to make the number of the atoms of the reactants equal to the number of the atoms of the products

(c) What are the limitations of a chemical equation?

Solution:

1. It does not state anything about the physical states of the product and the reactants, i.e. whether they are solids, liquids and gases.

2. It does not say anything about the concentration of reactants and products.

3. It does not inform about the time taken for the completion of the reaction.

4. It does not inform about the rate at which a reaction proceeds.

5. It does not inform about the heat changes during the reaction, i.e. whether the heat is given out of absorbed.

6. It does not inform about the conditions such as temperature, pressure, catalyst etc. which affect the reaction.

7. It does not inform about the nature of the reaction, i.e. whether it is reversible or irreversible.


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