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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 the 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) The molecular formula of Aluminium oxide – Al2O3
Valency of Al is 3
Another radical present is an 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 the variable valency? Give two examples of elements showing variable valency.
Solution:
Some elements have multiple valencies that mean 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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