Selina Chapter 5 Language of Chemistry ICSE Solutions Class 8 Chemistry

Selina Concise Solutions for Chapter 5 Language of Chemistry Class 8 Chemistry ICSE

Activity 1

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

Activity 2

2. Write the molecular formulae of:
1. Copper oxide
2. Iron (III) chloride
3. Sodium hydroxide
4. Iron (II) sulphide
5. Lead (II) oxide
6. Hydrogen nitrate (nitric acid)
7. Hydrogen sulphate (sulphuric acid)
8. Calcium hydroxide
9. Magnesium carbonate
10. Ammonium carbonate

Answer
1. Copper oxide – CuO
2. Iron (III) chloride – FeCl₃
3. Sodium hydroxide – NaOH
4. Iron (II) sulphide – FeS
5. Lead (II) oxide – PbO
6. Hydrogen nitrate (nitric acid) – HNO₃
7. Hydrogen sulphate (sulphuric acid) – H₂SO₄
8. Calcium hydroxide – Ca(OH)₂
9. Magnesium carbonate – MgCO₃
10. Ammonium carbonate – (NH₄)₂CO₃

Activity 3

3. Write the molecular formula for each of the following compounds:
1. Sulphur trioxide
2. Iron (II) sulphide and
3. Ammonia
Find the number and names of elements present in them and calculate their molecular masses.
Answer
1. Sulphur trioxide
1. A molecule of sulphur trioxide is represented by the formula 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. Molecular mass of sulphur trioxide (SO₃)
= 32 + 3×16
= 32 + 48 = 80 amu.

2. Iron (II) sulphide
1. A molecule of iron (II) sulphide is represented by the formula FeS.
2. The elements present in it are iron and sulphur.
3. One molecule of iron (II) sulphide has one atom of iron and one atom of sulphur.
4. Molecular mass of iron (II) sulphide (FeS)
= 55.5 + 32
= 87.5 amu.

3. Ammonia
1. A molecule of ammonia is represented by the formula 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. Molecular mass of ammonia (NH₃)
= 14 + 3×1
= 14 + 3
= 17 amu.

Exercise

1. Define:
(a) Radical
(b) Valency
(c) Molecular formula
Answer
(a) Radical: A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.
(b) Valency: It is the number of electrons donated or accepted by the valence shell of an atom during chemical combination.
(c) Molecular formula: It is a symbolic representation of a molecule. It shows the number of atoms of each element present in it. These atoms combine in whole number to form the molecule.

2. Give the symbols and valencies of following radicals:
(a) Hydroxide (b) Chloride
(c) Carbonate (d) ammonium
(e) Nitrate
Answers

3. Write the molecular formula for the oxide and sulphide of following elements.
(a) Sodium (b) Calcium
(c) Hydrogen
Answer
(a) Sodium oxide Na₂O
Sodium sulphide Na₂S
(b) Calcium oxide CaO
Calcium sulphide CaS
(c) Hydrogen oxide H₂O
Hydrogen sulphide H₂S

4. Write the molecular formulae for the following compounds and name the elements present.
(a) Baking soda (b) Common salt
(c) Sulphuric acid (d) Nitric acid
Answer
(a) Baking soda — NaHCO₃
Elements present in Baking soda are sodium, hydrogen, oxygen and carbon.
(b) Common salt — NaCl
Element present are: Sodium and chlorine.
(c) Sulphuric acid — H₂SO₄
Element present are: Hydrogen, sulphur and oxygen.
(d) Nitric acid — HNO₃
Elements present are: Hydrogen, nitrogen and oxygen.

5. The valency of aluminium is 3. Write the valency of other radicals present in the following compounds.
(a) Aluminium chloride
(b) Aluminium oxide
(c) Aluminium nitride
(d) Aluminium sulphate
Answer
Aluminium chloride — (AlCl₃) here valency of Al is 3.
Other radical – Chloride (Cl^-)
Valency of chloride = 1

(b) Aluminium oxide — (Al₂O₃)
Here valency of Al is 3
Other radical present = oxide (O^2-)
Valency of O^2- = 2

(c) Aluminium nitride — (Al N)
Here valency of aluminium = 3
Another radical = Nitride (N^3-)
Valency of nitride (N^3-) = 3

(d) Aluminium sulphate — Al₂(SO₄)₃
Here valency of aluminium is 3
Another radical = Sulphate (SO₄^2-)
Valency of (SO₄^2-) = 2

6. What is variable valency ? Give two examples of elements showing variable valency.
Answer
Certain elements exhibit more than one valency, that means they show variable valency.
Ferrous is written as Iron (II) and Ferric is written as Iron (III).

7. (a) What is a chemical equation?
(b) Why it is necessary to balance a chemical equation?
(c) What are the limitations of a chemical equation?
Answer
(a) Chemical Equation— A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formulae of the substances involved in the reaction.

(b) A chemical equation needs to be balanced so as to make the number of the atoms of the reactants equal to the number of the atoms of the products.

(c)

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

8. What are the ways by which a chemical equation can be made more informative?
Answer
A chemical equation can give more information in the following ways:

1. The physical state of the reactants and products can be indicated by putting (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous state.
2. Evolution or absorption of heat during the reaction can be denoted by adding or subtracting a heat term on the product side.
3. Temperature, pressure and catalyst can be indicated above the arrow (→ or =) separating the reactants and products.
4. Concentration of reactants and products are indicated by adding word (dil) for dilute and (cone) for concentrated before their formulae.
5. By the sign → or information about irreversible and reversible reactions can be obtained.

9. State the law of conservation of mass.
Answer
Law of conservation of mass: It states that mass can neither be created nor destroyed in a chemical reaction. During any change (physical or chemical), matter is neither created nor destroyed. However it may change from one form to another.

Experimental Verification of Law of Conservation of Mass

Requirements: H-shaped tube called Landolt’s tube, Sodium chloride solution, silver nitrate solution, etc.
Procedure: A specially designed H-shaped tube is taken. Sodium chloride solution is taken in one limb of the tube and silver nitrate solution in the other limb as shown in figure.
Both the limbs are now sealed and weighed. Now the tubes is averted so that the solutions can mix up together and react chemically. The reaction takes place and a white precipitate of silver chloride is obtained.

The tube is weighed again. The mass of the tube is found to be exactly the same as the mass obtained before inverting the tube.
Thus, this experiment clearly verifies the law of conservation of mass.

10. Differentiate between:
(a) Reactants and products
(b) A balanced and an unbalanced chemical equation
Answer
(a) Reactants and products
Reactants:

1. The substances that react with one another are called reactants.
2. Reactants are written on the left hand side of equation.

Products:

1. The new substances formed are called products.
2. Products are written on the right hand side of equation.

(b) A balanced and an unbalanced chemical equation
Balanced chemical equation:

1. A balanced chemical reaction is the one in which the number of atoms of each element on the reactant side is equal to the number of atoms of that element on the product side.
2. Example: H₂ + Cl₂ → HCl

Unbalanced chemical equation:

1. Number of elements on reactant side are not equal to the number of elements on product side.
2. Example: H₂ + Cl₂ → 2HCl

11. Balance the following equations:
(a) N₂ +  H₂  →  NH₃
(b) H₂ + O₂ →  H₂O
(c) Na₂O + H₂O →  NaOH
(d) CO + O₂  →  CO₂
(e) Zn + HCL  →  ZnCl₂ + H₂
Answer 
(a) N₂ +  3H₂  → 2NH₃
(b) 2H₂ + O₂  → 2H₂O
(c) Na₂O + H₂O → 2NaOH
(d) 2CO + O₂ → 2CO₂
(e) Zn + 2HCl → ZnCl₂ + H₂

12. Write balanced chemical equations for the following word equations:
(a) Iron + Chlorine → Iron (III) chloride
(b) Magnesium + dil sulphuric acid → Magnesium sulphate + water
(c) Magnesium + oxygen → Magnesium oxide
(d) Calcium oxide + water → Calcium hydroxide
(e) Sodium + chlorine → Sodium chloride
Answer
(a) Iron + Chlorine → Iron (III) chloride
4Fe + 3Cl₂ → 2F₂Cl₃

(b) Magnesium + dil sulphuric acid → Magnesium sulphate + water
2Mg + 2H₂SO₄ → 2MgSO₄ + 2H₂

(c) Magnesium + oxygen → Magnesium oxide
2Mg + O₂ → 2MgO

(d) Calcium oxide + water → Calcium hydroxide
CaO + H₂O → Ca(OH)₂

(e) Sodium + chlorine → Sodium chloride
2Na + Cl₂ → 2NaCl

13. What information do you get from the following chemical equation:
Zn(s) + 2HCl (dil) → ZnCl₂ (aq) + H₂(g)
Answer
This gives zinc chloride and hydrogen. The word equation is:
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
Formulae for the products are ZnCl₂ and H₂

Additional Questions

1. (a) Define chemical reaction.
(b) What is a chemical equation?
(c) Why do we need to balance chemical equations?
Answer
(a) Chemical reaction: Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
(b) Chemical equation: A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formula of the substances involved in the reaction.
(c) A chemical equation needs to be balanced so as to make the number of the atoms
of the reactants equal to the number of the atoms of the products.

2. State four conditions necessary for a chemical reaction to take place.
Answer
1. Close contact— For a chemical reaction to take place the reactants should be brought in close contact i.e., they should be mixed,
2. Solution form— Some substances react with each other only when they are mixed in the solution form. Example:

3. Heat— Some reactants need to be heated to undergo a chemical change. Example:

4. Light— Some reactions take place in the present of light. Example: 

3. Write balanced chemical equations for the reactions represented by word equations in the conditions for a chemical reaction.

Answer

(i) 2Na + H₂O ⟶ 2NaOH + H₂

(ii) AgNO₃ + NaCl ⟶ AgCl + NaNO₃

(iii) 2KClO₃ ⟶ 2KCl + 3O₂

(iv) 6 CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂

 

4. Balance the following chemical equations:

(i) Fe + O₂ ⟶ Fe₃O₄

(ii) Fe + H₂O ⟶ Fe₃O₄ + H₂

(iii) N₂ + O₂ ⟶ NO

(iv) Pb₃O₄ ⟶ PbO + O₂

Answer

(i) 3Fe + 2O₂ ⟶ Fe₃O₄

(ii) 3Fe + 4H₂O ⟶ Fe₃O₄ + 4H₂

(iii) N₂ + O₂ ⟶ 2NO

(iv) 2Pb₃O₄ ⟶ 6PbO + O₂

5. Balance the following equations. Also name the products formed. The first one is done for you.
Answer

6. State what you would observe when the following substances are heated in a glass test tube:
(i) Red lead
(ii) Copper (II) carbonate
Answer
(i) 2Pb₃O4 → 6PbO + O₂ ↑
The dark red lead as red powder changes to yellow colour. The yellow colour residue on further heating sticks to the tube and give reddish colour to glass. A colourless and odourless gas is evolved.

(ii) CuCO₃ → CuO + CO₂ ↑
The blue-green powder turns black and a colourless gas is evolved which extinguishes a burning flame.

7. Explain the following reaction with one suitable example for each.
(a) Combination reactions
(b) Decomposition reaction
(c) Displacement reaction
(d) Double decomposition reaction
Answer
(a) Combination reactions: In this reaction two or more substances combine to form a new substance, e.g. Burning of hydrogen in air.

(b) Decomposition reaction: In this reaction a substance breaks up on heating to form two or more simpler substances, e.g. Electrolysis of water.

(c) Displacement reaction: In this reaction a more reactive element displaces a less reactive element from its compound, e.g. Reaction of iron with copper sulphate.

(d) Double decomposition reaction: In this reaction two compounds in solution state react with each other to form two new substances by exchanging their radicals, e.g. Reaction of sodium hydroxide with dilute hydrochloric acid.

8. Name the type of chemical reaction shown by the following equations:

(a) CaCO₃ ⟶ CaO + CO₂

(b) 2Mg + O₂ ⟶ MgO

(c) Fe + CuSO₄ ⟶ FeSO₄ + Cu

(d) NaOH + HCl ⟶ NaCl + H₂O

(e) Fe₂O₃ + 2Al ⟶ Al₂O₃ + 2Fe

Answer

9. Write your observations and name the products when

(a) Zinc reacts with dilute hydrochloric acid.
(b) Iron nails are added to an aqueous solution of copper sulphate.
(c) An aqueous solution of barium chloride is added to dilute sulphuric acid.

Answer

(a) When zinc pieces are added to hydrochloric acid, zinc displaces hydrogen. As a result, zinc chloride and hydrogen gas are produced.

(b) When iron pieces are added to an aqueous solution of copper sulphate iron being more reactive displaces copper from copper sulphate solution to produce ferrous sulphate.

(c) An aqueous solution of barium chloride when mixed with dilute sulphuric acid produces a white insoluble solid barium sulphate and hydrochloric acid.

10. A solution of a substance ‘X’ is used for white washing.
(a) Name the substance ‘X’ and write its formula.
(b) Write the reaction of the substance ‘X’ named in (a) above with water.
Answer
(a) Calcium oxide CaO