Selina Chapter 5 Language of Chemistry Questions Answers Class 7 Chemistry

ICSE Solutions for Chapter 5 Language of Chemistry Class 7 Chemistry Selina Publisher

Exercise

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 chemical reactions to take place.

Answer

Conditions necessary for chemical reactions :

1. Close contact
2. Solution form
3. Heat
4. Light
5. Catalyst

3. Differentiate between:

(a) Reactants and products:

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

(b) Chemical reaction and chemical equation:

Chemical reaction	Chemical equation
Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.	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 balanced and a skeletal chemical equation:

Balanced Equation	Skeletal Equation
A balanced chemical equation is one in which the number of atoms each element on the reactant side is equal to the number of atoms of that element on the product side.	In a skeletal equation the number of atoms on reactant side are not equal to number of atoms of product side.

4. Write word equations for the following skeletal equations:

(a) KClO₃ → KCl + O₂

(b) Zn + HCl → ZnCl₂ + H₂

(c) FeCl₂ + Cl₂ → FeCl₃

(d) CO + O₂ → CO₂

(e) Ca + O₂ → CaO

(f) Na + O₂ → Na₂O

(g) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

(h) AgBr → Ag + Br₂

(i) KNO₂ → KNO₂ + O₂ 

Answer

(a) 2KClO₃ → 2KCl+ 3O₂

(b) Zn + 2HCl → ZnCl₂ + H₂

(c) 2FeCl₂ + Cl₂ → 2FeCl₃

(d) 2CO + O₂ → 2CO₂

(e) 2Ca + O₂ → 2CaO

(f) 4Na + O₂ → 2Na₂O

(g) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(h) 2AgBr → 2Ag + Br₂

(i) 2KNO₃ → 2KN0₂ + O₂

5. Balance the following chemical equations :

(a) FeS + HCl → FeCl₂ + H₂S

(b) Na₂CO₃ + HCl → NaCI + H₂O + CO₂

(c) H₂ + O₂ → H₂O

(d) Na₂O + H₂O → NaOH

Answer

(a) FeS + 2HCl → FeCl₂ + H₂S

(b) Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

(c) 2H₂+ O₂ → 2H₂O

(d) Na₂O + H₂O → 2NaOH

6. What information do you get from the equation H₂+ Cl₂  → 2HCl ?

Answer

(a) Hydrogen and chlorine molecules are the reactants.

(b) They are in gaseous form.

(c) The product is hydrogen chloride gas.

(d) Two molecules of hydrogen chloride are formed.

7. Write your observations for the following chemical reactions and name the products formed:

(a) When sugar is heated.

(b) When manganese dioxide is added to potassium chlorate and heated.

(c) When dilute acetic acid is poured on baking soda.

(d) When an aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate.

(e) When ammonium chloride is heated with sodium hydroxide.

(f) When water is added to quick lime?

Answer

(a) Black solid mass (charcoal) is formed along with water vapours.

(b) Manganese dioxide acts as a catalyst for the decomposition of potassium chlorate into potassium chloride and oxygen at a lower temperature.

(c) Sodium acetate, CO₂ and water is formed.

(d) A white insoluble solid precipitate of silver chloride is formed along with Sodium nitrate.

(e) When solid ammonium chloride is heated with sodium hydroxide solution, a gas ammonia is evolved which is recognised by its strong pungent smell.

(f) When water is added to quick lime, a large amount of heat energy is evolved.

8. Write symbolic representation for the following word equations and balance them :

(a) Calcium carbonate → Calcium oxide + Carbon dioxide

(b) Carbon + Oxygen → Carbon dioxide

(c) Calcium oxide + Water → Calcium hydroxide

(d) Aluminium + Chlorine → Aluminium chloride

(e) Iron + Sulphur → Iron sulphide

Answer

Objective Questions

1. Fill in the blanks:

(a) The substances which undergo chemical change are called _____

(b) The substances formed as a result of a chemical reaction are called _____

(c) During a chemical reaction transfer of _____ takes place.

(d) The basic conditions necessary for a chemical reaction is _____

(e) In some chemical reactions an insoluble _____ is formed when two solutions

are mixed.

Answer 

(a) reactants.

(b) products.

(c) energy

(d) close contact.

(e) precipitate

2. Write ‘true’ or ‘false’ for the following statements :

(a) No new substance is formed during a chemical reaction.

(b) Hydrogen sulphide has rotten egg smell.

(c) When potassium iodide solution is added to lead acetate solution a red precipitate is formed.

(d) A black residue is formed when sugar is heated.

(e) When iron and sulphur are heated together a grey mass is formed which is attracted by a magnet.

(f) A chemical equation gives only qualitative information of a chemical reaction : 

Answer 

(a) True

(b) True

(c) False

(d) True

(e) False

(f) False

Multiple Choice Questions

1. A chemical equation is a statement that describes a chemical change in terms of

(a) symbols and formulae

(b) energy

(c) number of atoms

(d) colours

Answer 

(a) symbols and formulae

2. Balancing a chemical equation is based on

(a) Law of conservation of mass

(b) Mass of reactants and products

(c) Symbols and formulae

(d) None of the above

Answer 

(a) Law of conservation of mass

3. Copper carbonate when heated, it turns :

(a) Blue

(b) Green

(c) Black

(d) Yellow

Answer

(c) Black

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 

Close contact: For a chemical reaction to take place the reactants should be brought in close contact i.e., they should be mixed,
Sodium + Water  ➝ Sodium  hydroxide + Water 

Solution form: Some substances react with each other only when they are mixed in the solution form, Example:

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

Answer 

a. 2Na + H₂O → 2NaOH + H₂ 

b. AgNO₃ + NaCl → AgCl + NaNO₃ 

c. 2KClO₃ → 2KCl + 3O₂ 

d. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ 

4. Balance the following equations :

a. Fe + O₂ → Fe₃O₄ 

b. Fe + H₂O → Fe₃O₄ +H₂ 

c. N₂  + O₂ → NO

d. Pb₃O₄ → PbO + O₂  

Answer 

a. 3Fe + 2O₂ → Fe₃O₄  

b. 3Fe + 4H₂O → Fe₃O₄ + 4H₂  

c. N₂ + O₂ → 2NO

d. 2Pb₃O₄ → 6PbO + O₂  

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

6. State what you would observe when the following substances are heated in a glass test tube: 

1. Red lead

2. Copper (II) carbonate Red lead

Answer 

1. 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.
2. 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 → 2MgO

(c) Fe + CuSO₄ → FeSO2 + Cu

(d) NaOH + HCl → NaCl + H₂O

(e) Fe₂O₃  + 2Al → Al2O₃  + 2Fe

Answer  

(a) CaCO₃ → CaO + CO₂ :Decomposition reaction

(b) 2Mg + O₂ → 2MgO :Combination reaction

(c) Fe + CuSO₄ → FeSO₄ + Cu :Displacement reaction

(d) NaOH + HCl → NaCl + H₂O :Double decomposition reaction

(e) Fe₂O₃ + 2Al → Al₂O₃ + 2Fe :Displacement reaction

9. Write your observations and name the products when

(a) Zinc reacts with dilute hydrochloric acid.

(b) ron 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