Language of Chemistry

Exercise

Question 1

(a) Define a chemical reaction.

(b) What is a chemical equation?

(c) Why do we need to balance chemical equations?

Answer

(a) Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.

(b) A chemical reaction can be represented with the help of symbols or the formulae of the elements and the compounds taking part in the reaction and the product formed, in the form of a chemical equation.

(c) A chemical equation needs to be balanced so as to make the number of atoms of the reactants equal to the number of atoms of the products because a chemical reaction is just a rearrangement of atoms. The atoms themselves are neither created nor destroyed during the course of a chemical reaction.

Question 2

State four conditions necessary for chemical reactions to take place.

Answer

The four conditions necessary for chemical reactions to take place are:

1. Close contact: For a chemical reaction to take place the reactants must come into close contact i.e. they should be mixed.
2. Solution form: Some substances react with each other only when they are mixed in solution form.
3. Heat: Some reactants need to be heated to undergo a chemical change.
4. Light: Some reactions take place in the presence of light.

Question 3

Differentiate between:

(a) Reactants and products

(b) Chemical reaction and chemical equation

(c) A balanced and a skeletal chemical equation

Answer

(a) Difference between reactants and products :

(b) Difference between chemical reaction and chemical equation :

(c) Difference between a balanced and a skeletal chemical equation :

Question 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) KNO₃ ⟶ KNO₂ + O₂

(j) FeS + 2HCl ⟶ FeCl₂ + H₂S

(k) Na₂CO₃ + 2HCl ⟶ 2NaCl + H₂O + CO₂

(l) 2H₂ + O₂ ⟶ 2H₂O

(m) Na₂O + H₂O ⟶ 2NaOH

Answer

(a) Potassium chlorate ⟶ Potassium chloride + Oxygen (gas)

(b) Zinc + Hydrochloric acid (dil.) ⟶ Zinc chloride + Hydrogen (gas)

(c) Ferrous chloride + Chlorine ⟶ Ferric chloride

(d) Carbon monoxide + Oxygen ⟶ Carbon dioxide

(e) Calcium + Oxygen ⟶ Calcium oxide

(f) Sodium + Oxygen ⟶ Sodium oxide

(g) Sodium hydroxide + Sulphuric acid ⟶ Sodium sulphate + Water

(h) Silver bromide ⟶ Silver + Bromine

(i) Potassium nitrate ⟶ Potassium nitrite + Oxygen (gas)

(j) Iron sulphide + Hydrochloric acid (dil.) ⟶ Iron chloride + Hydrogen sulphide (gas)

(k) Sodium carbonate + Hydrochloric acid (dil.) ⟶ Sodium chloride + Water + Carbon dioxide (gas)

(l) Hydrogen + Oxygen ⟶ Water

(m) Sodium oxide + Water ⟶ Sodium hydroxide

Question 5

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) When sugar is heated it first melts and then gets charred into a black residue. The products formed are charcoal and water vapour.

(b) When manganese dioxide is added to potassium chlorate and heated it decomposes to give potassium chloride and oxygen.

(c) When dilute acetic acid is poured on baking soda, a strong effervescence is observed. The products formed are sodium acetate, water and carbon dioxide.

(d) A white precipitate is formed when an aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate. The products formed are silver chloride and sodium nitrate.

(e) A strong pungent smelling gas is evolved when ammonium chloride is heated with sodium hydroxide. The products formed are sodium chloride, water and ammonia gas.

(f) A large amount of heat energy is evolved when water is added to quick lime. The product formed is calcium hydroxide.

Question 6

What information do you get from the equation

H₂ + Cl₂ ⟶ 2HCl?

Answer

We get the following information from the given chemical equation:

1. Hydrogen and Chlorine are reactants.
2. The reaction is irreversible.
3. The product formed is hydrochloric acid.
4. The equation is balanced i.e. number of atoms of each element on reactants and products sides are equal.

Objective Type Questions

Question 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 condition 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

Question 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.

Answer

(a) False
Correct Statement — New substance is formed during a chemical reaction.

(b) True

(c) False
Correct Statement — When potassium iodide solution is added to lead acetate solution, a yellow precipitate is formed.

(d) True

(e) False
Correct Statement — When iron and sulphur are heated together a grey mass is formed which is not attracted by a magnet.

Question 3a

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

1. Symbols and formulae
2. Energy
3. Number of atoms
4. Colours

Answer

Symbols and formulae

Reason — A chemical reaction can be represented with the help of symbols or the formulae of the elements and the compounds taking part in the reaction in the form of a chemical equation.

Question 3b

Balancing a chemical equation is based on:

1. Law of conservation of mass
2. Mass of reactants and products
3. Symbols and formulae
4. None of the above

Answer

Law of conservation of mass

Reason — Balancing a chemical equation is based on Law of conservation of mass i.e. the atoms themselves are neither created nor destroyed during the course of a chemical reaction.

Question 3c

Copper carbonate when heated turns:

1. Blue
2. Green
3. Black
4. Yellow

Answer

Black

Reason — Copper carbonate when heated produces copper oxide which is black in colour and carbon dioxide gas.

Question 3d

When lead acetate solution is added to potassium iodide solution a precipitate is formed which is:

1. Red
2. Yellow
3. White
4. Black

Answer

Yellow

Reason — When lead acetate solution is added to potassium iodide solution a yellow precipitate of lead iodide is formed.

Question 3e

This gas has a rotten egg smell:

1. Ammonia
2. Hydrogen
3. Hydrogen sulphide
4. Oxygen

Answer

Hydrogen sulphide

Reason — Hydrogen sulphide has rotten egg smell.

Question 3f

When sodium carbonate reacts with dilute hydrochloric acid, the gas evolved is:

1. Carbon dioxide
2. Nitrogen
3. Oxygen
4. Hydrogen

Answer

Carbon dioxide

Reason — When sodium carbonate reacts with dilute hydrochloric acid, sodium chloride, water and carbon dioxide gas is produced.