MAK LABS — Std 8 General Science Chapter 13 Chemical Change and Chemical Bond

1Complete the statement by filling the gaps using appropriate term from the terms given in the bracket. (slow, coloured, arrow, fast, smell, milky, physical, product, chemical, reactant, covalent, ionic, octet, duplet, exchange, sharing, equality sign)

(a) An arrow is drawn in between the reactants and products while writing the equation for a chemical reaction.

(b) Rusting of iron is a slow chemical change.

(c) The spoiling of food is a chemical change which is recognized from the generation of certain smell due to it.

(d) A colourless solution of calcium hydroxide in a test tube turns milky on blowing in it through a blow tube for some time.

(e) The white particles of baking soda disappear when put in lemon juice. This means that it is a chemical change.

(f) Oxygen is a reactant in respiration.

(g) Sodium chloride is ionic compound while hydrogen chloride is covalent compound.

(h) Electron duplet is complete in each hydrogen in a hydrogen molecule.

(i) Chlorine (Cl₂) molecule is formed by sharing of electrons between two chlorine atoms.

2Explain by writing a word equation.

(a) Respiration is a chemical change.

In respiration, glucose reacts with oxygen. New substances, carbon dioxide and water, are formed. Hence, respiration is a chemical change.

Glucose + Oxygen → Carbon dioxide + Water + Energy

(b) Hard water gets softened on mixing with a solution of washing soda.

Hard water contains salts of calcium and magnesium. Washing soda converts them into insoluble carbonates. These settle down. So the water becomes soft.

Calcium chloride + Sodium carbonate → Calcium carbonate + Sodium chloride
Magnesium chloride + Sodium carbonate → Magnesium carbonate + Sodium chloride

(c) Lime stone powder disappears on adding to dilute hydrochloric acid.

Lime stone is calcium carbonate. It reacts with hydrochloric acid. Calcium chloride is formed, which dissolves in water. Carbon dioxide escapes as gas. So the powder disappears.

Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water

(d) Bubbles are seen on adding lemon juice to baking soda.

Lemon juice contains citric acid. It reacts with baking soda and carbon dioxide gas is released. So bubbles are seen.

Citric acid + Sodium bicarbonate → Sodium citrate + Carbon dioxide + Water

3Match the pairs.

Group A	Group B
a. Photosynthesis	iii. Chemical change
b. Water	iv. Covalent bond
c. Sodium chloride	v. Ionic bond
d. Dissolution of salt in water	vi. Physical change
e. Carbon	ii. Reactant in combustion process
f. Fluorine	vii. Tendency to form anion
g. Magnesium	i. Tendency to lose electrons

4Show with the help of diagram of electronic configuration how the following compounds are formed from the constituent atoms.

(a) Sodium chloride (NaCl) – ionic bond

Sodium (2, 8, 1) gives one electron to chlorine (2, 8, 7). Na⁺ and Cl⁻ ions are formed. They attract each other and form an ionic bond.

Formation of the ionic bond in sodium chloride.

(b) Potassium fluoride (KF) – ionic bond

Potassium (2, 8, 8, 1) gives one electron to fluorine (2, 7). K⁺ and F⁻ ions are formed. They form an ionic bond.

Formation of the ionic bond in potassium fluoride.

(c) Water (H₂O) – covalent bonds

Oxygen (2, 6) shares one electron with each hydrogen atom. Two covalent bonds are formed. The octet of oxygen and the duplet of each hydrogen are complete.

Formation of covalent bonds in a water molecule (H – O – H).

(d) Hydrogen chloride (HCl) – covalent bond

Hydrogen and chlorine share one electron each. A single covalent bond is formed (H – Cl).

Formation of the covalent bond in hydrogen chloride.

13. Chemical Change and Chemical Bond

Exercises