CBSE Class 10 Science Acids, Bases and Salts Worksheet Set 10

Answer: Sodium hydrogen carbonate (baking soda), NaHCO₃.

Teacher's Note:
1. It neutralises excess hydrochloric acid in the stomach.
2. It is a mild base, so it does not harm the stomach lining.

Answer: When dissolved in water, ammonia forms hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. These OH⁻ ions make the solution basic.

Teacher's Note:
1. A base is judged by the OH⁻ it produces in water.
2. Ammonia does not itself need to contain OH.

Answer: Dilution.

Teacher's Note:
1. Mixing an acid with water decreases the concentration of H₃O⁺ ions per unit volume.
2. Always add acid to water, never water to acid, because the process is highly exothermic.

Answer: Tartaric acid (a mild edible acid).

Teacher's Note:
1. On heating or when wet, the two react and release CO₂.
2. The CO₂ makes cakes soft and spongy.

Answer: The fruit juice is acidic, because acids turn blue litmus red.

Teacher's Note:
1. Fruits such as lemon and orange contain citric acid.
2. Only acids change the colour of blue litmus to red.

Answer: Tooth enamel is calcium hydroxyapatite (a form of calcium phosphate), the hardest substance in the body.
It corrodes when the pH in the mouth falls below 5.5 because of acids formed by bacteria acting on sugar.
Toothpastes are basic, so they neutralise the acid and prevent tooth decay.

Teacher's Note:
1. Enamel is not soluble in water but is attacked by acids.
2. Rinsing the mouth after eating also helps.

Answer: A is chlorine and B is calcium oxychloride (bleaching powder).
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Teacher's Note:
1. Bleaching powder is made by passing chlorine over dry slaked lime.
2. It is also used to disinfect drinking water.

Answer: Dry blue litmus shows no change, because dry HCl gas does not release H⁺ ions.
Moist blue litmus turns red, because HCl dissolves in the moisture and ionises to give H⁺ ions.

Teacher's Note:
1. Acids show acidic character only in the presence of water.
2. The H⁺ ions are what turn blue litmus red.

Answer: The substance is copper sulphate pentahydrate, CuSO₄·5H₂O.
On heating it loses its water of crystallisation and becomes white anhydrous CuSO₄; on adding water or cooling it takes up moisture and turns blue again.

Teacher's Note:
1. The five water molecules are called water of crystallisation.
2. This change is used to test for the presence of water.

Answer: (a) Washing soda, Na₂CO₃·10H₂O.
(b) It is basic in nature.
(c) It is used in the glass, soap and paper industries and for washing clothes.

Teacher's Note:
1. Sodium carbonate forms a basic solution by hydrolysis.
2. It is made from baking soda by heating it and then recrystallising.

Answer: (i) Sodium chloride and water.
(ii) A is sodium hydroxide, NaOH. 2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂
The process is the chlor-alkali process. NaOH + HCl → NaCl + H₂O is a neutralisation reaction.

Teacher's Note:
1. Chlorine is formed at the anode and hydrogen at the cathode.
2. Each product has important uses in industry.

Answer: X is sodium hydrogen carbonate.
NaCl + H₂O + CO₂ + NH₃ → NaHCO₃ + NH₄Cl
It is mixed with a mild edible acid such as tartaric acid to make baking powder. On heating, CO₂ is released and makes the cake soft and spongy.

Teacher's Note:
1. Heating alone gives 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
2. Baking powder avoids the bitter taste of sodium carbonate.

Answer: (a)(i) Vinegar is acidic and neutralises the sting, so the wasp sting is basic. (ii) Toothpaste or milk of magnesia, which are mild bases.
(b) The oxide is basic: MO + H₂O → M(OH)₂. Basic solutions do not change blue litmus.

Teacher's Note:
1. A bee sting is acidic, so a mild base is used.
2. Never use strong bases on skin.

Answer: (a) Dip blue litmus paper (or use pH paper) in the river water; if it turns red, acid is present.
(b) Dissolving (the iron is reacting with the acid and being corroded).
(c) Iron reacts with the acid to give hydrogen gas, which causes fizzing: Fe + 2HCl → FeCl₂ + H₂.

Teacher's Note:
1. Melting is a physical change, while this is a chemical change.
2. The bubbles are hydrogen gas.

Answer: X is calcium carbonate and the gas is carbon dioxide.
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

Teacher's Note:
1. Limestone, chalk and marble are forms of calcium carbonate.
2. Excess CO₂ would make the milkiness disappear again.