CBSE Class 10 Science Acids, Bases and Salts MCQs Set 13

Answer: (b) 7. Pure water is neutral, with equal H⁺ and OH⁻ concentrations (10⁻⁷ mol/L each).

Teacher's Note:
1. A pH of 7 is neutral, below 7 acidic and above 7 basic.
2. The scale runs from 0 to 14.

Answer: (d) 10. Bases turn red litmus blue and have pH above 7.

Teacher's Note:
1. Acids turn blue litmus red, while bases turn red litmus blue.
2. Only option 10 is above 7.

Answer: (b) NaHCO₃. Baking soda is sodium hydrogencarbonate, NaHCO₃.

Teacher's Note:
1. Do not confuse it with washing soda, which is Na₂CO₃·10H₂O.
2. Baking soda is a mild non-corrosive base.

Answer: (d) 10. Washing soda is Na₂CO₃·10H₂O.

Teacher's Note:
1. Water of crystallisation is the fixed number of water molecules in one formula unit of the salt.
2. Washing soda is used to remove permanent hardness of water.

Answer: (b) CaSO₄·½H₂O. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O.

Teacher's Note:
1. It is obtained by heating gypsum at 373 K.
2. On mixing with water it sets into hard gypsum.

Answer: (a) dry slaked lime. Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.

Teacher's Note:
1. Bleaching powder is used for disinfecting drinking water and as a bleaching agent.
2. It is made with dry slaked lime.

Answer: (a) NaOH, Cl₂ and H₂. 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ on electrolysis of brine.

Teacher's Note:
1. Chlorine is released at the anode and hydrogen at the cathode.
2. Sodium hydroxide stays in the solution.

Answer: (a) Milk of magnesia. Mg(OH)₂ is a mild base that neutralises excess HCl in the stomach.

Teacher's Note:
1. Washing soda is too strongly basic for consumption.
2. Antacids relieve acidity and indigestion.

Answer: (b) formic acid. Ants inject methanoic (formic) acid; rubbing baking soda neutralises it.

Teacher's Note:
1. A mild base is used as the remedy for an acid sting.
2. Nettle leaves also inject methanoic acid.

Answer: (c) 5.5. Below pH 5.5, acid attacks the enamel (calcium phosphate).

Teacher's Note:
1. Toothpastes are generally basic to neutralise mouth acid.
2. Enamel is the hardest substance in the body.

Answer: (c) carbon dioxide. Metal carbonate + acid → salt + water + carbon dioxide.

Teacher's Note:
1. Carbon dioxide turns lime water milky.
2. Hydrogencarbonates also give the same gas.

Answer: (c) disappears. CaCO₃ reacts with more CO₂ and water to form soluble Ca(HCO₃)₂.

Teacher's Note:
1. Lime water first turns milky because of insoluble calcium carbonate.
2. Excess gas dissolves it again.

Answer: (b) add acid slowly to water with stirring. The process is highly exothermic and adding water to acid can cause splashing.

Teacher's Note:
1. Always add acid to water, never the reverse.
2. Stirring spreads the heat produced.

Answer: (c) CH₃COOH. Ethanoic acid ionises only partially in water.

Teacher's Note:
1. Strong acids ionise almost completely.
2. The strength depends on the number of H⁺ ions produced.

Answer: (a) pH = 2. Lower pH means higher hydrogen ion concentration.

Teacher's Note:
1. Each unit decrease in pH raises [H⁺] ten times.
2. pH 2 corresponds to 10⁻² mol/L.

Answer: (c) pink. Phenolphthalein is colourless in acid and pink in base.

Teacher's Note:
1. Methyl orange gives red in acid and yellow in base.
2. Choose an indicator by its colour change range.

Answer: (b) red. Methyl orange is red in acid and yellow in base.

Teacher's Note:
1. This is a synthetic indicator.
2. Natural indicators include litmus and turmeric.

Answer: (b) NaCl and H₂O. NaOH + HCl → NaCl + H₂O; a neutralisation reaction.

Teacher's Note:
1. Acid and base react to form salt and water.
2. The reaction is exothermic.

Answer: (c) green. Green corresponds to pH 7.

Teacher's Note:
1. Red-orange shows acids, blue-violet shows bases.
2. Universal indicator gives an approximate pH value.

Answer: (b) 3. pH = −log[H⁺] = −log(10⁻³) = 3.

Teacher's Note:
1. The solution is acidic as its pH is less than 7.
2. Use pH = −log[H⁺] directly.

Answer: (c) white. Removing the five water molecules from blue CuSO₄·5H₂O leaves white CuSO₄.

Teacher's Note:
1. Adding water restores the blue colour, a test for moisture.
2. The blue colour is due to the water of crystallisation.

Answer: (b) it has no H⁺ ions in the absence of water. Acids release H⁺ ions only in the presence of water.

Teacher's Note:
1. Moist litmus turns red with HCl gas.
2. Hence dry HCl gas is not acidic in behaviour.

Answer: (c) basic. Na₂CO₃ is a salt of a strong base (NaOH) and a weak acid (H₂CO₃), so its solution has pH above 7.

Teacher's Note:
1. Salts of strong acid and strong base are neutral.
2. Salts of weak acid and strong base are basic.

Answer: (c) NaCl. NaCl is formed from the strong acid HCl and the strong base NaOH.

Teacher's Note:
1. Its pH is about 7.
2. Hydrolysis does not occur in such salts.

Answer: (b) tartaric acid. On heating or adding water, the acid reacts with NaHCO₃ to release CO₂, which makes the cake rise.

Teacher's Note:
1. Baking soda alone leaves a bitter taste of sodium carbonate.
2. The acid neutralises it.

Answer: (c) 5.6. Normal rain water is slightly acidic at pH about 5.6 due to dissolved CO₂.

Teacher's Note:
1. Acid rain results from oxides of sulphur and nitrogen.
2. It damages buildings, soil and aquatic life.

Answer: (b) H₃O⁺. H⁺ ions combine with water molecules to form hydronium ions, H₃O⁺.

Teacher's Note:
1. Bases give OH⁻ ions in water.
2. A free H⁺ does not exist alone in solution.

Answer: (c) 373 K. CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O at 373 K (100 °C).

Teacher's Note:
1. Overheating gives anhydrous CaSO₄ which does not set with water.
2. Temperature control is therefore important.

Answer: (a) Both A and R are true and R is the correct explanation of A. Ions are the charge carriers, so R correctly explains A.

Teacher's Note:
1. Conduction in solutions is due to movement of ions.
2. Glucose and alcohol do not conduct, as they give no ions.

Answer: (a) Both A and R are true and R is the correct explanation of A. Contact with moisture hardens it, so R explains A.

Teacher's Note:
1. CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
2. Keep the container tightly closed.