CBSE Class 10 Chemistry Acids, Bases and Salts MCQs Set 18

Answer: (c) \( \ce{H2SO4} \)

Explanation:
1. A metal oxide reacts with an acid to give salt and water.
2. The salt is copper sulphate, so the acid is \( \ce{H2SO4} \).

Teacher's Note:
1. Metal oxides are basic.
2. Metal oxide plus acid gives salt plus water.

Answer: (a) Acid

Explanation:
1. Non-metal oxides are acidic.
2. They react with water to form acids, for example \( \ce{CO2 + H2O -> H2CO3} \).

Teacher's Note:
1. Non-metal oxide + water gives acid, metal oxide + water gives base.
2. Check by litmus.

Answer: (a) CaO

Explanation:
1. Quicklime is calcium oxide, CaO.
2. \( \ce{Ca(OH)2} \) is slaked lime and \( \ce{CaCO3} \) is limestone.

Teacher's Note:
1. Quicklime is made by heating limestone.
2. \( \ce{CaCO3 -> CaO + CO2} \).

Answer: (b) \( \ce{C2H5OH} \)

Explanation:
1. Acids give \( \ce{H+} \) in water.
2. Ethanol is an alcohol that does not ionise to give \( \ce{H+} \).

Teacher's Note:
1. Ethanol is neutral.
2. Hydrogen in the compound does not make it an acid.

Answer: (c) Sodium chloride

Explanation:
1. A strong electrolyte ionises completely in water.
2. NaCl does, while \( \ce{CS2} \) does not ionise, \( \ce{NH4OH} \) is weak and water is a very poor electrolyte.

Teacher's Note:
1. Salts of strong acids and bases are strong electrolytes.
2. Strong electrolytes carry current well.

Answer: (a) HCl

Explanation:
1. \( \ce{NaCl + H2SO4 -> NaHSO4 + HCl} \).
2. Hydrogen chloride reacts with ammonia to give white fumes of ammonium chloride.

Teacher's Note:
1. White dense fumes of \( \ce{NH4Cl} \) confirm HCl.
2. The reaction is used to prepare HCl.

Answer: (b) add the concentrated acid into water slowly with stirring

Explanation:
1. Dilution of an acid is highly exothermic.
2. Adding acid slowly to water spreads the heat and avoids splashing.

Teacher's Note:
1. Always add acid to water, never water to acid.
2. The heat released can make the mixture spurt.

Answer: (b) formic acid in the sting

Explanation:
1. Ant stings inject methanoic (formic) acid.
2. Rubbing a mild base like baking soda neutralises it.

Teacher's Note:
1. Nettle stings also contain methanoic acid.
2. Baking soda or calamine gives relief.

Answer: (b) A is an acid while B is a base

Explanation:
1. A pH below 7 is acidic and above 7 is basic.
2. So A (pH 6) is acidic and B (pH 8) is basic.

Teacher's Note:
1. pH less than 7 acid, equal to 7 neutral, more than 7 base.
2. Both are only weakly acidic and basic here.

Answer: (b) fourteen

Explanation:
1. At 25 degrees C, \( [H^+][OH^-] = 10^{-14} \).
2. Taking negative logarithm gives pH + pOH = 14.

Teacher's Note:
1. pH + pOH = 14 for dilute aqueous solutions at 25 degrees C.
2. Know this relation for conversion.

Answer: (b) acetic acid

Explanation:
1. Vinegar is a 5 to 8 percent solution of acetic (ethanoic) acid in water.

Teacher's Note:
1. Vinegar is used as a preservative in pickles.
2. Acetic acid is also called ethanoic acid.

Answer: (a) X

Explanation:
1. The salt XY is formed from the metal ion \( \ce{X+} \) (cation) and \( \ce{Y-} \) (anion).
2. So X is the cation part.

Teacher's Note:
1. The cation comes from the base and the anion from the acid.
2. Water is formed from \( \ce{H+} \) and \( \ce{OH-} \).

Answer: (c) basic

Explanation:
1. Bacteria produce acids that attack tooth enamel when the pH falls below 5.5.
2. Toothpastes are basic and neutralise these acids.

Teacher's Note:
1. Enamel is calcium phosphate and is attacked by acid.
2. Brushing removes food particles too.

Answer: (a) \( \ce{NaNO3} \)

Explanation:
1. \( \ce{NaNO3} \) is a salt of the strong acid \( \ce{HNO3} \) and strong base NaOH, so it is neutral.
2. \( \ce{CuSO4} \) is acidic, \( \ce{NaHCO3} \) and \( \ce{Ca(OH)2} \) are basic.

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

Answer: (c) hydrogen, chlorine and sodium hydroxide

Explanation:
1. Electrolysis of brine (the chlor-alkali process) gives chlorine at the anode, hydrogen at the cathode and sodium hydroxide in solution.
2. \( \ce{2NaCl + 2H2O -> 2NaOH + Cl2 + H2} \).

Teacher's Note:
1. Chlor-alkali process gives three useful products.
2. The name comes from chlorine and alkali.

Answer: (b) gives chlorine on exposure to the atmosphere

Explanation:
1. Bleaching powder, \( \ce{CaOCl2} \), reacts slowly with carbon dioxide and moisture of air to release chlorine.
2. So it smells of chlorine.

Teacher's Note:
1. It is made by passing chlorine over dry slaked lime.
2. It is used for bleaching and disinfecting water.

Answer: (c) \( \ce{NaHCO3} \)

Explanation:
1. Baking soda is sodium hydrogencarbonate, \( \ce{NaHCO3} \).
2. \( \ce{Na2CO3} \) is washing soda.

Teacher's Note:
1. Baking soda gives \( \ce{CO2} \) on heating, making cakes rise.
2. It is also an antacid.

Answer: (a) cutting the supply of air

Explanation:
1. Sodium bicarbonate and acid release carbon dioxide, which is heavier than air.
2. It covers the fire and cuts off the oxygen supply.

Teacher's Note:
1. Fire needs fuel, oxygen and heat.
2. Removing any one stops the fire.

Answer: (c) by heating gypsum at about 373 K

Explanation:
1. \( \ce{CaSO4.2H2O} \rightarrow \ce{CaSO4}\cdot\frac{1}{2}\ce{H2O} + \frac{3}{2}\ce{H2O} \) on heating gypsum to 373 K (100 degrees C).
2. The half-hydrate is plaster of Paris.

Teacher's Note:
1. Overheating gypsum gives dead burnt plaster that does not set.
2. So the temperature must be controlled.

Answer: (c) \( \ce{CaSO4.2H2O} \)

Explanation:
1. \( \ce{CaSO4}\cdot\frac{1}{2}\ce{H2O} + \frac{3}{2}\ce{H2O} \rightarrow \ce{CaSO4.2H2O} \).
2. It sets to a hard mass of gypsum.

Teacher's Note:
1. Setting is a hydration of plaster of Paris.
2. It is used for casts and statues.