CBSE Class 10 Chemistry Acids, Bases and Salts MCQs Set 05

Answer: (c) Sulphuric acid

Explanation:
1. Sulphuric acid ionises almost completely in water, so it is a strong acid.
2. Lactic, ascorbic and formic acids are weak organic acids.

Teacher's Note:
1. Mineral acids (HCl, \( \ce{H2SO4} \), \( \ce{HNO3} \)) are strong, most organic acids are weak.
2. Strength depends on the degree of ionisation, not on concentration.

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

Explanation:
1. HCl ionises completely in water.
2. Carbonic, ethanoic and methanoic acids are only partly ionised.

Teacher's Note:
1. Completely ionised in water means strong acid.
2. Carbonic acid is very weak.

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

Explanation:
1. Basicity is the number of replaceable hydrogen ions per molecule.
2. \( \ce{H2SO4} \) gives two \( \ce{H+} \), so it is dibasic. \( \ce{HCl} \) and \( \ce{HNO3} \) are monobasic and \( \ce{H3PO4} \) is tribasic.

Teacher's Note:
1. Basicity is the number of H+ ions an acid can release.
2. Count the ionisable hydrogen atoms.

Answer: (c) it is weakly ionised

Explanation:
1. The strength of an acid depends on how fully it ionises in water.
2. Acetic acid ionises only slightly, so it is weak.

Teacher's Note:
1. Weak acids ionise only partly.
2. The presence of -COOH does not by itself decide strength.

Answer: (d) they give pink colour with phenolphthalein

Explanation:
1. Acids give \( \ce{H+} \) in water, taste sour and turn blue litmus red.
2. Phenolphthalein stays colourless in acid. It turns pink only in a base.

Teacher's Note:
1. Phenolphthalein is pink in base, colourless in acid.
2. Methyl orange is red in acid and yellow in base.

Answer: (c) Nitric acid

Explanation:
1. Mineral acids are made from minerals.
2. Nitric acid is a mineral acid, the others are organic acids.

Teacher's Note:
1. HCl, \( \ce{H2SO4} \) and \( \ce{HNO3} \) are mineral acids.
2. Organic acids come from plants and animals.

Answer: (d) quicklime

Explanation:
1. Quicklime (CaO) reacts vigorously with water to form slaked lime, \( \ce{Ca(OH)2} \).
2. \( \ce{CaO + H2O -> Ca(OH)2} \) with release of heat.

Teacher's Note:
1. This reaction is highly exothermic.
2. Slaked lime is used for whitewashing.

Answer: (d) NaCl

Explanation:
1. KOH is a strong base, and ZnO and \( \ce{Al(OH)3} \) are amphoteric (they react with acids as bases).
2. NaCl is a neutral salt, so it is not basic.

Teacher's Note:
1. Salt of a strong acid and strong base is neutral.
2. Amphoteric substances act as acids or bases.

Answer: (c) \( \ce{K2SO4} \)

Explanation:
1. \( \ce{K2SO4} \) is formed from the strong base KOH and the strong acid \( \ce{H2SO4} \), so its solution is neutral.
2. KOH and \( \ce{Ca(OH)2} \) are bases and ZnO is an amphoteric oxide.

Teacher's Note:
1. Salt of strong acid and strong base has pH 7.
2. A salt is neither an acid nor a base.

Answer: (b) Alkalis are soluble in water but all bases are not

Explanation:
1. An alkali is a base that dissolves in water, such as NaOH and KOH.
2. Many bases like \( \ce{Cu(OH)2} \) and \( \ce{Fe(OH)3} \) are insoluble.
3. Ethanol contains OH but does not give \( \ce{OH-} \) ions.

Teacher's Note:
1. All alkalis are bases but not all bases are alkalis.
2. Having an -OH group does not make a compound a base.

Answer: (c) \( \ce{NH4OH} \)

Explanation:
1. NaOH, KOH and \( \ce{Ca(OH)2} \) are strong bases that ionise almost completely.
2. Ammonium hydroxide ionises only slightly, so it is weak.

Teacher's Note:
1. Strong bases are the hydroxides of Group 1 and heavier Group 2 metals.
2. Ammonia solution is the common weak base.

Answer: (b) changed first to red and then to blue

Explanation:
1. Dilute HCl turns blue litmus red.
2. Dilute NaOH turns red litmus back to blue.

Teacher's Note:
1. Litmus is red in acid and blue in base.
2. The change is reversible.

Answer: (c) colourless in acidic medium, pink in basic medium

Explanation:
1. Phenolphthalein is colourless in acid and in neutral solution.
2. It turns pink in a basic solution.

Teacher's Note:
1. Used to detect bases and as an indicator in titrations of acids with bases.
2. Methyl orange is yellow in base.

Answer: (d) a colourless and odourless gas is evolved with bubbles

Explanation:
1. \( \ce{Zn + 2HCl -> ZnCl2 + H2} \).
2. Hydrogen is a colourless, odourless gas that burns with a pop sound.

Teacher's Note:
1. Metal plus acid gives salt and hydrogen.
2. The test for hydrogen is a pop sound with a burning splinter.

Answer: (c) both salt and hydrogen gas

Explanation:
1. A metal replaces the hydrogen of the acid to form a salt and releases hydrogen.

Teacher's Note:
1. Metal + acid gives salt + hydrogen.
2. Nitric acid is an exception.

Answer: (c) sodium zincate and hydrogen gas

Explanation:
1. \( \ce{Zn + 2NaOH -> Na2ZnO2 + H2} \).
2. The products are sodium zincate and hydrogen.

Teacher's Note:
1. Zinc, being amphoteric, reacts with a strong base.
2. Hydrogen is again evolved.

Answer: (c) Al

Explanation:
1. Aluminium is amphoteric and reacts with NaOH to give sodium aluminate and hydrogen.
2. Mg, Cu and Ag do not react with alkali.

Teacher's Note:
1. Only Zn and Al among common metals react with alkalis.
2. This property is due to their amphoteric oxides.

Answer: (a) \( \ce{CO2} \)

Explanation:
1. Acid plus metal carbonate gives salt, water and carbon dioxide.
2. Carbon dioxide turns lime water milky.

Teacher's Note:
1. Brisk effervescence means \( \ce{CO2} \).
2. The same happens with bicarbonates.

Answer: (b) solid X is sodium bicarbonate and the gas evolved is \( \ce{CO2} \)

Explanation:
1. A gas that turns lime water milky is carbon dioxide.
2. Acids release \( \ce{CO2} \) from carbonates and bicarbonates, so X is sodium bicarbonate.
3. NaOH gives no gas, and sodium acetate and NaCl do not react with acetic acid.

Teacher's Note:
1. Lime water test is the confirmation for \( \ce{CO2} \).
2. Acid plus bicarbonate gives \( \ce{CO2} \).

Answer: (a) \( \ce{CaCO3} \)

Explanation:
1. \( \ce{Ca(OH)2 + CO2 -> CaCO3 + H2O} \).
2. Insoluble calcium carbonate gives the milkiness.

Teacher's Note:
1. Excess \( \ce{CO2} \) turns it clear again by forming soluble \( \ce{Ca(HCO3)2} \).
2. Lime water is a solution of calcium hydroxide.