CBSE Class 10 Chemistry Laboratory Preparation of Gases and Acids MCQs Set 12

Answer: (a) Manganese dioxide and concentrated hydrochloric acid

Explanation:
1. Manganese dioxide oxidises hydrochloric acid when the mixture is warmed.
2. MnO\( _2 \) + 4HCl \( \rightarrow \) MnCl\( _2 \) + 2H\( _2 \)O + Cl\( _2 \).
3. The acid must be concentrated; the dilute acid does not react.

Teacher's Note:
1. Chlorine is poisonous, so it is prepared in a fume cupboard.
2. Bleaching powder with dilute acid also gives chlorine.

Answer: (c) upward displacement of air

Explanation:
1. Chlorine is about 2.5 times heavier than air, so it sinks and the air is pushed up out of the jar.
2. It is fairly soluble in water, so it is not collected over water.

Teacher's Note:
1. Jars are held upright with the delivery tube reaching the bottom.
2. Hydrogen, being lighter, is collected by downward displacement of air.

Answer: (b) Yellowish green

Explanation:
1. Chlorine is a yellowish-green gas with a choking smell.
2. Its name comes from the Greek chloros, meaning pale green.

Teacher's Note:
1. Bromine vapour is reddish brown and iodine vapour is violet.
2. Chlorine irritates the nose and throat.

Answer: (a) remove hydrogen chloride vapour

Explanation:
1. Concentrated hydrochloric acid is volatile, so some HCl vapour is carried over with the chlorine.
2. HCl is very soluble in water, while chlorine is only fairly soluble, so the water holds back the HCl.

Teacher's Note:
1. Concentrated sulphuric acid in a second bottle can then dry the gas.
2. The gas should not be passed through water for too long.

Answer: (b) continues to burn with a red smoky flame

Explanation:
1. Chlorine combines with the hydrogen of the candle wax to form hydrogen chloride.
2. The carbon of the wax is left behind as black smoke, so the flame is red and smoky.

Teacher's Note:
1. This shows chlorine's strong affinity for hydrogen.
2. The candle burns because chlorine supports combustion of hydrogen.

Answer: (b) it burns spontaneously with white fumes

Explanation:
1. Antimony reacts vigorously with chlorine and catches fire without being heated.
2. The white fumes are antimony trichloride and pentachloride: 2Sb + 3Cl\( _2 \) \( \rightarrow \) 2SbCl\( _3 \).

Teacher's Note:
1. Dutch metal (copper-zinc alloy) also burns in chlorine.
2. Chlorine is a very active non-metal.

Answer: (a) orange-red

Explanation:
1. Chlorine is more reactive than bromine, so it displaces bromine: Cl\( _2 \) + 2KBr \( \rightarrow \) 2KCl + Br\( _2 \).
2. The liberated bromine gives an orange-red colour.

Teacher's Note:
1. With potassium iodide the displaced iodine gives a violet or brown colour.
2. Reactivity falls in the order Cl, Br, I.

Answer: (a) sulphur

Explanation:
1. Chlorine oxidises hydrogen sulphide: Cl\( _2 \) + H\( _2 \)S \( \rightarrow \) 2HCl + S.
2. The sulphur separates as a yellow solid.

Teacher's Note:
1. Chlorine acts as an oxidising agent here.
2. The same reaction occurs with bromine water.

Answer: (d) Hydrochloric acid and hypochlorous acid

Explanation:
1. Cl\( _2 \) + H\( _2 \)O \( \rightarrow \) HCl + HOCl.
2. Hypochlorous acid is unstable and decomposes to HCl and nascent oxygen, which does the bleaching.

Teacher's Note:
1. The bleaching action of chlorine needs moisture.
2. Dry chlorine does not bleach dry litmus paper.

Answer: (c) oxidation by nascent oxygen from hypochlorous acid

Explanation:
1. HOCl \( \rightarrow \) HCl + [O].
2. The nascent oxygen oxidises the coloured matter to a colourless product, so the bleaching is permanent.

Teacher's Note:
1. Sulphur dioxide bleaches by reduction, and the colour may return.
2. Chlorine bleaches cotton and paper pulp.

Answer: (d) sulphur dioxide

Explanation:
1. Cu + 2H\( _2 \)SO\( _4 \) \( \rightarrow \) CuSO\( _4 \) + 2H\( _2 \)O + SO\( _2 \).
2. The acid is reduced to sulphur dioxide, and copper is oxidised to copper sulphate.

Teacher's Note:
1. There is no reaction in the cold.
2. Dilute sulphuric acid does not act on copper.

Answer: (d) Burns in air with a blue flame

Explanation:
1. Sulphur dioxide is itself the product of burning sulphur and does not burn further.
2. A burning candle is put out in it, and it is very soluble in water giving sulphurous acid.

Teacher's Note:
1. Hydrogen sulphide, by contrast, burns in air.
2. SO\( _2 \) is collected by upward displacement of air.

Answer: (b) disappears

Explanation:
1. SO\( _2 \) is a reducing agent. It reduces purple permanganate to almost colourless Mn\( ^{2+} \) ions.
2. 2KMnO\( _4 \) + 5SO\( _2 \) + 2H\( _2 \)O \( \rightarrow \) K\( _2 \)SO\( _4 \) + 2MnSO\( _4 \) + 2H\( _2 \)SO\( _4 \).

Teacher's Note:
1. Hydrogen sulphide also decolourises acidified permanganate.
2. Dilute sulphuric acid is used to acidify the solution.

Answer: (a) chromium(III) sulphate

Explanation:
1. The orange dichromate ion is reduced by SO\( _2 \) to green chromium(III) ions.
2. K\( _2 \)Cr\( _2 \)O\( _7 \) + H\( _2 \)SO\( _4 \) + 3SO\( _2 \) \( \rightarrow \) K\( _2 \)SO\( _4 \) + Cr\( _2 \)(SO\( _4 \))\( _3 \) + H\( _2 \)O.

Teacher's Note:
1. This is a standard test for sulphur dioxide.
2. Orange to green shows reduction of chromium.

Answer: (a) an oxidising agent

Explanation:
1. SO\( _2 \) + 2H\( _2 \)S \( \rightarrow \) 2H\( _2 \)O + 3S.
2. Sulphur in SO\( _2 \) is reduced from +4 to 0, so SO\( _2 \) is the oxidising agent, and H\( _2 \)S is oxidised to sulphur.

Teacher's Note:
1. SO\( _2 \) can be both an oxidising and a reducing agent.
2. It reduces permanganate but oxidises hydrogen sulphide.

Answer: (b) magnesium oxide and sulphur

Explanation:
1. Magnesium takes the oxygen from SO\( _2 \): 2Mg + SO\( _2 \) \( \rightarrow \) 2MgO + S.
2. Some of the sulphur combines with magnesium to form magnesium sulphide.

Teacher's Note:
1. Magnesium is so reactive that it burns even in oxides such as carbon dioxide.
2. A burning candle is extinguished in SO\( _2 \).

Answer: (b) Ferrous sulphide and dilute hydrochloric acid

Explanation:
1. FeS + 2HCl \( \rightarrow \) FeCl\( _2 \) + H\( _2 \)S.
2. The gas can be prepared from a Kipp's apparatus.

Teacher's Note:
1. Dilute sulphuric acid also works.
2. Hydrogen sulphide is poisonous and must be prepared in a fume cupboard.

Answer: (b) lead sulphide

Explanation:
1. H\( _2 \)S + Pb(CH\( _3 \)COO)\( _2 \) \( \rightarrow \) PbS + 2CH\( _3 \)COOH.
2. Lead sulphide is black and insoluble.

Teacher's Note:
1. This is the standard test for H\( _2 \)S.
2. A silver coin is also blackened by H\( _2 \)S.

Answer: (d) black copper sulphide

Explanation:
1. CuSO\( _4 \) + H\( _2 \)S \( \rightarrow \) CuS + H\( _2 \)SO\( _4 \).
2. Copper sulphide is black and very insoluble even in dilute acids.

Teacher's Note:
1. Silver and lead salts also give black sulphides.
2. Cadmium sulphide is yellow.

Answer: (b) sulphur and water

Explanation:
1. 2H\( _2 \)S + O\( _2 \) \( \rightarrow \) 2H\( _2 \)O + 2S in a limited supply of oxygen.
2. In excess of oxygen, 2H\( _2 \)S + 3O\( _2 \) \( \rightarrow \) 2H\( _2 \)O + 2SO\( _2 \).

Teacher's Note:
1. A burning candle is put out in H\( _2 \)S but the gas itself burns.
2. The yellow deposit on the jar is sulphur.

Answer: (a) a reducing agent

Explanation:
1. H\( _2 \)S + 2FeCl\( _3 \) \( \rightarrow \) 2FeCl\( _2 \) + 2HCl + S.
2. Fe\( ^{3+} \) is reduced to Fe\( ^{2+} \) (pale green), and H\( _2 \)S is oxidised to sulphur.

Teacher's Note:
1. Hydrogen sulphide is a strong reducing agent.
2. It also reduces sulphuric acid and nitric acid.

Answer: (b) concentrated sulphuric acid in a glass retort

Explanation:
1. KNO\( _3 \) + H\( _2 \)SO\( _4 \) \( \rightarrow \) KHSO\( _4 \) + HNO\( _3 \).
2. The vapours are condensed in a receiver cooled with water. A glass retort with ground-glass joints is used because the acid attacks rubber and cork.

Teacher's Note:
1. Sodium nitrate can be used instead of potassium nitrate.
2. The temperature is kept low, as nitric acid decomposes on strong heating.

Answer: (a) nitrogen dioxide is dissolved in it

Explanation:
1. On heating, some acid decomposes: 4HNO\( _3 \) \( \rightarrow \) 4NO\( _2 \) + 2H\( _2 \)O + O\( _2 \).
2. The reddish-brown NO\( _2 \) dissolves and colours the acid yellow.

Teacher's Note:
1. Pure nitric acid is colourless.
2. Light also decomposes nitric acid, so it is kept in dark bottles.

Answer: (d) reddish-brown nitrogen dioxide

Explanation:
1. Cu + 4HNO\( _3 \) \( \rightarrow \) Cu(NO\( _3 \))\( _2 \) + 2H\( _2 \)O + 2NO\( _2 \).
2. The solution becomes blue because of copper(II) nitrate.

Teacher's Note:
1. With dilute acid the gas is nitric oxide, NO, which turns brown in air.
2. Nitric acid is a strong oxidising agent.