CBSE Class 7 Science Advanced Chemistry Air and Oxygen MCQs Set 04

Answer: (b) BDCA

Explanation:
1. Calcium is expensive: (i) → B.
2. Iron and steam react reversibly: (ii) → D.
3. Sodium reacts violently: (iii) → C.
4. Lead forms an insoluble coating of compounds: (iv) → A.
5. The order is B, D, C, A: option (b).

Teacher's Note:
1. Sodium is the violent one, and lead is the one with insoluble coating.
2. Iron and steam give a reversible reaction.

Answer: (c) (i) → (C), (ii) → (D), (iii) → (A), (iv) → (B)

Explanation:
1. Enamelling is a coating of a mixture of silicates: (i) → (C).
2. Tinning is coating with tin: (ii) → (D).
3. Electroplating is done by electrolysis: (iii) → (A).
4. Enamel paint is applied after first coating with red lead oxide: (iv) → (B).
5. This is option (c).

Teacher's Note:
1. Tinning and tin is the easy pair.
2. Electroplating needs electric current.

Answer: (a) 150 L

Explanation:
1. 21 L of oxygen is present in 100 L of air.
2. Volume of air = \( \dfrac{31.5 \times 100}{21} = 150 \) L.
3. The answer is 150 L.

Teacher's Note:
1. Set up the proportion: oxygen/air = 21/100.
2. Check: 21% of 150 = 31.5.

Answer: (a) Hydrogen and acetylene

Explanation:
1. Hydrogen burns in oxygen to give the oxyhydrogen flame (about 2800°C), used for cutting.
2. Acetylene burns in oxygen to give the oxyacetylene flame (about 3300°C), used for welding.
3. Nitrogen and helium do not burn, so the answer is hydrogen and acetylene.

Teacher's Note:
1. Both gases must be combustible.
2. Nitrogen and helium are inert.

Answer: (a) CO is oxidized.

Explanation:
1. In \(\ce{CO + H2O -> CO2 + H2}\) carbon monoxide gains oxygen and changes to carbon dioxide.
2. Gain of oxygen is oxidation, so CO is oxidized.

Teacher's Note:
1. Oxidation is gain of oxygen.
2. CO₂ is a product here, not a reactant.

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

Explanation:
1. Mercury(II) oxide, potassium nitrate and hydrogen peroxide all give oxygen on heating or with a catalyst.
2. Calcium carbonate gives carbon dioxide on heating, not oxygen.
3. The answer is CaCO₃.

Teacher's Note:
1. CaCO₃ gives CaO + CO₂.
2. Nitrates give oxygen when heated.

Answer: (c) Refrigerators

Explanation:
1. Smoke, soot, sulphur and nitrogen oxides from power plants, vehicles and industries irritate the lungs.
2. Refrigerators release CFCs, which damage the ozone layer, but they are not the source of these lung pollutants.
3. The answer is (c).

Teacher's Note:
1. Link the disease to smoke and fumes from burning fuels.
2. Refrigerators are linked with ozone depletion instead.

Answer: (d) CO

Explanation:
1. Carbon monoxide binds with haemoglobin about 200 times more strongly than oxygen and forms a stable compound.
2. This blocks oxygen transport and can cause suffocation and death.
3. The most harmful here is CO.

Teacher's Note:
1. Think about which gas blocks oxygen carriage in blood.
2. Poisoning from a closed room with a coal fire is by CO.

Answer: (b) ZnO

Explanation:
1. Zinc oxide is white and zinc is a bluish-white metal.
2. The reaction is \(\ce{ZnO + H2 -> Zn + H2O}\).
3. Lead oxide is yellow or red, copper oxide is black and tin oxide is white but tin is silvery, so the answer is ZnO.

Teacher's Note:
1. Match the colours of the oxide and of the metal.
2. CuO is black and gives reddish-brown copper.

Answer: (d) Al, Zn

Explanation:
1. Calcium reacts readily with cold water, so it is not a slow reactor.
2. Aluminium and zinc react only slowly with steam, and magnesium reacts with steam more quickly.
3. The answer is Al and Zn.

Teacher's Note:
1. Group the metals by reactivity: Ca high, Mg medium, Al and Zn lower.
2. Aluminium needs steam to break through its oxide layer.

Answer: (a) It reacts with water.

Explanation:
1. Carbon dioxide dissolves in water and reacts with it to form carbonic acid: \(\ce{CO2 + H2O -> H2CO3}\).
2. So it cannot be collected over water: option (a).

Teacher's Note:
1. Only insoluble gases are collected over water.
2. CO₂ is heavier than air, not water.

Answer: (a) reddish brown

Explanation:
1. \(\ce{2NO + O2 -> 2NO2}\): the colourless gas turns into brown fumes.
2. Nitrogen dioxide fumes are reddish brown.

Teacher's Note:
1. NO₂ is the brown gas seen in smog.
2. Chlorine is the greenish-yellow gas.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: high-energy radiation from the sun knocks electrons out of gas molecules, giving ions and free electrons in the thermosphere.
2. R is true: the same absorbed radiation heats this layer to as much as 2000°C.
3. The book treats the absorption of high-energy radiation as the common cause, so R is taken as the explanation: (a).

Teacher's Note:
1. Both statements point to absorption of high-energy radiation.
2. Ionisation and heating are both effects of that absorption.

Answer: (d) A is false, but R is true.

Explanation:
1. A is false: oxygen is not a greenhouse gas, and SO₂ and NO₂ cause acid rain rather than the greenhouse effect.
2. R is true: greenhouse gases such as CO₂, CH₄, water vapour and ozone absorb infrared radiation.
3. So A is false and R is true: (d).

Teacher's Note:
1. Typical greenhouse gases are CO₂, CH₄, O₃ and water vapour.
2. Check the examples in A one by one.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: auroras occur in the thermosphere.
2. R is true: the particles and radiation from the sun excite O₂ and N₂ molecules, which give out coloured light.
3. R is the reason for A, so the answer is (a).

Teacher's Note:
1. Auroras are glowing light, so ask what makes the gas glow.
2. Both oxygen and nitrogen give auroral colours.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: carbon monoxide binds to haemoglobin about 200 times more strongly than oxygen does, so it is far more poisonous.
2. R is false: carbon monoxide comes from incomplete combustion of fuels in vehicles and industries, not from deforestation.
3. So A is true and R is false: (c).

Teacher's Note:
1. Check the origin of CO: incomplete burning of carbon fuels.
2. Deforestation mainly reduces the CO₂ absorbed by plants.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: the pressure of air is lower on a mountain top than at sea level.
2. R is true: the air gets thinner with height, so there are fewer molecules above each point.
3. Lower density means lower pressure, so R explains A: (a).

Teacher's Note:
1. Pressure comes from the weight of the air above.
2. Less air above means less pressure.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: the orange dichromate turns green when SO₂ is passed.
2. R is true: chromium(III) sulphate, which is green, forms along with potassium sulphate.
3. The green colour is due to the new chromium compound, so R explains A: (a).

Teacher's Note:
1. Link the colour change to the new compound formed.
2. Cr³⁺ ions are green.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: sea level has the greatest column of air above it.
2. R is false: the troposphere is the densest layer, since it holds most of the atmosphere's mass.
3. So A is true and R is false: (c).

Teacher's Note:
1. The lowest layer is the densest.
2. Check whether R really says 'least' or 'maximum'.

Answer: (d) A is false, but R is true.

Explanation:
1. A is false: SO₂ is acidic, and it turns moist blue litmus red (it does not turn red litmus blue).
2. R is true: SO₂ dissolves in water to form sulphurous acid.
3. So A is false and R is true: (d).

Teacher's Note:
1. Acidic gases turn blue litmus red.
2. Basic gases such as ammonia turn red litmus blue.