CBSE Class 7 Science Advanced Chemistry Air and Oxygen MCQs Set 03

Answer: (c) exosphere

Explanation:
1. From the ground upwards the layers are troposphere, stratosphere, mesosphere, thermosphere and exosphere.
2. The outermost one is the exosphere (magnetosphere), and it absorbs the harmful ionizing radiations.
3. The answer is exosphere.

Teacher's Note:
1. Outermost means farthest from the earth's surface.
2. Do not confuse it with the stratosphere, which holds the ozone layer.

Answer: (d) Carbon dioxide

Explanation:
1. A gas heavier than air is collected by upward displacement of air, because it sinks to the bottom of the jar and pushes air out upwards.
2. Carbon dioxide (molar mass 44) is heavier than air (average about 29), whereas hydrogen and nitrogen are not heavier, and oxygen is collected over water.
3. The answer is carbon dioxide.

Teacher's Note:
1. Heavier than air means upward displacement of air.
2. Lighter gases such as hydrogen are collected by downward displacement of air or over water.

Answer: (b) tinning

Explanation:
1. Galvanization is coating with zinc, and electroplating uses electricity to deposit a metal.
2. Coating containers with a layer of tin so that the food does not react with the metal is called tinning.
3. The answer is tinning.

Teacher's Note:
1. The name of the process comes from the metal used: tin gives tinning, zinc gives galvanization.
2. Food cans are tin-coated for safety.

Answer: (a) BCDAE

Explanation:
1. Nitrogen is inert and dilutes the active oxygen: (B).
2. Oxygen forms acidic oxides with non-metals and basic oxides with metals: (C).
3. Helium is inert and present only in trace amounts: (D).
4. Carbon dioxide is acidic and neither burns nor supports burning: (A).
5. Water vapour is present in variable amounts: (E).
6. The order is B, C, D, A, E, which is option (a).

Teacher's Note:
1. Match the easy ones first: water vapour is the one with variable amount.
2. Helium is the trace inert gas.

Answer: (b) DCBAE

Explanation:
1. Going up from the earth's surface, the layers are troposphere, stratosphere, mesosphere, thermosphere and exosphere.
2. (D) fall of 6°C per km is the troposphere. (C) rise from −60°C to −15°C (inversion) is the stratosphere. (B) fall from −15°C to −120°C is the mesosphere. (A) rise up to 2000°C is the thermosphere. (E) high temperature from absorbing ionizing radiation is the exosphere.
3. The order is D, C, B, A, E: option (b).

Teacher's Note:
1. Read 'from above the earth's surface' as starting at the ground and moving upward.
2. Temperature falls, rises, falls, rises again as you go up.

Answer: (c) (i) → (D); (ii) → (E); (iii) → (A); (iv) → (C); (v) → (B)

Explanation:
1. Carbon monoxide combines with haemoglobin and reduces oxygen intake: (i) → (D).
2. More \(\ce{CH4}\), \(\ce{CO2}\) and \(\ce{O3}\) strengthen the greenhouse effect, causing global warming: (ii) → (E).
3. CFCs destroy the ozone layer: (iii) → (A).
4. Oxides of sulphur and nitrogen cause acid rain: (iv) → (C).
5. Suspended particulate matter helps form smog: (v) → (B).
6. This is option (c).

Teacher's Note:
1. Start with CFCs and ozone depletion, which is the surest pair.
2. SO₂ and NO₂ dissolve in rain to make acids.

Answer: (a) Corrosion of marble statues and monuments.

Explanation:
1. Global warming raises the earth's temperature, which melts ice caps, changes rainfall patterns and increases floods.
2. Corrosion of marble is caused by acid rain from SO₂ and NO₂, not by global warming.
3. The answer is (a).

Teacher's Note:
1. Link each effect to its cause: warming causes melting and unusual rain; acid rain causes marble corrosion.
2. The Taj Mahal's marble is damaged by acid rain.

Answer: (c) CFCs

Explanation:
1. Burning coal releases soot and ash (SPM), oxides of nitrogen and sulphur, and carbon monoxide from incomplete combustion.
2. CFCs come from refrigerators, air conditioners and aerosol sprays, not from coal.
3. The answer is CFCs.

Teacher's Note:
1. Coal contains carbon, a little sulphur and nitrogen compounds.
2. CFCs are man-made cooling and spray chemicals.

Answer: (a) Hydrogen

Explanation:
1. A reducing agent removes oxygen from a metal oxide.
2. Hydrogen combines with the oxygen of a metal oxide to form water, leaving the metal.
3. Nitrogen is inert, oxygen is an oxidising agent and carbon dioxide is already fully oxidised. The answer is hydrogen.

Teacher's Note:
1. Reduction of an oxide means removal of oxygen.
2. Example: CuO + H₂ gives Cu + H₂O.

Answer: (d) Haber's process

Explanation:
1. Nitrogen and hydrogen are combined at high pressure and about 450°C with an iron catalyst: \(\ce{N2 + 3H2 <=> 2NH3}\).
2. This industrial process is called Haber's process.

Teacher's Note:
1. Haber's process = ammonia; Contact process = sulphuric acid.
2. Hydrogenation means adding hydrogen to oils.

Answer: (c) \(\ce{CO}\) + \(\ce{H2}\)

Explanation:
1. Steam passed over red-hot coke gives carbon monoxide and hydrogen: \(\ce{C + H2O -> CO + H2}\).
2. This mixture is water gas, so the answer is CO + H₂.

Teacher's Note:
1. Water gas comes from water (steam) and carbon, so it has H₂ and CO.
2. Producer gas is the CO + N₂ mixture.

Answer: (a) Zn

Explanation:
1. Potassium reacts violently, iron reacts very slowly and aluminium is covered by an oxide layer.
2. Zinc reacts with dilute acid at a steady, moderate rate, which is why it is used in the laboratory.
3. The answer is Zn.

Teacher's Note:
1. Think of the lab method: zinc granules and dilute acid.
2. Too violent and too slow metals are not suitable.

Answer: (d) \(\ce{SO2}\)

Explanation:
1. Sulphur burns in air with a blue flame to give a gas with a sharp, suffocating smell.
2. Oxygen, carbon dioxide and hydrogen are not formed in this way and (except for SO₂) have no suffocating smell.
3. The gas is sulphur dioxide.

Teacher's Note:
1. A pungent choking smell points to SO₂.
2. Hydrogen burns but is odourless.

Answer: (c) thermosphere

Explanation:
1. The thermosphere contains ions and is used for long-distance communication, and satellites orbit in this layer.
2. So the answer is thermosphere.

Teacher's Note:
1. Ionised layers reflect radio and television signals.
2. Auroras also appear in the thermosphere.

Answer: (b) downward displacement of water.

Explanation:
1. Oxygen is only slightly soluble in water, and it is slightly heavier than air, so it is best collected over water by pushing the water out of the jar downwards.
2. The answer is downward displacement of water.

Teacher's Note:
1. Gases that are insoluble in water are collected over water.
2. This also keeps the oxygen free from air.

Answer: (c) Alloying

Explanation:
1. Mixing a metal uniformly with small amounts of another element to improve its properties is alloying.
2. Iron with a little carbon gives steel, so the process is alloying.

Teacher's Note:
1. Steel is an alloy of iron and carbon.
2. Galvanization and electroplating are coatings, not mixing.

Answer: (a) CBDEA

Explanation:
1. Proteins are made using nitrogen: (C).
2. Solid carbon dioxide (dry ice) is a refrigerant: (B).
3. Hydrogen is a chief component of fuel gases because of high heat of combustion: (D).
4. Water vapour forms fog, mist and clouds: (E).
5. Oxygen is used to make acids such as nitric and sulphuric acids: (A).
6. The order is C, B, D, E, A: option (a).

Teacher's Note:
1. Spot the easy pairs: fog goes with water vapour, proteins with nitrogen.
2. Dry ice is solid CO₂.

Answer: (d) CEADB

Explanation:
1. Exosphere: the outermost layer that absorbs ionizing radiation, (C).
2. Thermosphere: long-distance communication and auroras, (E).
3. Mesosphere: extends up to 85–90 km and burns meteorites, (A).
4. Stratosphere: little of the gases and aeroplanes fly here, (D).
5. Troposphere: holds most of the mass of the atmosphere, (B).
6. The order is C, E, A, D, B: option (d).

Teacher's Note:
1. Start from the outermost layer and come downward.
2. Meteorites burn in the mesosphere.

Answer: (a) (i) → (B); (ii) → (D); (iii) → (A); (iv) → (E); (v) → (C)

Explanation:
1. Hydrogen can be made by the action of steam on hot metals: (i) → (B).
2. Carbon dioxide is made by dilute acid acting on carbonates and hydrogen carbonates: (ii) → (D).
3. Water vapour is present in variable amounts in air: (iii) → (A).
4. Oxygen is made by heating oxides of low-reactivity metals such as HgO: (iv) → (E).
5. Nitrogen is the main component of air by volume and weight: (v) → (C).
6. This is option (a).

Teacher's Note:
1. Some entries give the method and some the amount present in air, so just match each gas to its sensible fact.
2. Mercury(II) oxide on heating gives oxygen.

Answer: (d) oxygen

Explanation:
1. CFCs contain carbon, chlorine and fluorine, and the chlorine atoms released from them destroy ozone.
2. Oxygen is the only element listed that is not part of the ozone-destroying chemicals.
3. The answer is oxygen.

Teacher's Note:
1. Look at the elements in CFCs (chlorofluorocarbons): C, Cl and F.
2. Ozone itself is made of oxygen, so this question treats oxygen as the harmless one.

Answer: (b) Oxide of sulphur

Explanation:
1. Paper making uses sulphur compounds in treating wood pulp, and sulphur dioxide is released.
2. So the pollutant is oxide of sulphur.

Teacher's Note:
1. Think of the sulphite process of pulp treatment.
2. CFCs come from cooling appliances.

Answer: (c) Oxygen

Explanation:
1. Liquid oxygen mixed with coal and petroleum jelly burns violently and is used as an explosive (liquid oxygen explosive).
2. The answer is oxygen.

Teacher's Note:
1. The oxidiser in an explosive mixture has to be oxygen.
2. Liquid nitrogen is used for freezing instead.

Answer: (d) hydrogenation

Explanation:
1. Hydrogen is added to unsaturated vegetable oils in the presence of a nickel catalyst to give solid fats (vanaspati).
2. This process is hydrogenation.

Teacher's Note:
1. Adding hydrogen to something is hydrogenation.
2. Vanaspati ghee is made this way.

Answer: (a) \(\ce{O2}\), \(\ce{H2}\)

Explanation:
1. Hydrogen burns in oxygen to give the oxyhydrogen flame, at nearly 2800°C.
2. This very hot flame welds metals and melts platinum and quartz, so the mixture is O₂ and H₂.

Teacher's Note:
1. Which two gases produce a very hot flame together?
2. Platinum and quartz need extremely high temperatures to melt.

Answer: (c) Pb

Explanation:
1. Lead reacts with sulphuric acid to form lead sulphate, which is insoluble and coats the surface of the metal.
2. This coating stops further reaction, so the answer is Pb.

Teacher's Note:
1. Look for the metal whose sulphate is insoluble.
2. This is why lead is not used to make hydrogen with dilute acid.

Answer: (b) \(\ce{SO2}\)

Explanation:
1. Sulphur dioxide turns the orange acidified potassium dichromate paper green.
2. The gas is SO₂.

Teacher's Note:
1. Colour change orange to green is the test for SO₂.
2. Chromium(III) sulphate is the green product.

Answer: (b) BCDA

Explanation:
1. Aeroplanes fly in the stratosphere: (i) → B.
2. Meteorites burn in the mesosphere: (ii) → C.
3. Radio waves are reflected by the ionosphere: (iii) → D.
4. Weather happens in the troposphere: (iv) → A.
5. The order is B, C, D, A: option (b).

Teacher's Note:
1. Match the weather phenomena with the lowest layer first.
2. The ionosphere is the layer of ions in the upper atmosphere.

Answer: (a) DBAC

Explanation:
1. Phosphine is absorbed by silver nitrate solution: D.
2. Hydrogen sulphide is absorbed by lead nitrate solution (black lead sulphide forms): B.
3. Carbon dioxide is absorbed by caustic potash: A.
4. Water vapour is absorbed by calcium chloride: C.
5. The order is D, B, A, C: option (a).

Teacher's Note:
1. Acidic gases (CO₂) go to alkali; moisture goes to a drying agent.
2. Lead nitrate gives black PbS with H₂S.

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

Explanation:
1. Alloys are used for tools and utensils: (i) → (C).
2. Enamelling protects cooking stoves and refrigerators: (ii) → (A).
3. Tinning is used for food containers: (iii) → (D).
4. Tar is coated on wooden or iron electric poles: (iv) → (B).
5. This is option (b).

Teacher's Note:
1. Tinning is for food cans, so (iii) → (D) removes options (a), (c) and (d) quickly.
2. Tar is thick and black, used outdoors.

Answer: (b) 195 L

Explanation:
1. Volume of N₂ = 78% of 250 L.
2. \( \dfrac{78}{100} \times 250 = 195 \) L.
3. The answer is 195 L.

Teacher's Note:
1. Percentage means per hundred.
2. 78% of 250 = 0.78 × 250.

Answer: (b) Preservation of food

Explanation:
1. Oxygen is used with other gases for cutting and welding, in making explosives such as cartridges, and for artificial breathing.
2. Food is preserved using nitrogen (liquid nitrogen) or carbon dioxide, not oxygen, which makes it spoil.
3. The answer is (b).

Teacher's Note:
1. Oxygen speeds up spoilage and burning.
2. Food packs are usually flushed with nitrogen.

Answer: (d) \(\ce{Fe2O3}\)

Explanation:
1. Water gas (CO + H₂) is mixed with steam over iron(III) oxide at about 500°C: \(\ce{CO + H2O -> CO2 + H2}\).
2. The catalyst is Fe₂O₃.

Teacher's Note:
1. The catalyst is an iron oxide, not a chloride.
2. This step increases the amount of hydrogen.

Answer: (d) All the above

Explanation:
1. Hydrogen peroxide decomposes quickly at room temperature when manganese dioxide is added, so heating is not required.
2. Manganese dioxide is the catalyst, and the catalyst should be pure.
3. All three statements are true, so the answer is (d).

Teacher's Note:
1. When all statements seem true, 'All the above' is likely.
2. Reaction: 2H₂O₂ gives 2H₂O + O₂ with MnO₂.

Answer: (c) Oxides of nitrogen

Explanation:
1. Oxides of nitrogen form nitric acid in rain, and they react with hydrocarbons in sunlight to form smog.
2. Oxides of sulphur cause acid rain but are not the main smog gases here, and hydrocarbons and carbon oxides do not cause acid rain.
3. The answer is oxides of nitrogen.

Teacher's Note:
1. Find the pollutant that appears in both lists.
2. Photochemical smog involves NOₓ.

Answer: (a) C and N

Explanation:
1. Carbon dioxide is needed for photosynthesis and nitrogen oxides help in nitrogen fixation, but in excess they cause global warming and acid rain.
2. So the pair is C and N.

Teacher's Note:
1. Think of needed gases: CO₂ for plants, nitrogen compounds for soil.
2. Both turn into pollutants when in excess.

Answer: (d) Brown to grey

Explanation:
1. Iron(III) oxide is a reddish-brown powder.
2. On reduction with hydrogen it forms iron, which is grey.
3. The change is brown to grey.

Teacher's Note:
1. Rust-coloured oxide goes to metallic iron.
2. Iron metal is grey.

Answer: (a) Na and K

Explanation:
1. Sodium and potassium react violently, even catching fire, with cold water.
2. Calcium reacts less vigorously, and magnesium reacts only with steam.
3. The answer is Na and K.

Teacher's Note:
1. Alkali metals are stored in kerosene because they react with water.
2. Magnesium reacts very slowly with cold water.

Answer: (b) \(\ce{O2}\) and \(\ce{H2}\)

Explanation:
1. Gases that are almost insoluble in water can be collected over water.
2. Oxygen and hydrogen are almost insoluble, whereas CO₂ and SO₂ dissolve in water.
3. The answer is O₂ and H₂.

Teacher's Note:
1. Soluble gases cannot be collected over water.
2. CO₂ and SO₂ both react with water.

Answer: (b) \(\ce{O2}\)

Explanation:
1. Alkaline pyrogallol absorbs oxygen and becomes brown.
2. So the gas is O₂.

Teacher's Note:
1. This is the usual test for oxygen in a gas mixture.
2. The solution is colourless before it absorbs oxygen.

Answer: (b) BADC

Explanation:
1. −60°C to −15°C is the stratosphere (B).
2. 17°C to −51°C is the troposphere (A).
3. Up to 2000°C is the ionosphere or thermosphere (D).
4. −15°C to −120°C is the mesosphere (C).
5. The order is B, A, D, C: option (b).

Teacher's Note:
1. The troposphere is the warm layer at ground level and cools with height.
2. The highest temperature belongs to the top layer.