CBSE Class 7 Science Advanced Chemistry Air and Oxygen MCQs Set 02

Answer: (c) usage of catalytic convertor

Explanation:
1. A catalytic converter has a platinum catalyst on a ceramic honeycomb.
2. It converts CO to \(\ce{CO2}\) and oxides of nitrogen to \(\ce{N2}\), which are less harmful.
3. So the answer is usage of catalytic converter.

Teacher's Note:
1. Scrubbers and precipitators are used in industries, not vehicles.
2. Catalytic converters are fitted in the exhaust of automobiles.

Answer: (b) \(\ce{H2O2}\), \(\ce{O2}\) and \(\ce{NO2}\)

Explanation:
1. In hydrogen peroxide oxygen has valency 1. It decomposes: \(\ce{2H2O2 -> 2H2O + O2}\), so Y is oxygen.
2. Oxygen reacts with the neutral oxide NO to form reddish-brown \(\ce{NO2}\): \(\ce{2NO + O2 -> 2NO2}\).
3. \(\ce{NO2}\) is acidic and turns blue litmus red.
4. So the answer is \(\ce{H2O2}\), \(\ce{O2}\) and \(\ce{NO2}\).

Teacher's Note:
1. Reddish-brown fumes are the clue to \(\ce{NO2}\).
2. Oxygen is neutral, whereas \(\ce{NO2}\) is acidic.

Answer: (c) of the presence of very few molecules or ions.

Explanation:
1. The few molecules there absorb radiation and become extremely hot.
2. But there are so few of them that they cannot transfer much heat to a thermometer.
3. So the answer is presence of very few molecules or ions.

Teacher's Note:
1. Temperature measured by a thermometer depends on how many particles hit it.
2. The thermosphere is not completely empty.

Answer: (d) all of the above

Explanation:
1. Warm air above cold air stops convection, so water vapour is not carried up.
2. Dust particles needed for condensation are also absent.
3. All three statements together give the reason, so the answer is all of the above.

Teacher's Note:
1. Clouds need water vapour, dust and rising air currents.
2. The stratosphere lacks all of these.

Answer: (d) carbon dioxide, water vapour

Explanation:
1. Carbon dioxide and water vapour are the most plentiful greenhouse gases in the air.
2. Their increase raises the earth's temperature.
3. So the answer is carbon dioxide, water vapour.

Teacher's Note:
1. Choose the pair of the biggest contributors.
2. Water vapour is a greenhouse gas as well.

Answer: (c) aluminium

Explanation:
1. Aluminium reacts with oxygen to form a thin layer of \(\ce{Al2O3}\).
2. This layer is chemically inert and protects the metal below, making it passive.
3. So the answer is aluminium.

Teacher's Note:
1. Iron rust flakes off, but aluminium oxide sticks tightly.
2. This is why aluminium is not used for making hydrogen from acids.

Answer: (b) sulphur, sulphur dioxide

Explanation:
1. Sulphur burns with a bright blue flame to give \(\ce{SO2}\).
2. \(\ce{SO2}\) turns orange acidified \(\ce{K2Cr2O7}\) paper green because of chromium sulphate.
3. So the answer is sulphur, sulphur dioxide.

Teacher's Note:
1. Blue flame together with the dichromate test points to sulphur.
2. Phosphorus gives white fumes and sodium a yellow flame.

Answer: (d) usage of refrigerators and air conditioners

Explanation:
1. Industries, coal and wood all release smoke, soot and ash, which are particulate matter.
2. Refrigerators and air conditioners release CFCs, which are gases that harm ozone, not particles.
3. So the answer is usage of refrigerators and air conditioners.

Teacher's Note:
1. Particulate matter means solid or liquid particles floating in air.
2. CFCs affect the ozone layer.

Answer: (c) global warming

Explanation:
1. Incomplete burning releases CO, NO and similar gases, leading to smog and acid rain.
2. Global warming is mainly due to the excess of \(\ce{CO2}\) from complete burning.
3. So the answer is global warming.

Teacher's Note:
1. Be careful to read the word 'doesn't' in the question.
2. Option (d) is incorrect because both (a) and (b) do happen.

Answer: (d) both (a) and (b)

Explanation:
1. Root nodules of leguminous plants have rhizobium bacteria.
2. These bacteria change atmospheric nitrogen into usable nitrogen compounds.
3. Both (a) and (b) are correct, so the answer is both (a) and (b).

Teacher's Note:
1. Nitrogen fixation by bacteria is a natural source of fertiliser.
2. Pea, gram and beans are legumes.

Answer: (b) Y is neutral while X is acidic.

Explanation:
1. An acidic gas turns blue litmus red, so X is acidic.
2. A neutral gas leaves both litmus papers unchanged, so Y is neutral.
3. So the answer is Y is neutral while X is acidic.

Teacher's Note:
1. Acids turn blue litmus red; bases turn red litmus blue.
2. Oxygen is an example of a neutral gas and \(\ce{CO2}\) of an acidic gas.

Answer: (b) ozone

Explanation:
1. Ozone near the ground is a pollutant and adds to global warming.
2. In the stratosphere it absorbs harmful UV rays and protects life.
3. So the answer is ozone.

Teacher's Note:
1. Location decides whether ozone is harmful or helpful.
2. CFCs and nitrous oxide deplete the ozone layer.

Answer: (b) \(\ce{H2O2}\), \(\ce{O2}\) and \(\ce{NO2}\)

Explanation:
1. Oxygen is univalent in hydrogen peroxide. \(\ce{2H2O2 -> 2H2O + O2}\) gives Y = oxygen.
2. Oxygen oxidises the neutral oxide NO: \(\ce{2NO + O2 -> 2NO2}\); the reddish-brown fumes are \(\ce{NO2}\) (Z).
3. So the answer is \(\ce{H2O2}\), \(\ce{O2}\) and \(\ce{NO2}\).

Teacher's Note:
1. Reddish-brown fumes mean \(\ce{NO2}\).
2. Z is acidic because it turns moist blue litmus red.

Answer: (a) 2 1 3 5

Explanation:
1. −60°C to −15°C is the stratosphere (2).
2. 17°C to −51°C is the troposphere (1).
3. −15°C to −120°C is the mesosphere (3), and up to 2000°C is the thermosphere (5).
4. So the sequence is 2 1 3 5.

Teacher's Note:
1. Link every temperature range with its layer from the notes.
2. Keep the order of the ranges given in the question.

Answer: (a) A → b, B → d, C → a, D → c

Explanation:
1. Red lead oxide coating is used on bridges and electric poles: A → b.
2. Enamel paint is used on car and bus bodies: B → d.
3. Electroplating is used for bicycle handles and rims: C → a.
4. Alloying (stainless steel) is used for surgical instruments and cutlery: D → c.
5. So the answer is A → b, B → d, C → a, D → c.

Teacher's Note:
1. Think of where you have seen each article.
2. Match one pair at a time and cut down the options.

Answer: (a) the active component of air oxidizes the filament

Explanation:
1. Air contains oxygen, an active gas that would oxidise the hot filament.
2. Argon and nitrogen are unreactive.
3. So the answer is the active component of air oxidizes the filament.

Teacher's Note:
1. Inert gases protect the filament from burning.
2. Filaments are usually made of tungsten.

Answer: (a) 3 2 4 1

Explanation:
1. \(\ce{O2}\): made by heating (decomposing) \(\ce{H2O2}\), a binary compound of hydrogen: 3.
2. \(\ce{CO2}\): carbon burns with sparkles and crackling: 2.
3. \(\ce{SO2}\): sulphur burns with a brilliant blue flame: 4.
4. \(\ce{P2O5}\): phosphorus burns with dense white fumes: 1.
5. So the order is 3 2 4 1.

Teacher's Note:
1. Match each oxide with the burning observation you have learned.
2. Write down the pairs before choosing an option.

Answer: (a) deep-sea diving

Explanation:
1. High pressure under water makes more oxygen dissolve in the blood of a diver.
2. The other activities involve a shortage of oxygen.
3. So the answer is deep-sea diving.

Teacher's Note:
1. Divers carry oxygen mixed with helium.
2. Too much oxygen causes nausea and dizziness.

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

Explanation:
1. The oxyhydrogen flame uses hydrogen gas, but the oxyacetylene flame is made by burning acetylene in oxygen; hydrogen gas is not one of its components. So A is false.
2. Hydrogen does burn easily (it is inflammable), so R is true.
3. So the answer is (d): A is false but R is true.

Teacher's Note:
1. Check the actual components of each flame.
2. A statement can be true on its own and still not explain a false assertion.

Answer: (a) neutralization

Explanation:
1. Lime water is basic and \(\ce{SO2}\) is acidic: \(\ce{Ca(OH)2 + SO2 -> CaSO3 + H2O}\).
2. A base and an acid forming a salt and water is neutralisation.
3. So the answer is neutralization.

Teacher's Note:
1. A scrubber removes acidic gases using a base.
2. The salt formed is removed as slurry.

Answer: (b) S, \(\ce{SO2}\), \(\ce{CaSO3}\)

Explanation:
1. The first shell holds 2 electrons, the penultimate shell 8, so the M shell has 8 − 2 = 6 electrons: configuration 2, 8, 6, which is sulphur.
2. \(\ce{S + O2 -> SO2}\) and \(\ce{SO2 + Ca(OH)2 -> CaSO3 v + H2O}\).
3. So the answer is S, \(\ce{SO2}\), \(\ce{CaSO3}\).

Teacher's Note:
1. Because the valence shell is the M shell, carbon cannot be the answer.
2. The milky substance is insoluble calcium sulphite.

Answer: (c) Tin doesn't react with the components of food material.

Explanation:
1. Zinc is more reactive than tin and reacts with acids in food.
2. Tin does not react with the components of food.
3. So the answer is tin doesn't react with the components of food material.

Teacher's Note:
1. A food container must not react with its contents.
2. Remember: tin is less reactive than zinc.

Answer: (c) \(\ce{CaCl2}\)

Explanation:
1. Anhydrous calcium chloride is deliquescent: it absorbs moisture and forms a solution.
2. This solution keeps dust and particles from rising into the air.
3. So the answer is \(\ce{CaCl2}\).

Teacher's Note:
1. Deliquescent substances take moisture from the air and dissolve in it.
2. NaCl and CaO are not used for this purpose.

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

Explanation:
1. \(\ce{CO2}\) first turns lime water milky (\(\ce{CaCO3}\)), and with excess \(\ce{CO2}\), \(\ce{CaCO3 + H2O + CO2 -> Ca(HCO3)2}\) makes the milkiness disappear. So A is true.
2. Calcium bicarbonate dissolves in water, so R is true.
3. R explains why the solution becomes clear again, so the answer is (a).

Teacher's Note:
1. Both statements are linked by the same reaction.
2. The solid dissolves because it is converted into a soluble salt.

Answer: (b) downward displacement of water and upward displacement of air, respectively

Explanation:
1. Oxygen is collected over water by downward displacement of water.
2. \(\ce{CO2}\) is heavier than air and very soluble, so it is collected by upward displacement of air.
3. So the answer is option (b).

Teacher's Note:
1. Soluble, heavy gases are collected by upward displacement of air.
2. Oxygen is only slightly soluble.

Answer: (c) \(\ce{SO2}\), \(\ce{CO2}\)

Explanation:
1. \(\ce{SO2}\) causes acid rain, so it pollutes both air and water.
2. \(\ce{CO2}\) is needed by aquatic plants in water, but in air it causes global warming.
3. So the answer is \(\ce{SO2}\), \(\ce{CO2}\).

Teacher's Note:
1. B must be a gas that is useful in water: that is carbon dioxide.
2. Both gases come from burning fossil fuels.

Answer: (b) air conditioners

Explanation:
1. CFCs attack ozone, and CFCs come from refrigerators and air conditioners.
2. So the answer is air conditioners.

Teacher's Note:
1. Ozone holes are caused by chlorine from CFCs.
2. The ozonosphere is part of the stratosphere.

Answer: (b) Y is neutral while X is acidic

Explanation:
1. An acidic gas turns blue litmus red, so X is acidic.
2. A neutral gas does not change either litmus, so Y is neutral.
3. So the answer is Y is neutral while X is acidic.

Teacher's Note:
1. Remember what each litmus colour change tells you.
2. Examples: \(\ce{CO2}\) is acidic and \(\ce{O2}\) is neutral.

Answer: (b) 3 2 4 5 1

Explanation:
1. Meteorites burn in the mesosphere (3).
2. Aeroplanes fly in the stratosphere (2).
3. Radio waves reflect from the ionosphere (4).
4. Harmful ionising radiation is absorbed in the exosphere (5).
5. Weather occurs in the troposphere (1).
6. So the sequence is 3 2 4 5 1.

Teacher's Note:
1. Take each activity one at a time and find its layer.
2. Write the numbers in the order of the activities.

Answer: (b) A → d, B → c, C → b, D → a

Explanation:
1. Tinned iron is used for food containers: A → d.
2. Enamelling is used for wash basins and ovens: B → c.
3. Oil or grease is applied to engine parts: C → b.
4. Galvanised iron is used for trunks and sanitary pipes: D → a.
5. So the answer is A → d, B → c, C → b, D → a.

Teacher's Note:
1. Link each method with the article you know.
2. Moving parts cannot be painted, so they are oiled.

Answer: (b) Mg

Explanation:
1. Magnesium is an active metal that reacts with nitrogen and keeps burning.
2. Fe, Cu and W do not burn in nitrogen.
3. So the answer is Mg.

Teacher's Note:
1. Only very reactive metals burn in nitrogen.
2. Magnesium gives magnesium nitride.

Answer: (b) 2 3 4 1

Explanation:
1. \(\ce{P2O5}\) dissolves in water to give phosphoric acid, a tribasic acid: 2.
2. \(\ce{CO2}\) does not support combustion but active metals still burn in it: 3.
3. \(\ce{O2}\) turns alkaline pyrogallol brown: 4.
4. \(\ce{SO2}\) turns acidified potassium dichromate green: 1.
5. So the order is 2 3 4 1.

Teacher's Note:
1. Match each substance with its test from the notes.
2. Phosphoric acid has three replaceable hydrogens.

Answer: (a) low atmospheric pressure

Explanation:
1. The atmospheric pressure outside falls as we go up.
2. The pressure inside the pen is higher, so ink is pushed out.
3. So the answer is low atmospheric pressure.

Teacher's Note:
1. Think about the difference of pressure across the pen.
2. Temperature plays no key role here.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Fuel gases like water gas and coal gas contain hydrogen because hydrogen gives a very high heat of combustion. So A is true.
2. Liquid hydrogen is indeed a very light fuel, so R is true.
3. But lightness is not the reason hydrogen is used in fuel gases, so R does not explain A. The answer is (b).

Teacher's Note:
1. The correct explanation for A is the high heat of combustion of hydrogen.
2. A true reason must connect directly to the assertion.

Answer: (d) death of aquatic animals

Explanation:
1. Acid rain acidifies rivers, lakes and seas and kills the aquatic life.
2. The other effects are due to global warming and ozone depletion.
3. So the answer is death of aquatic animals.

Teacher's Note:
1. Connect each hazard with its own effects.
2. Acid rain also eats away marble monuments.

Answer: (b) sulphur, sulphur dioxide

Explanation:
1. Sulphur burns with a bright blue flame and forms \(\ce{SO2}\).
2. \(\ce{SO2}\) turns acidified dichromate paper green.
3. So the answer is sulphur, sulphur dioxide.

Teacher's Note:
1. The blue flame and green paper are both signs of sulphur and its dioxide.
2. Phosphorus burns with white fumes.

Answer: (c) aluminium

Explanation:
1. Aluminium forms a thin, tough film of \(\ce{Al2O3}\) on its surface.
2. This film stops further reaction and makes the metal passive.
3. So the answer is aluminium.

Teacher's Note:
1. The oxide layer of aluminium sticks firmly.
2. Iron rust flakes off and exposes fresh metal.

Answer: (d) usage of refrigerators and air conditioners

Explanation:
1. Industries, coal and wood produce soot, ash and smoke particles.
2. Refrigerators and air conditioners release CFCs, which are gases.
3. So the answer is usage of refrigerators and air conditioners.

Teacher's Note:
1. Particulate matter is made of tiny solid or liquid particles.
2. CFCs damage the ozone layer rather than adding particles.

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

Explanation:
1. \(\ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O}\): the milkiness is due to calcium carbonate, so A is true.
2. Calcium carbonate is insoluble in water; that is why it makes the water milky. So R is false.
3. So the answer is (c): A is true but R is false.

Teacher's Note:
1. A precipitate forms only because the solid is insoluble.
2. This is the standard laboratory test for carbon dioxide.

Answer: (c) It is heavier than air and it is highly soluble in water.

Explanation:
1. Collection by upward displacement of air means the gas is heavier than air.
2. It cannot be collected over water, so it dissolves in water.
3. So the answer is heavier than air and highly soluble in water.

Teacher's Note:
1. Carbon dioxide is a good example of such a gas.
2. Insoluble gases can be collected over water.

Answer: (d) carbon dioxide, water vapour

Explanation:
1. \(\ce{CO2}\) and water vapour are the most abundant greenhouse gases.
2. Their rise in the air increases the earth's temperature.
3. So the answer is carbon dioxide, water vapour.

Teacher's Note:
1. Choose the pair with the biggest overall effect.
2. These two gases trap heat in the air.