CBSE Class 7 Science Advanced Chemistry Air and Oxygen MCQs Set 01

Answer: (b) decreases

Explanation:
1. Air gets thinner and less dense as we go up, so there is less air above to press down.
2. Therefore the atmospheric pressure decreases with height.

Teacher's Note:
1. Pressure is greatest at sea level (760 mm of Hg).
2. Picture the weight of the column of air above you: it becomes smaller as you climb.

Answer: (b) troposphere

Explanation:
1. In the troposphere the temperature falls from about 17°C to about −51°C as we rise, at about 6°C per km.
2. In the ionosphere and the stratosphere (ozonosphere) the temperature rises with height.
3. So the answer is the troposphere.

Teacher's Note:
1. Remember the pattern of temperature: falls, rises, falls, rises, from the troposphere upwards.
2. The troposphere is the only one of the lower layers where the temperature keeps falling.

Answer: (c) sulphur dioxide

Explanation:
1. Greenhouse gases include \(\ce{CO2}\), water vapour, \(\ce{N2O}\), \(\ce{CH4}\), \(\ce{O3}\) and CFCs.
2. Sulphur dioxide causes acid rain and breathing trouble but is not a greenhouse gas.
3. So the answer is sulphur dioxide.

Teacher's Note:
1. Learn the list of greenhouse gases once; every such question becomes easy.
2. \(\ce{SO2}\) is linked with acid rain, not with global warming.

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

Explanation:
1. Oxygen makes up nearly half of the earth's crust, mostly in silicates, carbonates and oxides.
2. So the answer is oxygen (option c).

Teacher's Note:
1. Silicon is the second most plentiful element, but oxygen comes first.
2. Oxygen is present in the crust mainly in combined form.

Answer: (a) alloying

Explanation:
1. Mixing a molten metal with other metals or non-metals to get a uniform mixture is alloying.
2. The other three are coating processes (chromium or nickel, tin and zinc).
3. So the answer is alloying.

Teacher's Note:
1. Stainless steel is an alloy of iron with carbon and chromium.
2. The word alloy always means a homogeneous mixture.

Answer: (c) \(\ce{H2O2}\)

Explanation:
1. \(\ce{KNO3}\) decomposes slowly and needs strong heating; \(\ce{HgO}\) gives poisonous mercury vapour and is costly.
2. Hydrogen peroxide with \(\ce{MnO2}\) gives oxygen quickly at room temperature.
3. So the answer is \(\ce{H2O2}\).

Teacher's Note:
1. Choose the method that is safe, cheap and works at room temperature.
2. \(\ce{MnO2}\) acts as the catalyst here.

Answer: (b) liquid oxygen

Explanation:
1. Liquid hydrogen is the fuel; it needs something that helps it burn.
2. Oxygen supports combustion, so liquid oxygen is used.

Teacher's Note:
1. Separate the fuel from the supporter of combustion.
2. Carbon dioxide cannot support burning, so it is never used for this.

Answer: (b) magnesium

Explanation:
1. Active metals such as Mg, Na and K take oxygen from \(\ce{CO2}\) and go on burning: \(\ce{2Mg + CO2 -> 2MgO + C}\).
2. Coke, sulphur and phosphorus do not.
3. So the answer is magnesium.

Teacher's Note:
1. Burning magnesium is so hot that it breaks up carbon dioxide.
2. This is why a \(\ce{CO2}\) extinguisher cannot be used on a magnesium fire.

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

Explanation:
1. Carbon dioxide is dissolved in soft drinks and soda water under high pressure.
2. It gives the sharp taste and the fizz.
3. So the answer is \(\ce{CO2}\).

Teacher's Note:
1. The gas escapes as bubbles when the bottle is opened because the pressure falls.
2. Solubility of \(\ce{CO2}\) increases with pressure.

Answer: (b) dry ice

Explanation:
1. Solid carbon dioxide, called dry ice, is very cold and leaves no liquid behind.
2. It is used to keep fish, meat and fruits fresh.
3. So the answer is dry ice.

Teacher's Note:
1. Dry ice turns straight into gas, so the food stays dry.
2. CFCs are refrigerants in machines, but they harm the ozone layer.

Answer: (a) deep-sea diving

Explanation:
1. Under the high pressure deep in the sea, more oxygen dissolves in the blood.
2. Mountaineering, flying and moon walks involve less oxygen, not more.
3. So the answer is deep-sea diving.

Teacher's Note:
1. That is why divers carry oxygen diluted with an inert gas such as helium.
2. Too much oxygen can cause nausea and dizziness.

Answer: (b) ozone

Explanation:
1. Ozone in the troposphere 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. The same gas can be harmful or helpful depending on where it is.
2. CFCs destroy the protective ozone layer.

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

Explanation:
1. Oxygen is only fairly soluble and is collected over water by downward displacement of water.
2. Carbon dioxide is very soluble and heavier than air, so it is collected by upward displacement of air.
3. So the answer is option (b).

Teacher's Note:
1. Soluble and heavy gases are collected by upward displacement of air.
2. Gases that are not very soluble are collected over water.

Answer: (d) formation of auroras

Explanation:
1. Acid rain, lightning with rain and bacteria in root nodules all produce nitrates in the soil.
2. Auroras are coloured lights caused by electrons from the sun striking gas molecules; they add no nitrates to the soil.
3. So the answer is formation of auroras.

Teacher's Note:
1. Auroras belong to the thermosphere, far above the soil.
2. Think of how nitrogen gets converted into nitrates to spot the odd one out.

Answer: (d) 45312

Explanation:
1. From top to bottom the layers are: exosphere (4), thermosphere or ionosphere (5), mesosphere (3), stratosphere (1) and troposphere (2).
2. So the order is 4 5 3 1 2.

Teacher's Note:
1. Start from the ground to remember: troposphere, stratosphere, mesosphere, thermosphere, exosphere.
2. Then simply reverse the list for decreasing altitude.

Answer: (a) 2431

Explanation:
1. Percentages by volume: nitrogen 78.084, oxygen 20.947, argon 0.934, carbon dioxide 0.033.
2. Increasing order: \(\ce{CO2}\) (2), argon (4), oxygen (3), nitrogen (1).
3. So the order is 2 4 3 1.

Teacher's Note:
1. Read the table of composition of air carefully before arranging.
2. Increasing means smallest first.

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

Explanation:
1. Nitrogen is the inactive part of air and weakens the action of oxygen, the active part.
2. So the answer is \(\ce{O2}\).

Teacher's Note:
1. Without nitrogen, burning and rusting would be much faster.
2. Oxygen is the active component of air.

Answer: (d) nitrogen dioxide

Explanation:
1. Methane, carbon dioxide and water vapour are greenhouse gases.
2. Nitrogen dioxide is not in the list of greenhouse gases; it causes acid rain and smog.
3. So the answer is nitrogen dioxide.

Teacher's Note:
1. Greenhouse gases: \(\ce{CO2}\), water vapour, \(\ce{N2O}\), \(\ce{CH4}\), \(\ce{O3}\) and CFCs.
2. Note that \(\ce{N2O}\) (nitrous oxide) is a greenhouse gas but \(\ce{NO2}\) is not on the list.

Answer: (a) Na

Explanation:
1. Sodium burns with a golden-yellow flame, magnesium with a dazzling white flame.
2. So the answer is Na.

Teacher's Note:
1. Flame colour is a quick way to recognise elements.
2. Sodium gives the golden-yellow colour of common salt flames.

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

Explanation:
1. Carbon dioxide absorbs infrared radiation and traps heat.
2. More \(\ce{CO2}\) in the air raises the earth's temperature.
3. So the answer is \(\ce{CO2}\).

Teacher's Note:
1. CO is poisonous and \(\ce{SO2}\) causes acid rain; neither is the main cause of warming.
2. \(\ce{N2}\) is an inert gas that has no heating effect.

Answer: (c) iron

Explanation:
1. Iron and steam give \(\ce{Fe3O4}\) and hydrogen, and the reaction can go both ways: \(\ce{3Fe + 4H2O <=> Fe3O4 + 4H2}\).
2. So the answer is iron.

Teacher's Note:
1. This is the reason iron is not used to prepare hydrogen from acid in the laboratory.
2. Reversible means products can change back into reactants.

Answer: (a) neon

Explanation:
1. Neon glows with a bright colour when electricity passes through it.
2. So the answer is neon.

Teacher's Note:
1. Helium is used for balloons and argon for bulbs.
2. Match each inert gas with its use.

Answer: (b) Pascal

Explanation:
1. Pressure is force per unit area, and the SI unit of pressure is the pascal (Pa).
2. So the answer is Pascal.

Teacher's Note:
1. Newton is the unit of force and joule the unit of energy.
2. 1 atm = 760 mm of Hg.

Answer: (d) oxygen

Explanation:
1. Lavoisier believed oxygen to be a part of every acid and named it oxygen, meaning acid former.
2. So the answer is oxygen.

Teacher's Note:
1. The name comes from Greek words meaning acid and producing.
2. Hydrogen means water former.

Answer: (b) stratosphere

Explanation:
1. The ozone layer lies between about 25 km and 40 km, inside the stratosphere.
2. So the answer is stratosphere.

Teacher's Note:
1. The stratosphere is also called the ozonosphere.
2. Ozone near the ground, in the troposphere, is a pollutant.

Answer: (d) chromium

Explanation:
1. Stainless steel is iron alloyed with carbon and 12% to 20% chromium.
2. Zinc and tin are used for coating, not for alloying.
3. So the answer is chromium.

Teacher's Note:
1. Alloying changes the whole metal, while coating covers only the surface.
2. Stainless steel is used for utensils and surgical tools.

Answer: (a) low atmospheric pressure

Explanation:
1. At higher altitudes the outside atmospheric pressure is lower.
2. The air pressure inside the pen is higher than outside, so ink is pushed out.
3. So the answer is low atmospheric pressure.

Teacher's Note:
1. High temperature is not the reason; temperature is usually lower on mountains.
2. Pressure difference between inside and outside causes the leak.

Answer: (b) air conditioners

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

Teacher's Note:
1. CFC stands for chlorofluorocarbons.
2. Substitutes for CFCs are used to save the ozone layer.

Answer: (d) death of aquatic animals

Explanation:
1. Acid rain makes rivers and seas acidic, which kills aquatic life.
2. Floods and unseasonal rain are effects of global warming; UV damage comes from ozone depletion.
3. So the answer is death of aquatic animals.

Teacher's Note:
1. Match each pollution hazard with its own effects.
2. Acid rain also damages marble monuments and soil.

Answer: (d) W

Explanation:
1. Active metals Na, Mg and K combine with nitrogen and keep burning.
2. Tungsten (W) does not react with nitrogen, so it stops burning.
3. So the answer is W.

Teacher's Note:
1. Burning in nitrogen needs a very reactive metal.
2. Magnesium forms magnesium nitride in nitrogen.

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

Explanation:
1. Upward displacement of air means the gas is heavier than air.
2. It cannot be collected over water, so it must dissolve well in water.
3. So the answer is heavier than air and highly soluble.

Teacher's Note:
1. Insoluble gases are collected over water; soluble gases are collected over air.
2. Heavy gases go by upward displacement of air; light gases by downward displacement.

Answer: (c) 21534

Explanation:
1. Inversion layer: stratosphere (2). Convection currents: troposphere (1). Auroras: ionosphere (5).
2. Absorbs ionising radiation: exosphere (3). Meteors burn: mesosphere (4).
3. So the sequence is 2 1 5 3 4.

Teacher's Note:
1. Link each phrase with the layer in the notes one by one.
2. Write the numbers in the same order as the phrases.

Answer: (b) 3241

Explanation:
1. Lightning first makes nitric oxide: \(\ce{N2 + O2 -> 2NO}\) (3).
2. Then nitrogen dioxide forms: \(\ce{2NO + O2 -> 2NO2}\) (2).
3. Then nitric acid forms in water droplets (4), and finally it reacts with carbonates in soil to give nitrates (1).
4. So the order is 3 2 4 1.

Teacher's Note:
1. Follow the chain of reactions: \(\ce{N2}\) to NO to \(\ce{NO2}\) to \(\ce{HNO3}\) to nitrates.
2. Each step needs the product of the earlier step.

Answer: (a) relatively thicker atmosphere in the mesosphere.

Explanation:
1. Meteorites burn because of friction with air molecules.
2. The mesosphere has a denser atmosphere than the thermosphere, so friction there is enough to burn them.
3. So the answer is relatively thicker atmosphere in the mesosphere.

Teacher's Note:
1. The thermosphere is so thin that there are too few molecules to cause burning.
2. Shooting stars are meteors glowing as they burn.

Answer: (d) of the absence of convection current.

Explanation:
1. In the stratosphere the temperature rises with height, so warm air lies above cold air and no convection currents form.
2. With no turbulence, flight is smooth.
3. So the answer is absence of convection currents.

Teacher's Note:
1. The troposphere has strong weather and air currents that make flying bumpy.
2. Temperature in the stratosphere increases, not decreases, with altitude.

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

Explanation:
1. \(\ce{SO2}\) pollutes air and, through acid rain, water as well, so A can be \(\ce{SO2}\) (or \(\ce{NO2}\)).
2. \(\ce{CO2}\) is needed in water by aquatic plants, but in air it causes global warming, so B is \(\ce{CO2}\).
3. So the answer is \(\ce{SO2}\), \(\ce{CO2}\).

Teacher's Note:
1. Look for the gas that is harmful only in air: that is carbon dioxide.
2. Only option (c) has \(\ce{CO2}\) in the second place as B.

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, spoiling it.
2. Tin does not react with the contents of food.
3. So the answer is tin does not react with food components.

Teacher's Note:
1. Food containers must be chemically safe.
2. Tin is less reactive than zinc.

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

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

Teacher's Note:
1. Use the electronic configuration to identify the element.
2. The milky precipitate is the insoluble salt, not the soluble hydrogen salt.

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

Explanation:
1. Marble (\(\ce{CaCO3}\)) with dilute HCl gives \(\ce{CaCl2}\), \(\ce{CO2}\) and water.
2. Anhydrous calcium chloride absorbs moisture and forms a solution, which keeps dust and particles from rising.
3. So Y is \(\ce{CaCl2}\).

Teacher's Note:
1. The question asks about the product that settles dust, not the gas.
2. \(\ce{CaCl2}\) is a drying agent that is deliquescent.

Answer: (a) neutralization

Explanation:
1. Lime is a base and the pollutants \(\ce{SO2}\) and \(\ce{NO2}\) are acidic oxides.
2. \(\ce{CaO + SO2 -> CaSO3}\) is a base reacting with an acid, which is neutralisation.
3. So the answer is neutralization.

Teacher's Note:
1. Base plus acid always means neutralisation.
2. The salt formed is removed as slurry.

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

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

Teacher's Note:
1. The hot tungsten filament would burn away in air.
2. Inert gases keep the filament safe.

Answer: (b) The cloud formation takes place in upper layers while mist formation takes place in lower layers of the troposphere.

Explanation:
1. Mist forms near the ground, in the lower part of the troposphere.
2. Clouds form higher up in the troposphere, and they form throughout the year.
3. So the answer is cloud in upper layers, mist in lower layers.

Teacher's Note:
1. Mist is like a cloud that forms at ground level.
2. Compare the heights at which condensation takes place.