CBSE Class 7 Science Advanced Chemistry Air and Oxygen Long Answer Questions Set 01

Long Answer Questions

Question 1. Describe the laboratory preparation of oxygen gas from hydrogen peroxide with a neat diagram.
Answer:
Principle: Hydrogen peroxide decomposes into water and oxygen at room temperature in the presence of manganese dioxide, which acts as a catalyst (it speeds up the reaction and lets it work at room temperature). \(\ce{2H2O2 ->[MnO2] 2H2O + O2 ^}\)

Procedure: Place \(\ce{MnO2}\) in a clean, dry conical flask closed with a stopper that has a thistle funnel and a delivery tube. Add \(\ce{H2O2}\) solution drop by drop through the thistle funnel.
Observation: Oxygen bubbles pass through the delivery tube into a gas jar filled with water and inverted over a beehive shelf in a water trough. The gas relights a glowing splinter.
Collection: By downward displacement of water.
Precautions: (i) Avoid heating the mixture. (ii) Use pure manganese dioxide.
In simple words: Hydrogen peroxide with a little \(\ce{MnO2}\) gives oxygen, which is collected over water.

Teacher's Note:
1. Draw and label the diagram: thistle funnel, conical flask, delivery tube, beehive shelf, gas jar and trough.
2. The glowing splinter test confirms oxygen.

Question 2. Mention the general methods of preparation of \(\ce{CO2}\) along with the chemical reactions.
Answer:
1. Burning of carbon fuels: \(\ce{C + O2 -> CO2 ^ + heat}\); \(\ce{CH4 + 2O2 -> CO2 ^ + 2H2O + heat}\)
2. Heating metal carbonates (except sodium and potassium carbonates): \(\ce{CaCO3 ->[\Delta] CaO + CO2 ^}\); \(\ce{CuCO3 ->[\Delta] CuO + CO2 ^}\); \(\ce{ZnCO3 ->[\Delta] ZnO + CO2 ^}\)
3. Heating metal hydrogen carbonates: \(\ce{2NaHCO3 -> Na2CO3 + H2O + CO2 ^}\)
4. Action of dilute acids on carbonates and hydrogen carbonates: \(\ce{Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2 ^}\); \(\ce{Ca(HCO3)2 + 2HNO3 -> Ca(NO3)2 + 2H2O + 2CO2 ^}\)
In simple words: Carbon dioxide can be made by burning carbon fuels, heating carbonates or hydrogen carbonates, or treating them with acids.

Teacher's Note:
1. Learn one equation under each method.
2. Sodium and potassium carbonates do not give \(\ce{CO2}\) on heating.

Question 3. Write a short note on rusting. What are the causes and effects of it? Explain different methods adopted to prevent rusting.
Answer:
Rusting: Iron in humid air slowly changes into a brown, flaky hydrated oxide called rust. \(\ce{4Fe + 3O2 -> 2Fe2O3}\); \(\ce{Fe2O3 + xH2O -> Fe2O3.xH2O}\) (rust)
Causes: exposure of iron to oxygen and moisture (humid air). Iron does not rust in dry air.
Effects: Rust crumbles away from the surface and weakens iron structures such as bridges, rails, machinery and household goods.
Prevention:
1. Coating: red lead oxide paint or tar on bridges and poles; enamel paint on doors, windows and vehicles; enamelling (a silicate coating) on cooker bodies, refrigerators, wash basins and ovens; oil or grease on moving machine parts.
2. Galvanisation: coating iron with zinc (buckets, tubs, trunks).
3. Tinning: coating with tin (food containers).
4. Electroplating: coating with chromium or nickel by electrolysis (bicycle handles, rims, bumpers).
5. Alloying: iron with carbon and chromium gives stainless steel (surgical tools, utensils, cutlery).
In simple words: Iron rusts in damp air; coating it with paint, zinc, tin, chromium or alloying it with chromium protects it.

Teacher's Note:
1. Write the formula of rust and the five prevention methods.
2. The common idea is to keep air and moisture away from the iron.

Question 4. Explain the significance of nitrogen, oxygen and water vapour present in the atmosphere.
Answer:
Nitrogen is the inactive part of air (about 78%). It dilutes the activity of oxygen, so that burning is not too fast. It is used to make ammonia and fertilisers, to preserve food, and plants and animals need it for making proteins (through nitrogen fixation).
Oxygen is the active part (about 21%). It is needed for respiration and combustion and so supports life. It is also used in cutting and welding, rockets, medicine and industries.
Water vapour with dust particles forms fog, mist and clouds and brings rain; it also acts as a greenhouse gas that keeps the earth warm.
In simple words: Nitrogen controls the action of oxygen, oxygen supports breathing and burning, and water vapour gives clouds and rain.

Teacher's Note:
1. Write two or three points for each gas.
2. Note the contrast between the inactive nitrogen and the active oxygen.

Question 5. Describe the laboratory method of preparation of hydrogen gas.
Answer:
Principle: A metal above hydrogen in the reactivity series gives hydrogen with dilute \(\ce{HCl}\) or dilute \(\ce{H2SO4}\). Zinc is preferred. \(\ce{Zn + H2SO4 -> ZnSO4 + H2 ^}\)

Procedure: Put granulated zinc in a flat-bottomed flask closed with an air-tight cork having two holes, with a thistle funnel in one and a delivery tube in the other. Add dilute sulphuric acid dropwise through the funnel.
Observation: Bubbles appear at first, then a colourless, odourless gas is given off.
Collection: By downward displacement of water.
Purification: Impurities such as \(\ce{H2S}\), \(\ce{SO2}\), \(\ce{PH3}\), \(\ce{AsH3}\) and \(\ce{CO2}\) (from impure zinc) are removed by passing the gas through silver nitrate solution, lead nitrate solution, caustic potash solution and then anhydrous \(\ce{CaCl2}\) (to dry it). Pure hydrogen is collected over mercury.
Precautions: air-tight apparatus, no flame nearby, collect the gas only after the air is expelled, and keep the funnel end almost at the bottom.
In simple words: Zinc with dilute sulphuric acid gives hydrogen, which is purified and collected over water.

Teacher's Note:
1. Zinc is chosen because the other metals are too violent, costly or unsuitable.
2. Draw the diagram neatly with labels.

Question 6. Describe the chemical reaction of \(\ce{CO2}\) with
(a) lime water
(b) metals like sodium, potassium and calcium
(c) nonmetals like carbon
Answer:
(a) Lime water: It forms a milky white precipitate of calcium carbonate. \(\ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O}\) With excess \(\ce{CO2}\) the precipitate dissolves, because soluble calcium hydrogen carbonate forms. \(\ce{CaCO3 + H2O + CO2 -> Ca(HCO3)2}\)
(b) Active metals: Sodium, potassium and calcium react with \(\ce{CO2}\) to form the metal carbonate and carbon. \(\ce{4Na + 3CO2 -> 2Na2CO3 + C}\) (Magnesium and aluminium give their oxides: \(\ce{2Mg + CO2 -> 2MgO + C}\).)
(c) Carbon: Over red-hot coke, \(\ce{CO2}\) is reduced to carbon monoxide, a very poisonous gas. \(\ce{C + CO2 ->[\Delta] 2CO}\)
In simple words: Carbon dioxide turns lime water milky, is reduced by active metals and by red-hot carbon.

Teacher's Note:
1. Do not forget the excess \(\ce{CO2}\) change in (a).
2. Carbon monoxide is poisonous.

Question 7. Write a short note on the following layers of atmosphere.
(a) troposphere
(b) stratosphere
Answer:
(a) Troposphere: The lowest layer, from the ground up to about 20 km. It holds most of the mass of the atmosphere. Air density and temperature decrease with height (from about 17°C to −51°C, a drop of 6°C per km, the lapse rate). Almost all weather events occur here. Its thickness varies from equator to poles.
(b) Stratosphere: The layer above the troposphere, up to about 50 km, holding about 19% of the gases. The temperature is nearly constant up to 25 km and then rises (from −60°C to −15°C) as UV radiation is absorbed (inversion layer). The ozone layer (25 km to 40 km) absorbs harmful UV rays and protects life. Aeroplanes fly in this layer.
In simple words: The troposphere is the weather layer where temperature falls with height; the stratosphere has the ozone layer and warms with height.

Teacher's Note:
1. Compare the temperature behaviour of the two layers.
2. Mention what happens in each layer: weather and flying.

Question 8. (a) What is air pollution?
(b) Write a short note on
(i) ozone depletion and
(ii) hazards of carbon monoxide
Answer:
(a) Air pollution: The change in the composition of air due to the release of harmful gases and particles, mainly by human activities, is called air pollution. A substance added to air that harms people and surroundings is an air pollutant (for example CO, CFCs and tropospheric ozone).
(b)(i) Ozone depletion: Stratospheric ozone absorbs harmful UV rays. CFCs reaching the stratosphere react with ozone and thin the layer. UV rays then reach the lower layers and harm plants and animals (skin cancer, cataract and crop damage). Prevention: use substitutes for CFCs in refrigeration and aerosols.
(ii) Hazards of carbon monoxide: CO combines with haemoglobin of blood about 200 times more strongly than oxygen and forms stable carboxyhaemoglobin. Oxygen supply to the body falls, causing suffocation, and it can be fatal. Prevention: burn less fossil fuel and avoid smoking.
In simple words: Air pollution means harmful additions to air; CFCs thin the ozone layer and CO blocks oxygen in the blood.

Teacher's Note:
1. Give a definition first, then the notes in order.
2. Carboxyhaemoglobin is the key word for CO.

Teacher's Note:
1. The gas passes through the four substances in this order.
2. Acidic gases are removed by a base such as caustic potash.

Question 10. Name the pollutants responsible for the following effects.
(a) reduction in intake of oxygen
(b) cause of acid rain
(c) greenhouse effect
(d) formation of smog
(e) depletion of ozone layer
(f) problems of digestion
(g) contamination of ground water
(h) respiratory disorders
Answer:
(a) Carbon monoxide.
(b) Sulphur dioxide and nitrogen oxides.
(c) Carbon dioxide (also methane, nitrous oxide, ozone and CFCs).
(d) Nitrogen oxides and suspended particulate matter.
(e) Chlorofluorocarbons (CFCs).
(f) Metals such as lead, mercury and arsenic.
(g) Fly ash.
(h) Sulphur dioxide and suspended particulate matter (also carbon monoxide).
In simple words: Each effect is linked with a particular pollutant; learn the table of pollutants.

Teacher's Note:
1. Revise the pollutant table from the notes once.
2. Some effects have more than one pollutant.

Question 11. X and Y are gases at room temperature. X is less reactive and is used to preserve food material, while "Y" is required for combustion. Metal "M" gets corroded when it combines with "Y" in the presence of "Z." An atom of "M" possesses 14 electrons in the second last shell and N is the last shell. Identify X, Y, Z and M.
Answer:
X is nitrogen: it is unreactive and is used in air-tight food packets to prevent oxidation.
Y is oxygen, which is required for combustion.
The metal M has 14 electrons in the M shell (second last) and the N shell as the last shell. The configuration 2, 8, 14, 2 is that of iron (Fe).
Iron is corroded by oxygen in the presence of water vapour (Z), forming rust. \(\ce{4Fe + 3O2 -> 2Fe2O3}\); \(\ce{Fe2O3 + xH2O -> Fe2O3.xH2O}\)
So X = nitrogen, Y = oxygen, Z = water vapour and M = Fe.
In simple words: X is nitrogen, Y is oxygen, Z is water vapour and M is iron, which rusts.

Teacher's Note:
1. Use the electronic configuration 2, 8, 14, 2 to find iron (26 electrons).
2. Rusting needs both oxygen and moisture.

Question 12. The compounds X and Y combine to form a compound Z. "Z" which is used for construction of monuments gets affected by acid rain. The compound X is used in white washing. Identify X, Y and Z and write the corresponding balanced chemical equations.
Answer:
X is used in whitewashing, so X is calcium hydroxide, \(\ce{Ca(OH)2}\) (slaked lime).
Y is carbon dioxide, \(\ce{CO2}\), and Z is calcium carbonate, \(\ce{CaCO3}\) (marble and limestone are used for monuments).
\(\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}\)
Acid rain attacks Z: \(\ce{CaCO3 + H2SO4 -> CaSO4 + H2O + CO2 ^}\). With excess \(\ce{CO2}\) and water, calcium carbonate also dissolves: \(\ce{CaCO3 + H2O + CO2 -> Ca(HCO3)2}\).
In simple words: Slaked lime plus carbon dioxide gives calcium carbonate, which acid rain eats away.

Teacher's Note:
1. Whitewashing material is slaked lime.
2. Marble is chemically calcium carbonate.

Question 13. A combines with B to form C and D. A and D are major contributors for global warming and both are carbon compounds. B is a supporter of combustion. Magnesium burns in the presence of B with a dazzling white flame and forms a product X which is basic in nature. With C, X forms a corresponding compound which turns red litmus paper to blue. Give the respective chemical reactions involved.
Answer:
A is methane, B is oxygen, C is water (vapour) and D is carbon dioxide. Methane and carbon dioxide are carbon compounds that cause the greenhouse effect.
\(\ce{CH4 + 2O2 -> CO2 + 2H2O}\)
Magnesium burns in oxygen with a dazzling white flame to form magnesium oxide (X), which is basic: \(\ce{2Mg + O2 -> 2MgO}\)
\(\ce{MgO + H2O -> Mg(OH)2}\)
Magnesium hydroxide is a base and turns red litmus blue.
In simple words: Methane burns in oxygen to give carbon dioxide and water; magnesium oxide and water give a base.

Teacher's Note:
1. Treat each clue separately to identify the substances.
2. Write the equations in the order of the story.

Question 14. A, B, C and D are four layers of atmosphere. A helps in long distance radio communications. In layer B most of weather phenomena take place. The pollutants released from air conditioners and aerosols deplete layer "D" which also gets depleted by nitrous oxide. Meteorites burn up in layer C. Identify the layers A, B, C and D and arrange them in an increasing order of height from the surface of earth.
Answer:
A = ionosphere (thermosphere): radio waves are reflected here.
B = troposphere: most weather phenomena occur here.
C = mesosphere: meteorites burn up here.
D = stratosphere: its ozone layer is depleted by CFCs (from air conditioners and aerosols) and by nitrous oxide.
Increasing order of height: B < D < C < A (troposphere, stratosphere, mesosphere, ionosphere).
In simple words: From the ground up: troposphere B, stratosphere D, mesosphere C, ionosphere A.

Teacher's Note:
1. Match each clue to a layer, then arrange by height.
2. Remember the order T, S, M, T from the ground upwards.

Question 15. The element "X" has only three shells in which the difference in number of electrons between the last shell and the second last shell is equal to the number of electrons in the first shell. This element "X" burns in oxygen to produce compound Y, which turns acidified potassium dichromate paper to green. Y turns moist blue litmus paper to red due to the formation of compound Z. Identify X, Y and Z.
Answer:
The first shell has 2 electrons; the second shell has 8. So the last (third) shell has 8 − 2 = 6 electrons, giving the configuration 2, 8, 6. X is sulphur.
Sulphur burns in oxygen to form sulphur dioxide (Y): \(\ce{S + O2 -> SO2}\). It turns acidified \(\ce{K2Cr2O7}\) paper green: \(\ce{3SO2 + K2Cr2O7 + H2SO4 -> K2SO4 + Cr2(SO4)3 + H2O}\)
\(\ce{SO2}\) dissolves in the moisture on litmus to form sulphurous acid, \(\ce{H2SO3}\) (Z), which turns blue litmus red: \(\ce{SO2 + H2O -> H2SO3}\)
In simple words: X is sulphur, Y is sulphur dioxide and Z is sulphurous acid.

Teacher's Note:
1. Use the shell clue to find the configuration 2, 8, 6.
2. The green dichromate paper is the test for \(\ce{SO2}\).

Question 16. Ozone present in the troposphere is a major pollutant. However, ozone formation in aqua purifiers makes drinking water pure and safe. Explain.
Answer:
Aqua purifiers have a source of UV rays. UV radiation converts oxygen into ozone: \(\ce{3O2 -> 2O3}\).
Ozone then breaks into oxygen and nascent (atomic) oxygen: \(\ce{O3 -> O2 + O}\). The nascent oxygen is a strong oxidising agent and kills harmful microorganisms in the water, so the water becomes pure and safe to drink.
In the troposphere, however, ozone is harmful to living things and adds to global warming, so it is a pollutant there.
In simple words: Ozone made inside a purifier releases atomic oxygen, which kills germs in water.

Teacher's Note:
1. The same chemical can be helpful or harmful depending on where it is.
2. Write both equations.

Question 17. Carbon dioxide is used for extinguishing fires caused by petroleum products but not fires caused by metals like Na or Mg. Explain.
Answer:
Carbon dioxide is non-combustible, does not support burning and is heavier than air. It covers a petroleum fire, keeps oxygen away and puts it out.
Active metals like Na and Mg react with carbon dioxide and take its oxygen, so they continue to burn: \(\ce{2Mg + CO2 -> 2MgO + C}\); \(\ce{4Na + 3CO2 -> 2Na2CO3 + C}\). Therefore, \(\ce{CO2}\) cannot extinguish such fires.
In simple words: Petroleum fires stop in carbon dioxide, but burning sodium or magnesium takes oxygen from it and goes on burning.

Teacher's Note:
1. Sand is usually used on fires of active metals.
2. Compare the two types of fire.

Question 18. Explain the principle behind the usage of electric precipitators in industries and catalytic converters in automobiles for controlling air pollution.
Answer:
Electric precipitator: The suspended particles in the smoke of power plants, cement and steel plants are charged. They are attracted to oppositely charged plates and collected, so smoke and ash entering the atmosphere are reduced.
Catalytic converter: It has a ceramic honeycomb with a platinum catalyst spread over it. The large surface area gives many reactive sites on which harmful gases are changed into less harmful ones: CO changes to \(\ce{CO2}\), and oxides of nitrogen change to nitrogen. These are then released into the atmosphere.
In simple words: A precipitator pulls dust out of smoke using electric charge; a catalytic converter turns poisonous exhaust gases into harmless ones.

Teacher's Note:
1. Precipitators handle particles; catalytic converters handle gases.
2. Mention the platinum catalyst.