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

Short Answer Questions

Question 1. Give any two chemical properties of \(\ce{CO2}\).
Answer:
1. It is acidic: it dissolves in water to form carbonic acid, which turns blue litmus red. \(\ce{CO2 + H2O -> H2CO3}\)
2. It turns lime water milky because insoluble calcium carbonate forms. \(\ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O}\)
(It is also neither combustible nor a supporter of combustion.)
In simple words: Carbon dioxide forms an acid with water and turns lime water milky.

Teacher's Note:
1. Pick properties that involve a reaction, not just appearance.
2. The lime water reaction is also the standard test for the gas.

Question 2. What is greenhouse effect?
Answer:
The earth's surface absorbs solar radiation and sends it back as heat (infrared rays). Some gases in the atmosphere, called greenhouse gases (\(\ce{CO2}\), water vapour, \(\ce{N2O}\), \(\ce{CH4}\), \(\ce{O3}\), CFCs), absorb this heat and re-radiate it in all directions.
This trapping of heat that keeps the earth's surface warm is the greenhouse effect. It supports life on the earth.
In simple words: Gases in the air hold some of the sun's heat near the earth, like the glass of a greenhouse.

Teacher's Note:
1. The effect is natural and useful; the problem is its excess.
2. When the gases increase, global warming results.

Question 3. Write a short note on acid rain.
Answer:
Gases like \(\ce{NO2}\) and \(\ce{SO2}\) are released from factory chimneys, burning of fossil fuels and volcanic eruptions. They dissolve in rain water and form nitric acid and sulphuric acid. Rain containing these acids is called acid rain.
\(\ce{2SO2 + O2 -> 2SO3}\)
\(\ce{SO3 + H2O -> H2SO4}\)
\(\ce{2NO2 + H2O -> HNO3 + HNO2}\)
Effects: loss of soil fertility, damage to crops, pollution of rivers and seas, and corrosion of marble statues and buildings.
In simple words: Rain made acidic by sulphur and nitrogen oxides is acid rain, which damages soil, water and monuments.

Teacher's Note:
1. Write the three equations to get full marks.
2. Remember \(\ce{SO2}\) and \(\ce{NO2}\) as the two culprits.

Question 4. Mention various methods of minimizing air pollution
Answer:
1. Dispose of waste products safely.
2. Use scrubbers and electrostatic precipitators in industries to stop harmful emissions.
3. Make eco-friendly chemicals.
4. Control the growth of population.
5. Use natural resources wisely.
6. Limit the use of plastic.
7. Stop deforestation and plant more trees.
In simple words: Cleaner industries, less waste and plastic, fewer vehicles' fumes and more trees all reduce air pollution.

Teacher's Note:
1. Give at least five points in an exam.
2. Planting trees is the easiest step each student can take.

Question 5. Explain the role of oxygen in the following:
(a) respiration
(b) cutting and welding
(c) explosion
Answer:
(a) Respiration: The oxygen we breathe in oxidises glucose in our cells to carbon dioxide and water and releases energy. \(\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy}\)
(b) Cutting and welding: Hydrogen or acetylene burning in oxygen gives very hot flames (about 2800°C and 3300°C), which melt metals so that they can be cut or joined.
(c) Explosion: A cartridge made of coal, petroleum jelly and liquid oxygen burns very fast and is used to blast rocks.
In simple words: Oxygen gives energy in the body, makes very hot flames for metalwork and drives fast burning in blasting.

Teacher's Note:
1. Oxygen is always the supporter of combustion in these uses.
2. Write the flame temperatures for full marks in (b).

Question 6. List out the important uses of \(\ce{CO2}\).
Answer:
1. Making chemicals such as washing soda, baking soda and white lead.
2. Making soft drinks and soda water tangy.
3. Medicine: mixed with oxygen as carbogen to stimulate breathing.
4. Dry ice as a refrigerant for meat, fish, fruit and vegetables.
5. Preserving food grains.
6. Photosynthesis, in which plants make glucose and release oxygen.
7. Fire extinguishers.
In simple words: Carbon dioxide is used for fizzy drinks, cooling food, putting out fires and by plants for making food.

Teacher's Note:
1. It is a favourite 'list the uses' question; learn at least five.
2. Its three fire-fighting properties are: non-combustible, a non-supporter of burning and heavier than air.

Question 7. Mention the ways of minimizing global warming.
Answer:
1. Reduce the use of fossil fuels such as petrol and diesel.
2. Stop deforestation and plant more trees.
3. Restrict the use of CFCs.
4. Use non-conventional (renewable) sources of energy.
In simple words: Burn less fuel, plant more trees and avoid CFCs to slow down global warming.

Teacher's Note:
1. Plants take in \(\ce{CO2}\), so trees reduce the greenhouse effect.
2. Give at least three points.

Question 8. Mention the advantage of existence of atmosphere.
Answer:
1. It contains the air we breathe, which supports life.
2. Its moisture and dust help in the formation of clouds and rain.
3. It protects us from harmful radiation of the sun (for example UV rays absorbed by the ozone layer).
4. Through the greenhouse effect it keeps the earth warm.
In simple words: The atmosphere gives us air, rain, protection from harmful rays and a warm earth.

Teacher's Note:
1. The question wants benefits, so list them one by one.
2. Mention the ozone layer when speaking about protection.

Question 9. Explain ozone depletion.
Answer:
Ozone in the stratosphere acts as a shield and absorbs highly energetic UV rays. Pollutants such as CFCs, coming from refrigerators, air conditioners and aerosols, rise to the stratosphere and react with ozone, which gradually thins the ozone layer and makes holes.
Then UV rays reach the lower layers and cause skin cancer, cataract and damage to plants and crops.
Prevention: use substitutes for CFCs in refrigeration and aerosols.
In simple words: CFCs destroy the protective ozone layer so that more harmful UV light reaches the ground.

Teacher's Note:
1. Explain the cause, the effect and the remedy to cover the whole question.
2. Nitrous oxide also depletes ozone.

Question 10. Mention the effects of acid rain.
Answer:
1. The soil loses its fertility.
2. Crops are damaged.
3. Rivers, lakes and seas are polluted and aquatic animals die.
4. Marble statues, monuments and buildings are corroded.
In simple words: Acid rain harms soil, crops, water life and monuments.

Teacher's Note:
1. Marble is calcium carbonate, which acids dissolve.
2. The Taj Mahal is a famous example of a monument at risk.

Teacher's Note:
1. Weight and volume percentages differ because gases have different masses.
2. The book gives oxygen as 23% by mass, which agrees with this table.

Question 12. Mention any four physical properties of \(\ce{CO2}\).
Answer:
1. It is colourless and odourless.
2. It is highly soluble in water, and its solubility increases with pressure.
3. It is heavier than air (vapour density 22).
4. It is acidic in nature and turns blue litmus red.
(Also: at very high pressure, suddenly expanding it through a small nozzle gives solid dry ice.)
In simple words: Carbon dioxide is a colourless, odourless, soluble, heavy and acidic gas.

Teacher's Note:
1. Vapour density 22 means it is 22 times as heavy as hydrogen.
2. Write four properties only; extra ones do not earn extra marks.

Question 13. Write a short note on nascent hydrogen.
Answer:
Newly formed, atomic hydrogen, written [H], is called nascent hydrogen. It is more active than ordinary hydrogen and can reduce compounds that ordinary hydrogen cannot, so it is a strong reducing agent.
It is usually produced by the action of a metal on an acid, for example zinc with dilute sulphuric acid.
Applications: (i) as a rocket propellant (it must be stored in liquid helium to prevent combining into \(\ce{H2}\)); (ii) it reduces nitrites to ammonia and arsenic compounds to arsine; (iii) it has a temporary bleaching action.
In simple words: Nascent hydrogen is freshly produced atomic hydrogen, which is a very strong reducing agent.

Teacher's Note:
1. Atoms of hydrogen soon join to form \(\ce{H2}\) molecules, so nascent hydrogen is short-lived.
2. Mention how it is made and what it does.

Question 14. Explain hydrogenation of oils.
Answer:
Hydrogenation is the addition of hydrogen to a carbon compound in the presence of a catalyst. \(\ce{H2C=CH2 + H2 -> H3C-CH3}\)
Vegetable oils such as groundnut and sunflower oil contain less hydrogen and are liquid at room temperature. When hydrogen is passed through them at high pressure and temperature in the presence of nickel catalyst, they become solid fats such as vanaspati. This is also called hardening of oils.
In simple words: Adding hydrogen to a liquid vegetable oil with nickel as catalyst turns it into a solid fat.

Teacher's Note:
1. Name the catalyst, nickel, in your answer.
2. Soap and vanaspati are made using this process.

Question 15. Mention any four important uses of hydrogen.
Answer:
1. Making ammonia by Haber's process.
2. Making hydrogen chloride and methyl alcohol.
3. Hardening vegetable oils into vanaspati by hydrogenation.
4. As a fuel (oxyhydrogen torch, rocket fuel, fuel cells).
5. As a reducing agent in extracting metals such as tungsten.
In simple words: Hydrogen is used for ammonia, vanaspati, fuel and extraction of metals.

Teacher's Note:
1. Choose any four from the list.
2. Hydrogen's high heat of combustion makes it a good fuel.

Question 16. Explain the process of nitrogen fixation.
Answer:
Plants cannot absorb free nitrogen of the air. Changing it into nitrogen compounds that plants can use is nitrogen fixation.
(i) By lightning: \(\ce{N2 + O2 -> 2NO}\); \(\ce{2NO + O2 -> 2NO2}\); \(\ce{4NO2 + 2H2O + O2 -> 4HNO3}\). Nitric acid falls with rain and reacts with metal carbonates in the soil: \(\ce{2HNO3 + MCO3 -> M(NO3)2 + CO2 + H2O}\). The nitrates are absorbed by plants.
(ii) By bacteria: Rhizobium bacteria in the root nodules of leguminous plants change atmospheric nitrogen into nitrogen compounds that enrich the soil.
In simple words: Nitrogen of the air is changed into nitrates by lightning and by bacteria in root nodules so that plants can use it.

Teacher's Note:
1. Give both the natural methods.
2. Pea and gram plants have the useful root nodules.