ICSE Class 7 Chemistry Important Questions Chapter 3 Air

Chemistry ICSE Class 7 Important Questions Chapter 3 Air

Question 1.
Name few common air pollutants.
Answer:
Fine particulates (smoke, soot and dust), carbon monoxide (CO), sulphur, dioxide (SO₂), nitrogen dioxide (NO₂) and chlorofluorocarbons (CFCs).

Question 2.
What is composition of air ?
Answer:

Contituent	Percentage by volume
Nitrogen	78%
Oxygen	21%
Carbon dioxide	0.03%
Noble gases (mainly argon)	0.9%
Water vapour	Variable
Dust particles and polluting gases	Variable

Question 3.
What are particulates ? What are their sources and harmful effects ?
Answer:
Very small fragments of solid materials suspended in air are called particluates. They include smoke, soot and dust particles. Smoke and soot are produced due to the burning of wood and fossil fuels. They carry minute solid particles that cause serious respiratory problems like asthma and bronchitis. Smoke when mixed with fog forms smog. It reduces visibility leading to serious road accidents. Particulates of lead oxide present in automobile exhaust can cause brain damage in children. Asbestos fibres can cause a disease of the lungs called silicosis.

Question 4.
What are the sources of the following air pollutants –
1. carbon monoxide
2. sulphur dioxide
3. nitrogen dioxide
Answer:
1. The incomplete burning of fossil fuels such as petrol and diesel leads to the formation of carbon monoxide.
2. When coal is burnt, sulphur dioxide is given off. Most sulphur dioxide emissions come from power generating plants which arecoal-based. Sulphur dioxide is also given off during volcanic eruptions.
3. The exhaust of automobiles running on petrol and diesel contain nitrogen dioxide. The burning of coal also generates nitrogen dioxide.

Question 5.
What are chlorofluorocarbons ? How does it lead to ozone depletion ?
Answer:
Chlorofluorocarbons (CFCs) used in aerosol sprays, refrigerators and air conditioners cause the depletion of the ozone layer in the atmosphere. Ozone depletion refers to the slow and steady decline of the ozone concentration in the earth’s atmosphere. The ozone layer absorbs most of the harmful ultraviolet rays of the sun and prevents them from reaching the earth’s surface. But due to a decrease in the concentration of ozone in the air, the harmful UV rays can enter the earth’s lower atmosphere to cause skin cancer, cataract in the eyes, sunburn, etc. It can also increase the earth’s temperature leading to global warming.

Question 6.
Write a note on acid rain.
Answer:
Acid rain – When fossil fuels are burnt, they discharge pollutants such as oxides of nitrogen and sulphur dioxide into the atmosphere. In the atmosphere, the oxides of nitrogen and sulphur dioxide combine with water vapour to form nitric acid and sulphuric acid. Eventually, these acids mix with rain, snow or fog and reach the earth’s surface as acid rain.
Harmful effects of acid rain :

1. harms aquatic life
2. damages plants
3. makes the soil acidic and thus reduces the fertility of the soil
4. damages certain building material such as marble.

Question 7.
Why is water acidified to prepare oxygen from it ?
Answer:
Oxygen is obtained by electrolysis of water. When electric current is passed through acidified water, it decomposes into hydrogen and oxygen. Now, pure water is almost a non-conductor of electricity. Hence, it is acidified by adding a few drops of dilute sulphuric acid to make it a conductor of electricity.

Question 8.
Why potassium nitrate is not generally used for the laboratory preparation of oxygen ?
Answer:
It is because :
(i) Strong heating is required for melting potassium nitrate (KN03).
(ii) The rate of evolution of oxygen is fairly slow.

Question 9.
List few physical and chemical properties of oxygen.
Answer:
Physical properties of oxygen –
Nature : colourless, odourless and tasteless gas
Density : slightly heavier than air
Solubility : slightly soluble in water
Becomes a liquid below – 183°C
Becomes a solid at about – 218.4°C.

Chemical properties of oxygen :
1. Action of litmus : Oxygen is neutral to litmus. It does not change its colour.
2. Combustibility : Oxygen is non-combustible but is a supporter of combustion.
3. Reactivity : Oxygen reacts with metals and non-metals to form the respective oxide. This is called oxidation reaction.
Example :

Question 10.
Name the types of oxidation.
Answer:
1. Spontaneous oxidation – Substance begins to bum on its own, without being ignited by a spark or flame.
Example :

2. Rapid oxidation : Substance reacts with oxygen rapidly to release a large amount of heat and light energy. Example : LPG petrol, kerosene show rapid oxidation.

3. Slow oxidation : Substance reacts with oxygen very slowly.
Example : Rusting of iron

Question 11.
What happens when :
1. a glowing splinter is lowered into a gas jar filled with oxygen.
Answer:
The splinter bursts into a flame.

2. a piece of sodium is lowered into a jar of oxygen.
Answer:
Sodium bums in oxygen with a bright yellow flame, forming sodium oxide.

Question 12.
Mention few’ uses of oxygen.
Answer:
Uses of oxygen :

1. Respiration : Oxygen is a life – supporting gas. Oxygen cylinders are used by fire-fighters, deep-sea divers, miners, etc for respiration.
2. Burning : Oxygen is necessary for the burning of fuels to produce heat and light energy.
   
3. Medical purpose : In hospitals, oxygen-filled cylinders are used to give oxygen to patients. A mixture of 95% oxygen and 5% carbon dioxide called carbogen is given to the patients to stimulate natural breathing.
4. For welding and cutting purposes : Burning of a mixture of oxygen and hydrogen produces a flame with a very high temperature (about 2800°C) known as oxy-hydrogen flame. It is used for cutting and welding metals and glass.
5. As a rocket propellant – Liquid oxygen (LOX) is used as a rocket propullant as there is no oxygen in outer space.

Question 13.
How is oxygen renewed in air ?
Answer:
Oxygen in the air is used up in respiration by animals, burning of fuels and rusting of iron, etc, still its amount in the atmosphere remains constant because the carbon dioxide released during respiration and burning of fuels is utilized by green plants to produce their food by photosynthesis and release oxygen into the air. Thus, in nature, oxygen and carbon dioxide are consumed and produced continuously.

Question 14.
How do you prepare carbon dioxide in the laboratory ?
Answer:
Aim : To prepare carbon dioxide in the laboratory by the action of dilute hydrochloric acid on marble chips (CaCO₃)
Materials required : A Woulfe’s bottle, a delivery tube, corks with a hole, a thistle funnel with a stopper, a gas jar, marble chips, dilute hydrochloric acid.

Procedure :

1. Put some marble chips (calcium carbonate) in a Woulfe’s bottle.
2. Set up the apparatus as shown
3. Add dilute hydrochloric acid to the marble chips with the help of the thistle funnel such that the marble chips are completely immersed in the acid.

Observation :
Marble chips react with dilute hydrochloric acid to produce carbon dioxide gas with brisk effervescence.

Collection : Pure dry carbon dioxide is collected by the upward displacement of air since it is heavier than air.

Question 15.
Write few physical and chemical properties of carbon- dioxide.
Answer:
Physical properties of carbon dioxide – Nature : colourless, odourless and non-poisonous gas .
Density : 1.5 times heavier than air
Solubility : Fairly soluble in water at ordinary pressure
Liquefaction : Can be easily liquefied under pressure. Carbon dioxide solidifies on cooling at – 78°C to form a white solid, which is commonly called dry ice. Chemical properties of carbon dioxide

1. Carbon dioxide turns blue litmus red. This shows that the gas is acidic in nature.
2. Carbon dioxide is neither combustible nor a supporter of combustion.
3. It dissolves in water under pressure to give a weak acid called carbonic acid.
   
4. It reacts with alkalis to give the corresponding salts and water.
   
5. When carbon dioxide gas is passed through lime water, it turns milky. If excess of carbon dioxide is passed through the milky suspension, the solution becomes clear.

Question 16.
Mention few uses of carbon dioxide.
Answer:
Uses of carbon dioxide :

1. In carbonated drinks – As it is easily soluble in water under pressure, carbon dioxide is used in the manufacture of carbonated drinks.
2. As a refrigerant for preservation of foods as dry ice (solid carbon dioxide)
3. In the manufacture of urea (fertilizer).
4. In the food industry to make baking powder.
5. Green plants use carbon dioxide to prepare their own food by photosynthesis.

Question 17.
Mention few harmful effects of global warming and steps to reduce it.
Answer:
Harmful effects of global warming :
1. Global warming could cause the melting of the polar ice caps leading to a rise in sea levels, causing floods.
2. There could be an increase in heat waves and a greater risk of drought.
3. There could be more rainfall and more intense winds (cyclones).
4. Climate change could lead to the extinction of many animals. The greenhouse effect/global warming can be reduced by –
(a) planting more trees
(b) reducing the burning of fossil fuels

Question 18.
Write two chemical properties of nitrogen.
Answer:
(i) Nitrogen is neither combustible nor a supporter of combustion.
(ii) Nitrogen is chemically inert or unreactive under ordinary conditions. However, it reacts with certain metals of form metal nitrides.

Question 19.
Define nitrogen cycle. Name the main steps.
Answer:
The process of conversion of atmospheric nitrogen into essential nitrogen compounds and decomposition of these compounds back to gaseous nitrogen is called the nitrogen cycle. Its main steps are :

1. Fixation of nitrogen
2. Assimilation of nitrogen
3. Transfer of nitrogen to soil
4. Release of nitrogen into the air

Question 20.
List uses of noble gases.
Answer:
Uses of noble gases

1. Helium is used to fill instrument carrying balloons for weather studies.
2. Deep – sea divers use helium-oxygen mixture for breathing.
3. Neon is widely used in advertising signs.
4. Argon is filled in electric bulbs. It prevents the oxidation of tungsten filaments as it is unreactive.
5. Radon is radioactive and used in the treatment of cancer.
6. Krypton and xenon are used in high intensity flashlamps for photography.

Question 21.
Fill in the blanks :
1. Smoke when mixed with fog forms ………….
Answer:
Smoke when mixed with fog forms smog.
2. …………….. is given out as air pollutant from coal-based
power plants.
Answer:
Sulphur dioxide is given out as air pollutant from coal-based power plants.

3. Full form of CNG is …………..
Answer:
Full form of CNG is compressed natural gas.

4. There is a massive …………… in the ozone layer right above Antarctica.
Answer:
There is a massive ozone hole in the ozone layer right above Antarctica.

5. In the combined state …………… is the most abundant element on the earth.
Answer:
In the combined state, oxygen is the most abundant element on the earth.

6. Scheele and priestley used ……………… in the preparation of oxygen.
Answer:
Scheele and priestley used mercury (II) oxide in the preparation of oxygen.

7. The lowest temperature at which a substance starts burning is known as …………….
Answer:
The lowest temperature at which a substance starts burning is known as ignition temperature/kindling temperature.

8. To prevent rusting, metals such as ………….. and …………….. are electroplated on iron.
Answer:
To prevent rusting, metals such as nickel and chromium are electroplated on iron.

9. Respiration is an example of ………….. reaction.
Answer:
Respiration is an example of oxidation reaction.

10. ………….. discovered carbon-dioxide by burning charcoal in air.
Answer:
Jan Baptist van Helmont discovered carbon-dioxide by burning charcoal in air.

11. Carbon dioxide dissolves in water under pressure to give a weak acid called …………..
Answer:
Carbon dioxide dissolves in water under pressure to give a weak acid called carbonic acid. ‘

12. The ………….. gas produced by mixing baking powder in dough makes the dough rise and become soft.
Answer:
The carbon dioxide gas produced by mixing baking powder in dough makes the dough rise and become soft.

13. Carbon-dioxide is …………… than air while nitrogen is slightly, than air
Answer:
Carbon-dioxide is heavier than air while nitrogen is slightly, lighter than air.

14. Due to global warming, it has been predicted that there could be a …………… C rise in average temperatures over the next 100 years.
Answer:
Due to global warming, it has been predicted that there could be a 4°C rise in average temperatures over the next 100 years.

15. The process of converting free atmospheric nitrogen into nitrogen compounds such as nitrates and ammonia is called ……………
Answer:
The process of converting free atmospheric nitrogen into nitrogen compounds such as nitrates and ammonia is called nitrogen fixation.

16. …………… gases are also called rare gases.
Answer:
Noble gases are also called rare gases. .

17.  ………….. bacteria transform soil ammonia into nitrates while …………… bacteria break down nitrates and return nitrogen in to the air.
Answer:
Nitrifying bacteria transform soil ammonia into nitrates while denitrifying bacteria break down nitrates and return nitrogen in to the air.

18.  …………… is a chemical reaction involving the addition of oxygen.
Answer:
Oxidation is a chemical reaction involving the addition of oxygen.

19. Oxygen reacts with metals and non metals of form ………….
Answer:
Oxygen reacts with metals and non metals of form oxides.

20. The …………. and ………… flames are used for cutting and welding purposes.
Answer:
The oxy-hydrogen and oxyacetylene flames are used for cutting and welding purposes.

21. Carbon-dioxide reacts with alkalis to give ………….. and ………….
Answer:
Carbon-dioxide reacts with alkalis to give carbonates and bicarbonates. .

22. The amount of waters vapour present in the air at a place depends on the ………… and …………. of the place.
Answer:
The amount of waters vapour present in the air at a place depends on the season and location of the place.

Question 22.
Which of the rare gases is not present in air ?
Answer:
Radon (Rn)

Question 23.
Write reaction for (i) respiration in animals (ii) photosynthesis in plants.
Answer:

Question 24.
Name two human activities responsible for global warming. ‘
Answer:
(i) Deforestation i.e. cutting of trees
(ii) Burning of fossil fuels

ICSE Class 7 Chemistry Important Questions