Air Atmosphere

Air consists of:

1. only oxygen
2. only nitrogen
3. only carbon dioxide
4. all of these.

Answer

all of these.

Reason — The main gases present in air are nitrogen and oxygen. Besides these two gases, air also contains small quantities of carbon dioxide, water vapour, dust particles, smoke and traces of inert gases like helium, neon, argon, etc.

The most abundant gas in Earth's atmosphere is:

1. oxygen
2. nitrogen
3. carbon dioxide
4. argon

Answer

nitrogen

Reason — The most abundant gas in Earth's atmosphere is Nitrogen. The amount of nitrogen in air is 78%.

Which of the following gases supports burning?

1. oxygen
2. nitrogen
3. carbon dioxide
4. water vapour

Answer

oxygen

Reason — The 'active part' of air, which is used up when substances burn, is called oxygen. It is present about one-fifth of the total volume of the air.

Air is a mixture because:

1. the components of air retain their individual properties.
2. energy change takes place when the components of air are mixed together.
3. the composition of air remains the same everywhere.
4. the components of air cannot be separated by simple physical methods.

Answer

the components of air retain their individual properties.

Reason — Air is a mixture because the components of air retain their individual properties. Other options are not true because no energy change takes place, the composition of air varies from place to place and the components of air can be separated by physical methods.

Oxygen in air is:

1. 4/5 parts
2. 1/5 parts
3. 1/3 parts
4. negligible

Answer

1/5 parts

Reason — The 'active part' of air i.e., oxygen is present about one-fifth (1/5 parts) of the total volume of the air.

Write the chemical name of:

(a) Lime water

(b) The white insoluble solid formed on reaction of carbon dioxide with lime water.

Answer

(a) Calcium hydroxide

(a) Calcium carbonate

What is atmosphere ?

Answer

The earth is surrounded by a thick blanket of air called the atmosphere that extends upto a height of about 320 kilometres above the surface of the earth.

Why can't we see air ?

Answer

We cannot see air, because it is colourless, odourless, tasteless, and transparent gaseous matter.

What is wind?

Answer

Fast moving air is called wind. We can feel the presence of wind.

What would have happened if there would have been no atmosphere around the earth ?

Answer

1. Atmosphere protects us from harmful rays of the sun. We could be burnt or frozen if there was no atmosphere around the earth.
2. In the absence of the atmosphere, the earth would get very hot during the day and very cold at night that we would not be able to survive.
3. There would be no life on earth under such a condition.

What do you observe when

(a) Ice cold water is filled in a glass tumbler.

(b) A burning candle is covered with an inverted jar.

(c) Carbon dioxide gas is passed through lime water.

(d) A beam of light is allowed to enter in a closed dark room through a small hole.

Answer

(a) When ice cold water is filled in a glass tumbler, we will observe that fine water droplets get deposited on the outer wall of the glass tumbler. This is due to the cold surface of the glass tumbler, the water vapour present in air get condensed as water droplets. This proves that air contains water vapour.

(b) When we cover the burning candle with an inverted jar, we will notice that after a few minutes, the flame gets extinguished. This is because the oxygen in the air trapped in the inverted jar is used up and the candle does not get any more oxygen for burning.

(c) Carbon dioxide reacts with lime water to form white insoluble calcium carbonate which gives milky white appearance to the solution. This confirms the presence of carbon dioxide in air.

(d) A beam of light is allowed to enter in a closed dark room through a small hole. We will observe randomly moving dust particles in the beam of light. This confirms the presence of dust particles in the air.

Why is air called a mixture ? Give five facts in support of your answer.

Answer

Air is called a mixture because it is made up of several different gases, dust particles, water vapour, smoke and other impurities that are physically combined, not chemically bonded.

Five facts which show that air is a mixture :

1. The composition of air varies from place to place and from time to time.
2. No energy change occurs when the components of air are mixed together.
3. The components of air retain their individual properties.
4. Air cannot be represented by a formula as its constituents are not in a fixed proportion.
5. The components of air can be separated by simple physical methods.

What are the main components of air ? Write down the composition of the three main gases present in air by volume.

Answer

The main gases present in air are nitrogen and oxygen. Besides these two gases, air also contains small quantities of carbon dioxide, water vapour, dust particles and traces of inert gases like helium, neon, argon, etc. Air also contains smoke as a pollutant.

Rust is:

1. hydrated iron oxide
2. hydrated copper sulphate
3. anhydrous iron oxide
4. none of the above

Answer

hydrated iron oxide

Reason — The reddish brown powdery mass that gets coated on the surface of some iron object when kept in moist air for a few days is called as rust. It is chemically know as hydrated iron oxide (Fe₂O₃.xH₂O)

Photosynthesis is a process in which plants:

1. take in oxygen and give out carbon dioxide
2. take in carbon dioxide and give out oxygen
3. take in nitrogen and give out oxygen.
4. none of the above

Answer

take in carbon dioxide and give out oxygen

Reason — Photosynthesis is a process through which green plants prepare their food in presence of sunlight with the help of carbon dioxide and water. In this process carbon dioxide is consumed and oxygen is released.

Carbon dioxide + Water $\xrightarrow[\text{chlorophyll}]{\text{light}}$ Glucose + Oxygen

Fuels which do not leave any residue on burning are:

1. coal and wood
2. coal and LPG
3. wood and CNG
4. LPG and CNG.

Answer

LPG and CNG.

Reason — LPG And CNG are modern day fuels which do not leaves any residue on burning.

Respiration :

1. is a slow process
2. is a natural and continuous process
3. takes place at body temperature
4. all of the above

Answer

all of the above

Reason — Respiration is slow, natural and continuous process, and takes place at body temperature.

Which of the following is common in combustion and respiration ?

1. oxygen
2. release of heat and light
3. natural process
4. nitrogen.

Answer

oxygen

Reason — Oxygen present in air is required for respiration of all living organisms and without oxygen, burning cannot take place. Hence oxygen is common in combustion and respiration.

Which of the gas present in air is required for the synthesis of proteins in plants ?

1. Oxygen
2. Nitrogen
3. Ozone
4. Carbon dioxide

Answer

Nitrogen

Reason — Nitrogen is needed to prepare a vital nutrient 'protein'.

Which component of air controls the climatic conditions?

1. Inert gases
2. Oxygen
3. Water vapour
4. Nitrogen

Answer

Water vapour

Reason — Water vapour present in the air provides moisture and helps in predicting climatic conditions of a particular area. Its amount varies from place to place from time to time.

Name two important processes supported by oxygen present in air.

Answer

Two important processes supported by oxygen present in air are given below:

1. Respiration
2. Burning (combustion)

Name the gas used in:

(a) weather balloons

(b) traffic lights

(c) electric bulbs

Answer

(a) Helium

(b) Neon

(c) Argon

State the full forms of LPG and CNG.

Answer

The full forms of LPG and CNG are given below:

1. LPG — Liquefied Petroleum Gas
2. CNG — Compressed Natural Gas

Define the following:

(a) Respiration

(b) Photosynthesis

(c) Combustion.

Answer

(a) Respiration is a chemical process that takes place in all living beings. In this process, oxygen that we breathe in with air reacts with the digested food in our body. This results in the release of energy, carbon dioxide- and water.

(b) Photosynthesis is a process in which carbon dioxide is used by green plants to prepare their food. Oxygen is released during this process which is then used for respiration by both plants and animals.

(c) Burning (Combustion) is a process in which a substance reacts chemically with oxygen and gets oxidised, with the release of energy in the form of heat and light.

What are fuels? Give two examples of modern fuels.

Answer

Substances which are burnt to produce large amount of energy in the form of heat and light are known as fuels.
Examples :

1. Liquefied Petroleum Gas (LPG)
2. Compressed Natural Gas (CNG)

Give two uses of the following components present in air:

(a) oxygen

(b) nitrogen

(c) carbon dioxide

(d) water vapour

Answer

(a) Oxygen

1. For respiration in plants and animals
2. Oxygen is an essential component of burning or combustion

(b) Nitrogen

1. Nitrogen is needed to prepare a vital nutrient protein.
2. Food packaging and storing food.

(c) Carbon dioxide

1. Dry ice(solid carbon dioxide) is used as a refrigerant.
2. Carbon dioxide is used to prepare fizzy drink like soda-water.

(d) Water vapour

1. Water vapour present in the air provides moisture for both plants and animals.
2. It also helps in predicting climatic conditions of a particular area.

How is air useful to:

(a) water boats

(b) agriculture

(c) windmills

(d) scooters and cars?

Answer

(a) Air helps in the movement of water boats by providing air resistance.

(b) Air speeds up drying up of wet agricultural products like grains, pulses, fruits and air helps in pollination of flowers and dispersal of seeds.

(c) Windmills move due to the movement of air. They are also used to generate electricity.

(d) Air is filled in tyres of vehicles like cars, scooters, trucks, buses, bicycles, etc. for their smooth movement.

What is the importance of dust particles present in air ?

Answer

Dust particles present in air cause rain as water vapour get condensed on these particles.

Give reason:

Aquatic animals and plants are able to survive in water.

Answer

Oxygen present in air is required for respiration by all living organisms. Aquatic plants and animals use the air (oxygen) dissolved in water for respiration.

Give reason:

A burning candle stops burning if covered with a glass tumbler.

Answer

A burning candle stops burning if covered with a glass tumbler because the oxygen in the air trapped in the inverted glass is used up and the candle does not get any more oxygen for burning.

Give reason:

Mountaineers and divers carry oxygen cylinders with them.

Answer

At higher altitudes, the air is less dense or thin and breathing becomes difficult. Similarly, air becomes less dense below the surface of the earth. That is why mountaineers, astronauts, miners and divers carry oxygen cylinders with them.

Give reasons:

When water is heated, we see bubbles rising up.

Answer

When water is heated, we see bubbles rising up because these bubbles come from the air dissolved in water.

State two similarities and two differences between respiration and burning.

Answer

The similarities and differences between respiration and burning are given below:

Define rusting. What are the two necessary conditions for rusting of iron ? Give the chemical name of rust.

Answer

Rusting is a chemical process which takes place in presence of moist air. When some iron objects like iron scraps or bright iron nails are kept in moist air for a few days, their surface gets coated with a reddish brown powdery mass which is commonly called rust.

Conditions for rusting :

1. Presence of Air (oxygen)
2. Presence of Moisture (water)

Chemical name of rust : Hydrated iron oxide (Fe₂O₃.xH₂O)

Name the processes which maintain the balance between oxygen and carbon dioxide in the air. How is it done?

Answer

The main process in nature that helps in balancing the quantities of oxygen and carbon dioxide in air is photosynthesis that occurs in green plants. Photo means light and synthesis means putting together.

It is a process through which green plants prepare their food in presence of sunlight with the help of carbon dioxide and water. The food is prepared in the form of glucose (a carbohydrate) with the release of oxygen gas.

Carbon dioxide + Water $\xrightarrow[\text{chlorophyll}]{\text{light}}$ Glucose + Oxygen

• In photosynthesis, plants use carbon dioxide from the air, water from the soil and give out oxygen.
• This process takes place only in green plants because they contain a green pigment called chlorophyll which is able to trap solar energy from the sun.

(a) Why is nitrogen important to all living beings?

(b) What is nitrogen fixation?

Answer

(a) Importance of Nitrogen

1. Nitrogen is needed to prepare a vital nutrient 'protein'. Every living being needs proteins for its growth.
2. Nitrogen dilutes the effect of oxygen present in air. Thus, it controls combustion as it does not support burning.
3. Manufacture of fertilizers and various nitrogenous products.
4. Nitrogen is used in the food industry for food packaging and storing food in containers as it is inert and prevents food from becoming stale.

(b) Nitrogen fixation is the process by which free nitrogen of air is converted into nitrogen compounds like nitrites and nitrates which are useful plant nutrients to produce proteins.

Air pollution is caused due to :

1. cutting of green plants
2. smoke given out by factories
3. gases like carbon monoxide, sulphur dioxide, etc.
4. all of the above.

Answer

all of the above

Reason — Air pollution is the presence of harmful substances in the air that can affect human health, animals, plants, and the environment. It is caused by cutting green plants, smoke given out by factories and harmful gases like carbon monoxide and sulphur dioxide.

The gases which cause acid rain are:

1. sulphur dioxide and oxygen
2. nitrogen and oxygen
3. carbon dioxide and water vapour
4. nitrogen dioxide and sulphur dioxide.

Answer

nitrogen dioxide and sulphur dioxide

Reason — Gases like sulphur dioxide and nitrogen dioxide (pollutant) get mixed in rain water to form acid rain.

Which of the following is not a greenhouse gas?

1. Carbon dioxide
2. Sulphur dioxide
3. Methane
4. Nitrous oxide

Answer

Sulphur dioxide

Reason — Green house gases are carbon dioxide, methane, nitrous oxide, which traps more heat from the sunlight. In contrast, sulphur dioxide is not a green house gas.

Common air pollutants that cause respiratory diseases are:

1. ozone
2. inert gases
3. dust and smoke
4. water vapour

Answer

dust and smoke

Reason — Common air pollutants like dust and smoke cause respiratory, skin and eye problems.

The gas responsible for ozone depletion is:

1. oxygen
2. chlorofluorocarbons
3. carbon dioxide
4. sulphur dioxide

Answer

chlorofluorocarbons

Reason — Air pollutants like chlorofluorocarbons (CFCs) reacts with the ozone in the atmosphere and reduce the density of ozone in the stratosphere.

Fill in the blanks:

(a) The layer of air around the earth is called the ............... .

(b) Although we cannot see air, we can ............... it.

(c) Air is a ............... of gases.

(d) Plants and animals maintain the balance of ............... and ............... in air.

(e) Polluted air is ............... for health.

(f) The supporter of combustion in air is ............... .

(g) Green plants need ............... to prepare food.

(h) Oxygen is used in ............... of the food to get ............... .

(i) Aquatic plants and animals use ............... in water.

(j) The reddish brown powder on the surface of iron nails exposed to air and moisture is called ............... .

Answer

(a) The layer of air around the earth is called the atmosphere.

(b) Although we cannot see air, we can feel it.

(c) Air is a mixture of gases.

(d) Plants and animals maintain the balance of oxygen and carbon dioxide in air.

(e) Polluted air is harmful for health.

(f) The supporter of combustion in air is oxygen.

(g) Green plants need carbon dioxide to prepare food.

(h) Oxygen is used in digestion of the food to get energy.

(i) Aquatic plants and animals use dissolved oxygen in water.

(j) The reddish brown powder on the surface of iron nails exposed to air and moisture is called rust.

Write 'true' or 'false' for the following:

(a) Air is a compound. ...............

(b) Carbon dioxide is given out during photosynthesis. ...............

(c) Respiration needs nitrogen. ...............

(d) The composition of air was discovered by Lavoisier. ...............

(e) The major component of air is oxygen. ...............

Answer

(a) False
Correct Statement — Air is a mixture.

(b) False
Correct Statement — During photosynthesis, oxygen is released and carbon dioxide is absorbed by plants to make food.

(c) False
Correct Statement — Respiration needs oxygen.

(d) True

(e) False
Correct Statement — The major component of air is nitrogen (78 %).

Match the following

Answer

What is air pollution ?

Answer

Air pollution is the presence of harmful substances in the air that can affect human health, animals, plants, and the environment.

Name two air pollutants which

(a) affect our health,

(b) cause acid rain,

(c) cause global warming.

Answer

(a) Dust, smoke

(b) Sulphur dioxide, Nitrogen dioxide

(c) Carbon dioxide, methane

Name three greenhouse gases.

Answer

Three greenhouse gases are given below:

1. Carbon dioxide
2. Methane
3. Nitrous oxide

What is meant by ozone depletion?

Answer

Ozone depletion refers to the gradual thinning and reduction of the ozone layer in the earth's stratosphere due to the release of certain harmful chemicals, especially CFCs (chlorofluorocarbons).

Mention five causes of air pollution.

Answer

Five causes of air pollution are given below:

1. Increase in human population.
2. Indiscriminate cutting of forests (deforestation).
3. Burning of fossil fuels.
4. Emission of harmful gases from vehicles.
5. An increase in the number of industries near the residential areas.

State four steps to be taken to control air pollution.

Answer

Four steps to be taken to control air pollution are given below:

1. Growing more trees and plants in big cities and towns. Trees clean the atmosphere by using carbon dioxide and releasing oxygen. Trees also help in bringing more rains that make the atmosphere free of pollutants.
2. Factories and power houses should be provided with tall chimneys so that smoke and gases go high up into the air.
3. Unleaded petrol should be used in vehicles. Vehicles should be regularly checked for pollution control. Nowadays, CNG (Compressed Natural Gas) is being used as a fuel in all the public transport vehicles to reduce pollution.
4. Using smokeless sources of energy, like solar energy and electrical energy in place of the conventional fossil fuels. Smokeless chulhas use biogas and solar energy.

Study the diagram and name the processes A, B and C.

Answer

1. A is Respiration — Respiration is a chemical process that takes place in all living beings. In this process, oxygen that we breathe in with air reacts with the digested food in our body. This results in the release of energy, carbon dioxide and water.
2. B is Photosynthesis — Photosynthesis is a process in which carbon dioxide is used by green plants to prepare their food. Oxygen is released during this process which is then used for respiration by both plants and animals.
3. C is Burning of fuels — Burning (Combustion) is a process in which a substance reacts chemically with oxygen and gets oxidised, with the release of energy in the form of heat and light.

In an experiment done with a burning candle, the apparatus was set up as shown below. Initially, the candle burns but after sometime it gets extinguished.

Look at the diagram and answer the questions given below.

(a) Label A and B.

(b) Why does the candle burn in (I) but gets extinguished in (II) ?

(c) Why does the water level rise after sometime ?

(d) What conclusion is drawn by the experiment show above ?

Answer

(a) In the given diagram A and B are:
A → Air
B → Inactive air

(b) The candle burn in (I) because of the presence of 'active air' that contains oxygen, which is used by the candle to burn. The candle does not burn in case of (II), because the jar consist of 'inactive air', consist mainly nitrogen, which does not support burning.

(c) When the candle is lit in the jar, it starts burning using the active air present in the jar, upto one-fifth part of the jar containing oxygen helps the candle to burn. After sometime the candle gets extinguished and the water level rises upto one-fifth part of the jar to compensate the loss of oxygen.

(d) This proves that air contains both oxygen and nitrogen. It clearly explains that air is made up of an 'active' part and an 'inactive' part. The 'active' part of air, which is used up when substances burn, is about one-fifth of the total volume of the air. It is a gas called oxygen. The 'inactive' part, which does not support burning, consists mainly of a gas called nitrogen, constituting 4/5th of air by volume.