Chapter 7 – Air : A Mixture Of Gases

Selina Concise Chemistry 2023 solutions for class 7 Chemistry. Chapter 7 – Air: A Mixture Of Gases is provided here with simple step-by-step explanations. These solutions for Air: A Mixture Of Gases (6th Chapter) are extremely popular among class 10 students. The Air: A Mixture Of Gases solution is handy for quickly completing your homework and preparing for exams.

EXERCISE – I

Question 1

Give one use for each of the following inert gases :

1. argon
2. helium
3. neon
4. radon
5. krypton
6. xenon

Answer

1. Argon is filled into electric bulbs to prevent the oxidation of their filaments.
2. It is used in filling up weather observation balloons.
3. Neon is used for making advertisement sign boards.
4. It is used for the treatment of Cancer.
5. It is used in photography.
6. It is also used in photography.

Question 2

Answer the questions put against each of the following constituents of air :

1. Nitrogen: Explain its significance for plants and animals.
2. Oxygen: What is the percentage proportion of oxygen in air ? Why is oxygen
   called active air.
3. Carbon dioxide: “Although carbon dioxide plays no role in respiration, all life would come to an end if there is no carbon dioxide in the air.” Support this statement with relevant facts.
4. Water vapours: Explain their role in modifying the earth’s climate.

Answer

(a) Nitrogen:

• Significance for plants: Essential nutrient for plant growth and part of chlorophyll.
• Significance for animals: Key component of amino acids and proteins.

(b) Oxygen:

• Percentage proportion in air: Approximately 21%.
• Why called active air: Highly reactive nature and necessary for respiration.

(c) Carbon dioxide:

• Although not involved in respiration, it plays a critical role in regulating Earth’s temperature as a greenhouse gas.

(d) Water vapour:

• Acts as a greenhouse gas and helps regulate Earth’s temperature.
• Plays a critical role in the water cycle, which affects weather patterns and precipitation.

Question 3

Define the following terms :

1. pollutants
2. acid rain
3. Global warming
4. smog

Answer

1. Pollutants: Substances that contaminate or alter the natural state of the environment and can have harmful effects on living organisms.
2. Acid rain: Precipitation, such as rain, snow or fog, that has become acidic due to the release of sulfur dioxide and nitrogen oxides into the atmosphere, primarily through human activities like burning fossil fuels.
3. Global warming: The gradual increase in the Earth’s surface temperature due to the buildup of greenhouse gases in the atmosphere, primarily carbon dioxide and methane, which trap heat and cause the Earth’s temperature to rise.
4. Smog: A type of air pollution that results from the combination of smoke, fog, and other atmospheric pollutants, which can create a thick, hazy layer in the atmosphere and have harmful effects on human health and the environment.

Question 4

“Air is a mixture”. Support this statement by citing at least three pieces of evidence.

Answer

“Air is a mixture” The following are in evidence which proves that air is a mixture.

1. The composition of air varies from place to place and from time to time.
2. The components of air retain their individual properties.
3. Liquid air has no definite boiling point.
4. No energy exchange occurs when the components of air are mixed with each other.

Question 5

What is air pollution? What are the harmful effects of sulphur dioxide, nitrogen dioxide and hydrogen sulphide present in the air?

Answer

Air Pollution: Air pollution is the presence of harmful substances, called pollutants, in the air we breathe.

• Sulphur dioxide: respiratory problems, acid rain, damage to forests, crops, and water bodies.
• Nitrogen dioxide: respiratory problems, reduced lung function, increased risk of respiratory infections.
• Hydrogen sulphide: irritation of the eyes, nose, and throat, headaches, dizziness, nausea, and at high concentrations, it can be deadly.

Question 6

1. What are the causes of air pollution?
2. Suggest five measures to prevent air pollution.

Answer

The causes of air pollution are

• Burning of fossil fuels.
• Industrial processes.
• Transportation emissions.
• Agricultural activities.
• Natural sources like dust and wildfires.
• Household activities like cooking and cleaning.
• Construction activities.

Here are five measures to prevent air pollution:

1. Promote the use of public transportation, bicycles or walking.
2. Increase the use of clean and renewable energy sources like solar and wind power.
3. Enforce regulations and standards on industries to reduce emissions and control pollution.
4. Encourage the use of cleaner technologies in transportation and industry.
5. Create awareness among the general public about the harmful effects of air pollution and promote actions to reduce personal contribution to air pollution, like reducing energy consumption, properly disposing of waste, and planting trees.

Question 7

1. What is nitrogen fixation?
2. What are the two ways in which nitrogen fixation occurs?
3. Explain the conversion of nitrogen into nitrates during lightning.

Answer

(a) Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that can be used by living organisms, such as ammonia (NH₃) or nitrate (NO₃^–). Nitrogen fixation is an important natural process that plays a critical role in the global nitrogen cycle and is essential for the growth and survival of many organisms.

(b) There are two ways in which nitrogen fixation occurs:

• Biological nitrogen fixation: It is carried out by certain types of bacteria, which have the ability to convert atmospheric nitrogen into ammonia, which can be used by plants and other organisms.
• Industrial nitrogen fixation: It is done by the Haber-Bosch process, which is a method of producing ammonia from atmospheric nitrogen and hydrogen gas under high pressure and temperature.

(c) During lightning, nitrogen gas reacts with oxygen to form nitrogen oxides, which dissolve in rainwater to form nitric and nitrous acids. These react with minerals in the soil to form nitrates, which are important nutrients for plants, aiding the nitrogen cycle.

EXERCISE – II

Question 1

Name :

1. The most abundant element in the earth’s crust.
2. A chemical called oxygenated water.
3. A metal highly resistant to rusting.
4. A mixture of oxygen and carbon dioxide used for artificial respiration.
5. Two substances from which oxygen can be obtained at a large scale.
6. An oxide and a carbonate containing oxygen.
7. Two substances which undergo rapid oxidation.

Answer

1. The most abundant element in the earth’s crust is oxygen.
2. Oxygenated water is also known as hydrogen peroxide.
3. A metal highly resistant to rusting is stainless steel.
4. A mixture of oxygen and carbon dioxide used for artificial respiration is called Carbogen.
5. Two substances from which oxygen can be obtained at a large scale are air and water.
6. An oxide and a carbonate containing oxygen are iron oxide (rust) and calcium carbonate (limestone).
7. Two substances which undergo rapid oxidation are magnesium and hydrogen gas.

Question 2

1. Taking hydrogen peroxide, how would you prepare oxygen in the laboratory?
2. What is the role of manganese dioxide in the preparation of oxygen?
3. Write the balanced chemical equation for the above chemical reaction.
4. Why is hydrogen peroxide preferred in the preparation of oxygen gas?
5. Why is oxygen collected by the downward displacement of water?
6. What happens when a glowing splinter is introduced in a jar containing oxygen?
7. What happens when oxygen gas is passed through an alkaline pyrogallol solution?

Answer

1. To prepare oxygen in the laboratory using hydrogen peroxide, we can mix hydrogen peroxide with a catalyst such as manganese dioxide, which will cause the hydrogen peroxide to decompose into water and oxygen gas. The oxygen gas can be collected by the downward displacement of water.
2. Manganese dioxide acts as a catalyst in the preparation of oxygen by hydrogen peroxide. It provides a surface for the decomposition of hydrogen peroxide and speeds up the reaction.
3. The balanced chemical equation for the decomposition of hydrogen peroxide into water and oxygen gas is:
   2 H₂O₂ → 2 H₂O + O₂
4. Hydrogen peroxide is preferred in the preparation of oxygen gas because it does not require heating to decompose, and the rate of oxygen evolution is moderate and can be controlled. Additionally, hydrogen peroxide is a safe chemical to handle compared to other chemicals that can be used to generate oxygen.
5. Oxygen is collected by the downward displacement of water because oxygen is only slightly soluble in water, which allows it to be collected over water without fear of excessive dilution. Oxygen is also slightly heavier than air, so it cannot be collected over the air.
6. When a glowing splinter is introduced into a jar containing oxygen, the splinter rekindles, but the gas does not catch fire. This is because oxygen is not flammable, but it does support combustion by reacting with the fuel.
7. When oxygen gas is passed through an alkaline pyrogallol solution, the solution turns brown. This is because oxygen reacts with pyrogallol in the presence of an alkali to form brown-coloured compounds, such as pyrogallol brown. This reaction is used to test for the presence of oxygen gas.

Question 3

1. What happens when
   1. mercuric oxide and
   2. potassium nitrate are heated ?
2. Why is potassium chlorate not used for laboratory preparation of oxygen ?

Answer

(a)

1. When mercuric oxide is heated, it decomposes to give mercury and oxygen.
2. Potassium nitrate on heating gets converted into molten potassium nitrite with the release of oxygen.

(b) Potassium chlorate needs heating for quite some time (to a high temperature) before it decomposes.

Question 4

What are oxides? Give two examples for each of metallic and non-metallic oxides.

Answer

Oxides are binary compounds formed by the chemical combination of a substance metal or a non-metal with oxygen.

Examples :

Metal:

1. Sodium oxide (Na₂O).
2. Calcium oxide (CaO).

Non-metal:

1. Sulphur dioxide (SO₂).
2. Carbon dioxide (CO₂).

Question 5

Name the three types of oxidation processes. In which of these large amount of heat and light energy are produced?

Answer

Oxidation can be categorised into three types :

1. Spontaneous oxidation
2. Rapid oxidation
3. Slow oxidation

Out of the above said three types, rapid oxidation produces a large amount of heat and light energy.

Question 6

What do you observe when the following substances are heated and then tested with moist blue and red litmus – paper?

1. Sulphur
2. Phosphorus
3. Calcium
4. Magnesium

Answer

1. Sulphur: Blue litmus turns red.
2. Phosphorus: Blue litmus turns red.
3. Calcium: Red litmus turns blue.
4. Magnesium: Red litmus turns blue.

Question 7

Complete and balance the following chemical equations.

Answer

Question 8

1. Give four uses of oxygen.
2. How is oxygen naturally renewed in air?

Answer

(a) Four uses of oxygen are:

1. Oxygen is used for respiration by all living beings, including humans, animals, and plants.
2. Oxygen is used in combustion reactions, such as burning fuels for energy and heating.
3. Oxygen is used in the production of steel and other metals through the process of smelting.
4. Oxygen is used in medical treatments to help patients with respiratory problems.

(b) Oxygen is naturally renewed in the air through the process of photosynthesis. During photosynthesis, green plants use energy from sunlight to convert carbon dioxide and water into glucose and oxygen. The oxygen produced by photosynthesis is then released into the atmosphere, which helps to maintain the level of atmospheric oxygen. Additionally, oxygen is constantly being cycled through the processes of respiration and combustion, which also help to maintain the level of atmospheric oxygen.

Question 9

1. What is rust ?
2. State at least two ways of prevent rusting.

Answer

(a) Rust is a hydrated form of ferric oxide (Fe2O₃ . x H₂O) that forms a brownish-red coating over iron when it is exposed to air and moisture. The exact value of x in Fe₂O₃ . x H2O can vary, but it is typically between 1 and 3.

(b) Two ways to prevent rusting are:

1. Painting: Applying a layer of paint on the surface of iron objects creates a barrier that prevents air and moisture from coming into contact with the iron surface, thus preventing rust formation. Red lead, oil paint, and enamel coating are some of the types of paints that can be used to prevent rusting.
2. Coating: Another way to prevent rusting is to apply a layer of coating on the surface of iron objects. Coal tar is one such coating that is used for protecting the lower parts of ships and bridges. It forms a tough, impermeable layer that prevents air and moisture from coming into contact with the iron surface, thus preventing rust formation.

Question 9

State two differences between : Rusting and burning.

Answer

OBJECTIVE TYPE QUESTIONS

Question 1

Fill in the blanks :

1. ____ is the most abundant inert gas present in the air.
2. Oxides of sulphur and nitrogen combine with rainwater to form ____ and ____ which cause ____.
3. ____ and ____ are the most common air pollutants.
4. ____ discovered the oxygen gas.
5. Oxygen occupies about ____ of air by volume.

Answer

1. Argon
2. sulphuric acid, nitric acid, acid rain
3. NO₂, CO
4. Joseph Priestly
5. 21 %

Question 2

Match the following :

Answer

Multiple Choice Questions

Question 1

A fuel when used releases least amount of pollutants in the air.

1. sulphur dioxide
2. chlorofluorocarbon
3. smoke
4. CNG

Answer

CNG

Explanation

CNG (Compressed Natural Gas) is the fuel that releases the least amount of pollutants in the air. It emits lower levels of harmful pollutants like carbon monoxide, nitrogen oxides, particulate matter, sulfur dioxide, and chlorofluorocarbon than gasoline, diesel, or other fossil fuels.

Question 2

The natural way of adding oxygen to air which involves green plants is called

1. photosynthesis
2. respiration
3. burning
4. dissolution

Answer

photosynthesis

Explanation

The natural way of adding oxygen to air which involves green plants is called photosynthesis. During photosynthesis, plants use sunlight, water, and carbon dioxide to produce oxygen and glucose. This process plays a crucial role in balancing the oxygen and carbon dioxide levels in the atmosphere, sustaining life on earth.

Question 3

Which one of the following is most likely to be corroded?

1. a stainless steel cup-board
2. a galvanised iron bucket
3. an iron hammer
4. a tin plated iron box

Answer

An iron hammer

Explanation

An iron hammer is most likely to be corroded because iron is highly susceptible to corrosion due to its tendency to react with oxygen and moisture in the air. Stainless steel has high corrosion resistance, galvanized iron has a protective coating, and tin plating can also provide some corrosion resistance.

Question 4

The process by which the oxidation of food in our body takes place is

1. photosynthesis
2. respiration
3. decomposition
4. combustion

Answer

respiration

Explanation

The process by which the oxidation of food in our body takes place is respiration. During respiration, glucose is broken down into carbon dioxide and water, releasing energy that is used by the body to perform various functions. This process occurs in cells and is necessary for the survival of living organisms.