Air and Atmosphere

1. What is Air?

Air is a mixture of gases that forms the Earth's atmosphere. It is colourless, odourless, and tasteless.

Air occupies space and has weight. This can be demonstrated by simple experiments.

Worked Example: How can you show that air occupies space?

Invert a glass into a bucket of water. The water does not fill the glass completely. This shows that air occupies space inside the glass.

2. Composition of Air

Air is not a single substance but a mixture of several gases.

Percentage Composition (by volume)

Worked Example: In a sample of 100 L of air, how much oxygen is present?

Oxygen is 21% of air. So, volume of oxygen = 21/100 x 100 L = 21 L.

Common Mistake: Thinking air is mostly oxygen. Nitrogen makes up 78% of air, while oxygen is only 21%.

3. Uses of the Major Components

Nitrogen (N2) — 78%

• Diluent: Reduces the reactivity of oxygen (without nitrogen, oxygen would make everything burn too fast).
• Manufacture of ammonia: Used to make fertilisers.
• Food preservation: Nitrogen is used in packaging to prevent spoilage.
• Liquid nitrogen: Used as a coolant and in medical cryosurgery.

Oxygen (O2) — 21%

• Respiration: Essential for all living organisms to release energy from food.
• Combustion: Supports burning.
• Medical use: Given to patients with breathing difficulties.
• Welding: Oxygen-acetylene torches cut and weld metals.

Exam Focus (2 marks): 'Why is nitrogen important even though we do not breathe it like oxygen?'

Nitrogen acts as a diluent, controlling the rate of combustion. Without nitrogen, the high concentration of oxygen would make fires burn uncontrollably. Nitrogen is also essential for making fertilisers for plant growth.

Carbon Dioxide (CO2) — 0.03%

• Photosynthesis: Plants use CO2 to make food.
• Fire extinguishers: CO2 is heavier than air and smothers fires.
• Carbonated drinks: CO2 gives fizz to soft drinks.
• Dry ice: Solid CO2 used as a coolant.

4. The Atmosphere

The atmosphere is the layer of air surrounding the Earth. It extends up to about 500 km.

Layers of the Atmosphere

Importance of the Atmosphere

1. Provides oxygen for breathing and carbon dioxide for plants.
2. Protects us from harmful UV rays (ozone layer).
3. Regulates Earth's temperature (greenhouse effect).
4. Burns up meteoroids before they reach the surface.
5. Enables sound to travel.

5. Air Pollution

Causes of Air Pollution

Effects of Air Pollution

• Health problems: Asthma, bronchitis, lung cancer.
• Acid rain: Sulphur dioxide and nitrogen oxides react with water vapour to form acids that damage buildings, soil, and water bodies.
• Global warming: Excess CO2 traps heat, raising Earth's temperature.
• Ozone layer depletion: CFCs damage the ozone layer, increasing UV radiation.
• Smog: Harmful combination of smoke and fog.

Common Mistake: Thinking global warming and ozone depletion are the same. Global warming is caused by greenhouse gases (CO2, methane). Ozone depletion is caused by CFCs. They are different problems.

6. Prevention of Air Pollution

1. Use public transport: Reduces vehicle emissions.
2. Plant more trees: Trees absorb CO2 and release oxygen.
3. Use clean fuels: CNG, LPG, and electric vehicles produce less pollution.
4. Industrial filters: Factories should install scrubbers and filters.
5. Recycle and reduce: Less manufacturing means less pollution.
6. Avoid burning garbage: Use proper disposal methods.

Exam Focus (3 marks): 'Suggest three measures to reduce air pollution in cities.'

(1) Promote the use of public transport and carpooling. (2) Plant more trees along roads and in open areas. (3) Encourage the use of electric vehicles and clean fuels like CNG.

7. Self-Test

1. What is the composition of air by percentage?
2. State two uses each of: (a) Nitrogen (b) Oxygen (c) Carbon dioxide.
3. Name the layer of the atmosphere: (a) Where weather occurs (b) That contains the ozone layer.
4. List four causes of air pollution.
5. What is acid rain? How is it formed?
6. Differentiate between global warming and ozone depletion.
7. Suggest two ways individuals can help reduce air pollution.

8. Answers to Self-Test

1. Nitrogen 78%, Oxygen 21%, CO2 0.03%, Argon 0.93%, other gases 0.04%.
2. (a) Nitrogen: fertiliser manufacture, food preservation. (b) Oxygen: respiration, combustion. (c) CO2: photosynthesis, fire extinguishers.
3. (a) Troposphere (b) Stratosphere.
4. Vehicle emissions, factory smoke, burning of fossil fuels, construction dust.
5. Acid rain is rain containing acids formed when SO2 and NOx react with water vapour in the atmosphere.
6. Global warming: increase in Earth's temperature due to greenhouse gases. Ozone depletion: thinning of the ozone layer due to CFCs.
7. (1) Use public transport or carpool. (2) Plant trees. (3) Avoid burning garbage.