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Gases In The Air | Grade 7 Science Lesson

The gases found in the air are nitrogen, oxygen, carbon dioxide, noble gases and water vapour.

Water vapour is sometimes not included in the composition of air. This is because its proportion varies. In humid countries, there is a higher percentage of water vapour in the air.

Balancing oxygen and carbon dioxide

The composition of these gases remains almost constant. The air will not run out of oxygen and carbon dioxide. This is due to the oxygen and carbon dioxide cycle.

Plants and animals use oxygen during respiration. During plant growth the rate of photosynthesis is higher than the rate of respiration. This way, plants produce more oxygen than they use. Carbon dioxide is released by animals and plants during respiration. Carbon dioxide from the atmosphere is used up by plants during photosynthesis.

Properties of gases

The gases in the air have many properties. These properties make them useful in many ways in our daily life.


Oxygen is the second most abundant gas in the air and is essential for life. Oxygen has no smell. In other words, it is odourless. It is also colourless. It supports life and supports combustion also.

When a glowing splint is inserted into a test tube containing oxygen, it bursts into flames (rekindles). This can be used as a test for the presence of oxygen.

In 1774, an English chemist called Joseph Priestly first isolated a gas which he called ‘super air’. When he breathed in this gas, he felt relaxed and light-headed. This ‘super air’ was oxygen!

Carbon dioxide

Carbon dioxide is the densest gas in air. It is used by plants for photosynthesis. It is odourless, colourless and does not support combustion. When fossil fuels are burnt, they produce carbon dioxide. Due to the increased use of fossil fuels, the percentage of carbon dioxide in the air has increased.

When carbon dioxide is passed through limewater (calcium hydroxide), it forms a white precipitate called calcium carbonate.


Nitrogen is an unreactive gas. It is abundant in the air. Nitrogen is odourless, colourless and does not support combustion.


It is the lightest of all gases. Hydrogen is colourless and odourless. It is insoluble in water, highly flammable and burns in air to produce water vapour. When zinc reacts with hydrochloric acid bubbles of hydrogen gas are given off. Other metals such as magnesium and iron can also react with acids to produce hydrogen.

Noble gases

Noble gases are also known as inert gases: helium, neon, argon, krypton, xenon and radon are the noble gases. These gases are unreactive, colourless and do not support combustion.

Water vapour

The percentage of water vapour in the air varies with temperature and pressure. Water vapour is odourless and colourless. Water or its vapour will turn blue cobalt chloride paper to pink.

Anhydrous copper sulfate is a white powder. It turns blue when water is added, it will also turn blue slowly when exposed to air, proving that air contains water vapour.

Uses of gases in the air

Oxygen is the most important of all gases in the air. Oxygen supports combustion. It is also used during respiration by plants and animals. When gases burn in pure oxygen, they burn at very high temperatures. Such temperatures are often high enough to melt most metals.

Carbon dioxide is present in the air only in small amounts, but it has many uses. Carbon dioxide is used in photosynthesis, fire extinguishers, the food industry and in the making of dry ice.

Nitrogen is generally unreactive. It is used as a preservative in the food industry and to make fertilizers.

Noble gases are inert, so they have many different uses in daily life. Helium is lighter than air and is used in airships and party balloons. Neon is used in advertising signs because it is able to produce bright and colourful lights. Argon gas is used in bulbs and it prevents the filament from burning.

Hydrogen is produced on a large scale because it has a variety of industrial and commercial uses. When hydrogen burns, it produces large amounts of energy. Hydrogen, as a fuel, has the advantage of burning with no pollution products as its only by-product is water. Hydrogen, being the lightest gas, is also ideal for weather balloons that go high up in the atmosphere. Hydrogen is used in the manufacture of margarine from vegetable oils. The oil becomes a solid when hydrogen is bubbled through at a certain temperature in the presence of a nickel catalyst.