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Science

Preparing gases and testing for them

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Gases such as hydrogen, oxygen, carbon dioxide, ammonia and chlorine can be made using chemical reactions. These gases are collected in different ways, depending upon their density and solubility in water. They can be identified using different tests.

Upward and downward delivery

Upward delivery

Upward delivery

If a gas is less dense than air, it is often more convenient to collect it in a gas jar or test tube by upward delivery. The gas produced in a chemical reaction is passed through a delivery tube into the gas jar, where it rises and takes up the space at the top of the jar - pushing the air in the jar down, and out at the bottom.

This works well for hydrogen and ammonia, which are both less dense than air. The equipment must be used in a fume cupboard when ammonia is collected, because ammonia is harmful and has a sharp, choking smell.

Downward delivery

Downward delivery

If a gas is denser than air, it is often more convenient to collect it in a gas jar or test tube by downward delivery. The gas produced in a chemical reaction is passed through a delivery tube into the gas jar, where it sinks and pushes the air out of the top.

This works well for carbon dioxide and chlorine, which are both denser than air. The equipment must be used in a fume cupboard when chlorine is collected, because chlorine is toxic and has a sharp, choking smell.

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