Different chemical reactions can be used to test for common gases.
Hydrogen is produced by many different chemical reactions:
Hydrogen is less dense than air, so it is usually collected by upward delivery.
Pure hydrogen burns with a colourless or pale blue flame. When the test for hydrogen is carried out, the hydrogen needs to be mixed with a little air.
|Test for hydrogen||Result|
|Place a lighted splint in a test tube of the gas||Gas burns with a squeaky 'pop'|
|Test for oxygen||Result|
|Place a glowing splint in a test tube of the gas||The splint relights|
The test for chlorine can use either type of litmus paper, but blue litmus paper is used most commonly. The litmus paper must be damp - the water dissolves some of the chlorine so that it can react with the indicator on the litmus paper.
|Test for chlorine||Result|
|Place damp blue litmus paper in a test tube of the gas||The blue litmus paper is bleached|
This test shows that chlorine is a powerful bleach.
An alternative test for chlorine is to place damp starch-iodide paper into a test tube of chlorine. A positive result for chlorine is indicated by the paper turning from white to blue-black.