Tests for gases

There are several different tests to detect and identify gases and the ions in compounds. It is important that the test for any gas or ion is unique. The results of a test must let you determine which gas or ion is present, rather being uncertain about which one it is.


Oxygen supports combustion. If oxygen is present in a test tube, a glowing splint relights when it is held inside.


Hydrogen ignites in air. If hydrogen is present in a test tube, a lighted splint held near its mouth ignites with a squeaky pop.

Carbon dioxide

Carbon dioxide reacts with calcium hydroxide solution to produce a white precipitate of calcium carbonate. Limewater is a solution of calcium hydroxide. If carbon dioxide is bubbled through limewater, the limewater turns milky or cloudy white.

Carbon dioxide in limewater
Carbon dioxide in limewater


Chlorine is an acidic gas that also acts as a bleach. Damp litmus paper is bleached white when it is placed in chlorine.

If damp blue litmus paper is used, the paper turns red then white.


Glowing splint held in a test tubeSplint relightsOxygen is present
Lighted splint held in a test tubePop sound heardHydrogen is present
Gas bubbled through limewaterLimewater turns milky or cloudy whiteCarbon dioxide is present
Damp litmus paper held in a test tubePaper turns whiteChlorine is present