Why is it unsafe to collect too much hydrogen in a test tube before testing it with a lit splint?

Hydrogen gas (`~H_2` ) is very explosive in air. The reaction for the combustion of hydrogen in air is:

`~2H_2` + `~O_2`  -> `~2H_2O`

Energy is released from this reaction in the form of heat, light, sound, and kinetic energy. The "pop" heard during a positive hydrogen splint test is a tiny explosion of hydrogen gas. If there is a high concentration of hydrogen gas in the tube, the amount of energy produced by the reaction will be greater. This may result in an unsafe explosion of hydrogen gas. 

The best "pop" is usually achieved from a mixture of hydrogen and oxygen that contains 20% - 40% hydrogen by volume.

Splint tests can also be used to test for other types of gases including oxygen, nitrogen, and water vapor.