What is the test for hydrogen gas?

The test for hydrogen gas is characterized by the distinctive "pop" sound it makes when ignited. When holding a burning splint at the open end of a test tube containing hydrogen, the gas reacts with the flame, producing a brief explosive sound. This reaction is a result of hydrogen being a highly flammable gas that reacts with oxygen in the air when ignited. This method is commonly taught in chemistry demonstrations to confirm the presence of hydrogen, as it is specific and easily observable.

The other options do not accurately indicate the presence of hydrogen gas. For example, holding a lighted match to observe if it burns does not provide clear evidence of hydrogen, as many gases may catch fire, but they won't create the specific popping sound. Additionally, injecting a cotton swab into the gas does not provide a reliable or safe method for testing, and placing a lit candle in the gas does not guarantee the distinct reaction indicative of hydrogen. Therefore, the auditory cue from the pop is a unique and definitive test for hydrogen.