Question

Why does hydrogen gas burns with a pop sound?

Answer 1

Impure Hydrogen gas burns with pop sound. The reason is the highly combustible nature of hydrogen.
Impure Hydrogen means, the laboratory prepared one, that is not purified. It contains traces of oxygen, nitrogen, and water vapour, which are the impurities.

When, a spark is introduced to impure H2 gas, a highly exothermic reaction occurs. Due to this a sudden temperature rise happens,and the gases expand on heating. This expansion occurs very fast, in milliseconds. This sudden expansion of these hot gases, casuses a compression on the surrounding gases, producing a sound wave, that we hear as a pop sound.

If hydrogen is 100% pure, still, if exposed to atmosphere, it becomes impure by diffusion. If you supply very small quantities of pure H2, there wont be any pop sound, as the gases expand much slowly, as hydrogen supply is low.