We hear a “pop” sound when a burning splinter gets extinguished by hydrogen gas because hydrogen gas is highly inflammable gas that ignites like an explosion. Another reason is that the ignition temperature of hydrogen gas is very low, so it does not required to be heated much, and as you know, air is a bad conductor of heat, the hydrogen gas remains cool. As soon as the high temperature of the burning splinter touches the hydrogen gas, it increases its temperature to more than its ignition temperature and readily ignites it resulting in an explosion.
And everyone knows that when something explodes in a medium like air, it produces a sound and the sound depends upon the amount and energy of the substance.
Hydrogen has a very high energy(calorific value) but as it is so light and less dense, there is not a huge amount of it that is used in the lab. So, it explodes but as it is in a less amount, it instead produces a “pop” sound.
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