Why do we hear a pop sound when hydrogen gas is burning ?

 Hydrogen gas is very much flammable . When we pass hydrogen gas through a lighted match stick , we hear a pop sound .

What is actually happening is hydrogen undergoes chemical reaction where it combines with oxygen in the air thus forming water in vapour form and enormous amount of energy .

This energy is in the form of heat . We know that heat causes the gases to expand . Since a lot of heat is generated  , the adjacent air expands so quickly that the gas molecules break the sound barrier .

If the process is done in a small way  ,then we hear just the pop sound . If it is carried in a big way  , then we can hear the bang sound .