Why Does Hydrogen Make That 'Pop' Sound Upon Burning?

When two molecules of hydrogen combine with one molecule of oxygen, they undergo a chemical reaction to produce two molecules of water—and also release a lot of energy. This reaction is, therefore, exothermic in nature. Hydrogen is also called “water-former” because of this reaction. The energy released in this reaction is mostly in the form of heat. This heat imparts kinetic energy to molecules of the gaseous mixture and causes them to expand. The air surrounding the gases also starts expanding very rapidly. This sudden and quick expansion of gas molecules causes a break in the sound barrier. Thus, we hear a pop sound when hydrogen is burned due to the explosion.