# Why does hydrogen burn with a pop sound?

Our teacher made us to do an experiment with zinc granules and hydrochloric acid.

$$\ce{2HCl + Zn -> ZnCl2 + H2}$$

We collected the hydrogen gas that was evolved in a test tube and on ignition, it burned with a "pop" sound. Why does hydrogen show this behavior? Why doesn't it burn more quietly like other fuels (gasoline, coal, wood, etc.)?

## 1 Answer

We may agree that during the combustion, regardless if the one of wood, gasoline, or in your example, hydrogen gas, is an exothermic reaction. In a simplified view, products of these reaction are gaseous water; in the processes burning organic matter varying relative amounts of carbon monoxide and carbon dioxide are to be considered, too.

The gaseous products formed take up a volume, much more than their starting materials in the condensed state (liquid, or solid). Because of the local heat, these gases aim to expand rapidly. In contrast to burning wood for a bonfire, the volume offered by a test tube is confined except for the mouth of the test tube, where a decompression to reach ambient pressure may take place. This small scale explosion initially occurs at the interface of air's oxygen at the mouth of your test tube filled with hydrogen, yet this frontier actually propagates up to the closed end of the test tube, too. Sometimes, especially with lengthier test tubes, this may be seen from the outside, too.

The combustion of hydrogen is worth a special note, because -- as correctly spot by airhuff -- formally there is volume contraction along

$$\ce{2H2 + O2 -> 2H2O}$$

which is more than compensated by the thermally caused expansion of the gaseous water.

Gasoline driven combustion engines work by the same principle; spark plugs locally ignite a mixture of finely dispersed fuel and air, triggering a rapid combustion (explosion) where the reaction products again aim to expand. As they operate at a larger scale as the test tube experiment, the generated sound is much more intense, and these vehicles need a silencer, too.

While your bonfire equally yields $\ce{CO2}$ and water, these products may expand easily; there is no pressure generation, hence there is no sudden expansion (at least, not at this scale), and hence we may perceive the bonfire as (in comparison to the other examples) as peacefully calm.

I suggest to take a view on videos about explosions of hydrogen gas, and an appealing demonstration how it looks in the inner of such an Otto engine here. In the later example (the display of the combustion starts around 2:51 min and again at 6:00 min), gasoline was replaced and a high speed camera looks through a special lid, yet the example still shows nicely the propagation of this frontier of explosion.

• Big +1 for the first video illustrating the rapid expansion of the explosion (plus it's just really cool). The only nitpick I have is the first sentence of the second paragraph about the expansion due to the phase change. In the case of burning $\ce{H2}$ and $\ce{O2}$, we're converting gas to gas, and contracting from 3 moles down to 2 at that (2 hydrogens plus one oxygen going to just 2 waters) so I don't think that's relevant. But the rest of that paragraph hits it; the rapid thermal expansion in a confined area. Anyway, nice answer. Apr 23 '17 at 23:41
• @airhuff Your comment is completely justified. Had I written the chemical equation first... Apr 24 '17 at 12:31
• Presumably the reason the explosion is not bigger is that the explosive chain reaction depends on exact pressure/temp and composition. If you want to try a demo on this. Find a thin tin used for say granulated coffee, make a 2mm hole in lid and one in base for a rubber tube. Place on retort stand, fill with hydrogen from cylinder until you are sure its full, a minute or so, then remove tube close cylinder and get cylinder well away. Light hydrogen at hole in lid. Will burn then go out as it explodes. Lid is light , flies off, no damage. Have done this is lectures several times. Great fun. Apr 24 '17 at 12:42