The Hot Chocolate Effect

By Anupum Pant

Have you ever tried tapping the bottom of a pan full of boiling soup? I do it all the time. And unless I hear that low hollow sound, I don’t consider the soup as cooked.

Tapping the bottom of a pan, with boiling soup in it, makes a significantly hollow sound. The frequency of sound that comes from such a tap seems to be much lower than what you’d actually hear if you tapped the bottom of a pan with same amount of cold still water. This effect has a name and is called the hot chocolate effect. Here is how it works.

Water is about 800 times denser than air. Also since air is 15,000 times more compressible than water, sound travels faster in water, than in air. Sound travelling faster in a medium (water in this case), creates a standing wave that has a higher frequency than a standing wave created in a column of some other medium which is less dense (like air).

Boiling liquid, or specifically boiling soup has a lot of air bubbles trapped in it. As a result, the average density of the liquid + air concoction decreases, and the compressibility becomes much higher. This makes the sound travel much slower in it.

So, the sound that comes from tapping the bottom of a container full of thick boiling liquid with a lot of air bubbles trapped in it makes that low-pitched sound. I find it extremely satisfying. Moreover, it is good to know that they  have a name for it!