Science Questions

Why does hot water sound different to cold water when poured?

Dear Chris, Hello, my name is Tommy Czaczkes, and I am a PhD student at the University of Sussex. My question is; why does boiling water sound different to room temperature water when poured into a mug? Yours, Tommy Czaczkes Tommy Czaczkes

Chris -   If you've noticed, when you turn the shower on the morning, the cold water comes through from the pipe first and it will splash and sound different against the bottom of the shower compared with when the hot water, which comes along shortly afterwards, comes in; the note will change.  This is a real observation; your ears aren’t deceiving you.

The reason for it is that water changes its viscosity - its stickiness - according to its temperature.

If you could zoom in with a really powerful microscope and look at some water molecules, what you’d see is they are shaped like miniature boomerangs.  At the apex of the boomerang you would see an oxygen atom and on each of the arms, you'd see hydrogen atoms. 

Oxygen loves electrons, so it pulls the electrons of itself and the hydrogen towards itself very tightly, and that makes the oxygen a bit minus. The hydrogens are correspondingly therefore a bit plus. 

As a result, when water molecules are sitting together side by side in solution, the positively charged hydrogens are attracted to the negatively charged oxygens of an adjacent molecule and this is called hydrogen bonding.  It makes water sticky, and it gives it some of its special properties that in fact help it to make life happen on Earth. So it’s pretty important that this happens. 

But, when you heat the water up, the particles start to move much more quickly.  They have more kinetic energy which is a function of the temperature.  This means that they're zipping past each other much faster.  They're therefore gluing onto each other less well and this makes the water runnier or less viscous. 

So when it comes splashing out of the shower and hits the shower pan, the water fragments into smaller particles and makes a higher pitched splashing noise than when it goes into the cup or goes into the sink when it’s cold. 

Have a listen next time you're in the bath for the shower and you will see that the note is different.

March 2010