Can you tell the difference between hot water and cold water by listening?

04 October 2016

Question

Can you tell the difference between hot water and cold water being poured by listening?

Answer

We put Sam's question to Caroline Steel...

Chris - Now just a bit of background because this does sound rather odd like we've all gone mad. But, actually the idea here is that you can tell apart hot and cold water just by the sound when you pour them. Now I'll ask Caroline about the science behind this in a minute, but let's give you at home a go and also everyone here in the studio an opportunity to try this.

So what I have here is I have got two empty glasses. So here's one, here's the other. I've also got two filled glasses and one of them has cold water and the other has hot water. And I'm going to pour one of the glasses at a time into one of the empty glasses and I want you to listen to the sound and we'll see if we can pick up a difference.

I'm just going to move the microphone because, for obvious reasons, I don't want to demolish the studio. Here we go, so this is sample number one. Are we ready, here we go?

[Pouring]

Okay - keep that in mind.

Okay - here is sample number two.

[Pouring]

Right. did we notice. Judith you're a physicist aren't you - did you notice any difference?

Judith - I have to say I don't think I did. I'm not sure I was supposed to but I don't think I did.

Chris - Caroline?

Caroline - I think the first one made a slightly higher pitch noise.

Chris - So, therefore, would you think that was the hot one or the cold one?

Caroline - I think that was the hot one would be my guess.

Chris - Would you concur with that Heidi - what do you think?

Heidi - I could hear the pitch was slightly higher on the first one.

Chris - You are right, the first one was the hot one but now tell us why that should be the case?

Caroline - This is because hot water is slightly less viscous than cold water, so it's a bit more runny. And that's because molecules of water have boiled and they're hotter than the cold water, move around faster, so they form slightly smaller droplets. So that when you pour hot water, these slightly smaller droplets, as they splash against the container or against the water that's already in the container, they make a slightly higher pitch noise. So if you were to say pour custard, you'd expect it to sound a bit lower and it makes bigger droplets which make kind of a more low sound. But, obviously, the difference between hot and cold water is fairly small so I'm really surprised that we managed to hear that one, especially as there are so many factors.

Chris - Max?

Max - What about the difference between hot and cold custard?

Caroline - Well I imagine hot custard is - well we know that don't we. Hot custard is more runny than cold custard so it would make a slightly higher pitched noise.

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