…why does hot water, poured from a kettle*, sound so different to cold water poured from a kettle*?

I was making a brew this morning and due a distraction, didn’t turn on the kettle. I got my mug c/w teabag and poured in the water. I knew immediately by the sound, that the water was not hot.

An explanation would be bloody lovely.

*other vessels are available

What’s the difference in sound?

Hot water probably provides a different environment for sound wave propogation.

Could be a few things:

The heat from hot water causes the air (partially) trapped in the teabag to expand, which makes the bag float and so the water is hitting that, rather than the mug.

The heat will cause the mug to expand slightly, possibly causing high frequency noise as minor defects in the mug move against each other, which you subconsciously pick up on?

Is it to do with Brownian Motion?… or is that what keeps blocking trap 2 at work.

oh yeah – the bag inflates when hot water hits it, you can hear the air hissing out of it.

It’s something I’ve noticed as well. I can tell when the water running from our hot tap changes from cold to hot (we have a combi-boiler and the first few seconds run cold until the boiler fires up).

I believe what we are sensing (via sound) is the change in viscosity which accompanies the change in temperature.

What he said – as you say, you can hear that the water is different even in a sink (eg no tea bag).

More to the point, why does it freeze faster than cold water.

I can tell when the water running from our hot tap changes from cold to hot (we have a combi-boiler and the first few seconds run cold until the boiler fires up).

That could be noise resonating up and down the pipe changing as one end gets hot. I can’t tell the difference in our house, we have a hot water tank.

More to the point, why does it freeze faster than cold water.

It doesn’t, that’s an urban myth.

Or at least, it’s a misunderstanding. Hot water will cool at a faster rate than cold water as differential to the ambient temperature is greater. Its rate will slow as it cools, a bit like the graph above. Ie, it’ll cool faster to start with but it won’t freeze sooner than something that was colder to start with.

it won’t freeze sooner than something that was colder to start with

It can do.

http://www.theguardian.com/commentisfree/2012/jun/28/why-does-hot-water-freeze-faster

OP, it totally does! Would love to know why. Viscosity chart is interesting.

I also noticed the other day that after pouring a cuppa, the pitch of the teaspoon clinking against the cup rose – to a point – as the cup heated up.