[royden (OP)]

Royden Hodgskin asked the Naked Scientists:
   
When I turn on the shower 1st thing in the morning, I can hear when the hot water arrives from the geyser as it makes a slightly different sound to the initial water.

Why is this?

What do you think?

[Chemistry4me]

I wish I had any idea of what you are talking about. Unfortunately, I don't, so I'll take a guess. If I still have my facts correct [], the speed of sound in gas and liquid phase is dependent on their temperatures. The frequency of resonance is related to the length of the pipes and the speed of sound. So hence as the temperature of the water rises the frequency increases. Someone who knows physics will be able to tell you more.

[Chemistry4me]

Ah, I've found this link that may help: http://www.answers.com/topic/speed-of-sound#Dependence_on_the_properties_of_the_medium

[dentstudent]

There is also the fact that hot water and cold water have a different viscosity rating which changes according to the energy within the water, i.e., how hot it is. Hot water is more fluid than cold water - have you ever noticed this when pouring out boiling water from a kettle? It could be that the fluidity of the water changes the pitch of the sound as it leaves the shower head. Also the fact that the hotter water will probably minutely change the size of the holes in the shower head allowing a different volume of water to pass through, which will also modify the pitch of the sound.

[Chemistry4me]

Well, you're a more clever fellow than I. []
You notice this effect before Mr. dentstudent?

[dentstudent]

Yup. Also with cold cups of tea - the tea always looks "stickier" when it's cold than when it's piping hot!

[royden (OP)]

Thx, I think that explains the different sounds I'm hearing. Its most probably the showerhead holes changing size thats responsible.

[Bored chemist]

The change in the size of the holes in the showerhead will be very small. The water changes density a bit as it gets hotter, but not much.
The viscosity and surface tension change a lot with temperature and I think they affect the size of the drops that form. My guess is that those two effects are much more important to the sound than the size of the holes.