[Shirley]

Shirley asked the Naked Scientists:
When first filling my bathtub with water I can tell when the water coming out changes from cold to warm because the sound of the water coming out changes.  Why does the sound change when the water temperature changes?
What do you think?

[Titanscape]

Because the hot water heats the metal pipe, and it expands, changing the vibrations. Deeper tone. Wider aperture.

[chris]

This is all to do with viscosity. Hot water is less viscous (sticky) than cold water, so when it comes out of the tap it makes a higher-pitched splash than water at a lower temperature.

You can demonsrate this for yourself at the kitchen sink. Take two mugs of the same size and fill one with hot water and the other with cold.

In turn, pour the mugs into the empty sink from the same height and listen to the sound the water makes as it hits the sink bottom. The hotter liquid will sound "splashier" because it is less sticky.

Chris

[lyner]

I'd go for Chris' answer rather than titanscape's. The coeff of expansion of copper is tiny and wouldn't alter resonances much.

[Chrisglen]

I don't agree with the viscosity answer. The sound in the piping itself changes not only the splash.