Ever wondered when you turn on the cold water tap followed by the hot water tap why there is a notable difference in sound (assuming flow rate and pressure are more or less the same)?
It’s actually to do with the viscosity (or thickness) of the water at different temperatures. Hot water is less viscous than cold water because its molecules are moving around more rapidly (think of the water in a kettle bubbling) and therefore hot water is less dense than cold water.
This difference in density has an effect on the sound it makes when poured. Try it next time you pour yourself a glass of water or a cup of coffee. You will probably notice that hot water is more high-pitched.
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