Why do we hear a difference between hot and cold water? It’s all down to the frequency of the sound. There are three sources of sound during pouring: the resonance of the air, the vibration of the container and water as a whole, and the water sounds (including bubbles). Hot and cold water both produce these same frequencies but with different strengths. When cold water is poured the dominant sound is the vibration of the container and water, whereas when hot water is poured the resonance of the air is the dominant sound.

OK, but why is there a difference in the strength of these frequencies? Most people think the difference is due to the viscosity (thickness or gloopiness) of the water. Believe it or not cold water is 5 times more viscous than hot water. This affects the size of the water bubbles produced, the more viscous the solution the larger the bubbles and the lower the resonant frequency (the natural vibrating frequency of an object). It is also possible that the steam produced by the hot water has an impact on the frequency of the air in the container. Either way, there is definitely room for future research!

To find out more, don’t forget to listen to our episode on the science behind Blue Planet II.