system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does running cold water sound different from running hot water?
gpt
The differences in density and viscosity are the causes. 

Cold water will have a higher pitch due to the stronger surface tension and hot water will have a lower pitch due to the weaker surface tension breaking up the water before it hits the glass or whatever you may be pouring it into. 

That's why coffee or something warm will have a low sound and a cold glass of water will sound higher.