Why does hot water go up and cold water

go down?

Any object or substance that is less dense than a fluid will float in that fluid, so hot water rises (floats) in colder water ... Any object or substance that is more dense than a fluid will sink in that fluid, so cold water sinks in warmer water. Hot air rises and cold air descends for exactly the same reason.

Step-by-step explanation:

Any object or substance that is less dense than a fluid will float in that fluid, so hot water rises (floats) in colder water ... Any object or substance that is more dense than a fluid will sink in that fluid, so cold water sinks in warmer water. Hot air rises and cold air descends for exactly the same reason.