Question

Which is heavier cold water or warm water? If And why?

Answer 1

Both are of same mass technically.
But hot water is less dense and has more kinetic energy than cold water.

Answer 2

Cold water is denser than hot water. Therefore, it is cold water that is heavier than hot water (for equal volumes of water). Hot water is lighter than the same volume of cold water, not heavier.  

 

The reason for the decrease in density as the temperature increases is that the water molecules cannot get so close together at higher temperatures. The molecules have more kinetic energy and take up more space as they move around in the liquid.