What weighs more hot or cold water?

Answer 1

The question makes an erroneous assumption. Depending on what you mean by "hot", hot water is notheavier than cold water.

If we look at common household temperatures and you weigh one gallon of room-temperature water (~70ºF) and compare it to one gallon of hot water (~120ºF), you will find that the hot water weighs less than the colder water. That's because the warmer water is less dense -- there aren't as many water molecules per unit volume -- and fewer molecules weigh less than more molecules of the same substance.

On the other hand, water behaves a little differently than many other substances, in that if you cool it below it's point of maximum density (about 39ºF) it starts to become /less/ dense. So if you compare a gallon of water at, say, 34ºF to a gallon of water at 39ºF, the warmer water will weigh morebecause in this range it is more dense than the colder water.

But I'm not sure anyone would call water at 39ºF "hot". :)

Answer 2

Neither. They are both equally the same.