Which is heavier, hot water or cold water?

Cold Water is heavier. the same thing with air hot air rises and cold air sinks.

Hot water is less dense than cold water.
http://www.exploratorium.edu/science_explorer/watertrick.html

The colder water is the denser it gets, until you get down around 4 degrees celsius, then it gets less dense again. So, for example, water that is 2 degrees celsius is less dense than water which is 4 degrees, but more dense than water which is 7 degrees. So there is no absolute answer unless you specify that the water is well above freezing point.

http://www.simetric.co.uk/si_water.htm

Neither. One pound of cold water weighs the same as one pound of hot water. The difference is in the amount of space they take up. And water is odd. As Lieberkuhn says, water gets denser (more compact) down to 4 degrees, then starts expanding again. This is actually very important. If it didn't, all the ice would be down at the bottom of the sea. As it is, it floats. But in terms of weight, that doesn't change with temperature. Remember, a pound of feathers weighs the same as a pound of lead.....