water

Sure.

Water, as other substances, expands with temperature increase.

Therefore, its density becomes lower as does its weight for the same volume.

Water also expands with temperature decreases.

It doesn't water-down my answer, as:

- Water is the only substance where the maximum density does not occur when solidified. As ice is lighter than water, it floats.

- Water is the only substance on Earth that exists in all three physical states of matter: solid, liquid and gas.

You have to be careful what you actually mean by this question. The mass of a particular amount of water does not change due to temperature. However, the density changes.

If you take two beakers of water, each with the same number of molecules in them, then the weight will be the same. If you then alternately heat and cool the two samples, they will still weigh exactly the same (mass). However, the will have different densities (assuming they are not under pressure) and if you allow them to interact, the cooler water will sink to the bottom and the warmer water will rise. This is not because of mass, but because of density and displacement.

Interestingly, once the water gets really cold and freezes it will become less dense than water and float. This is because the water molecules are spread out differently in a frozen state than in a liquid state.

If by "heavier" you mean "mass", then the answer is No.

If by "heavier" you mean "relative density", then the answer is Yes (assuming the samples are under the same pressure conditions).

not sure if this helps, but this is my favourite dissertation on density

George McFly: Lorraine, my density has bought me to you.
Lorraine Baines: What?
George McFly: Oh, what I meant to say was...
Lorraine Baines: Wait a minute, don't I know you from somewhere?
George McFly: Yes. Yes. I'm George, George McFly. I'm your density. I mean... your destiny.

Yeah, sometimes people are a bit dense and sometimes they carry weight..