water mass

[Ratlady]

Hi guys!

I'm trying to find an answer to a simple 'Did U Know' statement.

The statement is 'Did you know: hot water is heavier than cold water?'

Now it's all very well me quoting that to a 10-year old girl & impressing her with my knowledge - what if she says 'Why?'?!?!

My field of scientific interest is relativity & quantum mechanics, so I could explain it by way of the Einsteinian effects of the hot water molecules' increased speed resulting in increased mass, but I somehow suspect that's a teensy bit too complicated!

Any of you brainy people know the answer to this one? Any of you fancy making something up?? As long as it sounds good I don't care!

I'll pop back in now & then & see if anyone's thought of anything.

Thanks![/i]

[THF]

I'd imagine the hot water molecules would have more energy, bounce around more and take up more space in the volume making it lighter than the denser cold water. The hot water would lose more molecules to evaporation as well, which would have a slight effect. I don't think relativity would come into it until molecules would be travelling at speed of light which is only seen in electrons in heavy metals such as thallium

[RobJim]

You say 'hot water is heavier than cold water'. What do you mean by that? Do you mean per unit volume? Or do you mean per molecule? Or what?

I am guessing you mean per molecule, based on the idea that each molecule has more energy in hot water and energy is equivalent to mass. To a little girl though it's going to seem more intuitive to think by volume, in which case the hot water is less heavy. Also, I don't know if heat adds weight, even if it equivalent to mass.

If you're going by energy is equivalent to mass, the only way I can imagine trying to explain that would to be to tell the little girl that the warmth has weight.