Is hot water heavier than cold water?

06 March 2011


Is hot water heavier than cold water? I was wondering this while watching ice float in my drink.


Chris - Hot water is actually a little bit heavier than cold water because, as Einstein told us, E=mc2. So if E, the energy in the water, goes up because it's hotter, then mass, m, must also go up to keep the equation balanced [c, the speed of light in a vaccuum, doesn't change]. So there will be a very subtle and very tiny increase in mass of the hot water, compared to the cold water.

The reason the ice floats is actually because it's a lot less dense than the water. The ice is made of water but, because water expands when it freezes, the ice is pushing a bigger volume - and hence a bigger mass - of water out of the way than the ice itself weighs. For this reason the ice is actually feeling a bigger push "up"(called buoyancy) from the water underneath than the ice weighs itself, which makes it float.


Photons are light. Not heat.
It is well known that cold water is heavier because the molecules are more densely packed than hot water. This answer is incorrect. The correct answer is available from many sources.

If you add energy to a system then the mass increases: E=mc^2; the information provided is correct.

Imagine a water molecule and an anti-water molecule interacting – basically a matter anti-matter interaction. Each respective water molecule would be transformed into heat (photons). Thus mass is made up of photons. So add more photons – add more mass. A hot water molecule has more mass (weight) than a cold water molecule because its electrons will absorb photons (mass) when heated.

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