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.


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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