Science Questions

Is hot water heavier than cold water?

Sun, 6th Mar 2011

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Question

Doreen, Werrington asked:

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

Answer

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 itís 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 itís a lot less dense than the water.  The ice is made of water but itís pushing a bigger volume of water out of the way, than the ice itself weighs.  For that reason, itís actually feeling a big push up from the water underneath it which makes it float, so that's the reason. 

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