weight of water

[sonic81]

why is hot water heavier than cold water???

[wildwood]

You've got it the wrong way around..... I think.

The molecules in hot water are moving about like crazy (agitated) so take up more space resulting in less mass per pint.

[dundurn]

Not quite. Cold water is more dense than warmer water down to 4 degrees C at which point it begins to get less dense again. This is because below this temperature water starts to develop a crystalline structure which is fully developed at 0 degrees C as water transitions to ice. It's because of this decrease in density that ice floats.

[hailthechimp]

Remember sonic,hot water is still losing mass through evaporation so it would be less in weight then its cold counterpart.

[dundurn]

All water loses mass due to evaporation. Even ice can sublimate without going through a water phase. Don't confuse loss of mass with reduction in density - they are quite different things. Imagine a sealed balloon completely filled with warm water and placed in a cold bath. It can't lose mass through evaporation but will nevertheless float on the surface of the bath because it is less dense than its surroundings.

[Rev. Green]

You need to say whether you are comparing the same volume or the same number of "molecules" of hot and cold water.

Hot water is usually lighter than cold water for the same volume ( but see Dundurn's remark about 4 degrees).

Hot water is very, very slightly heavier than cold water for the same number of "molecules" because of the mass of the extra heat.

(I've written molecules within quotes because liquid water forms more complex structures than the familiar molecule comprising two hydrogen atoms and one oxygen atom.)

[Teddio]

A vague question. A "hot" water molecule will have more vibrational kinetic energy then a "cold" water molecule and will, therefore have more mass by virtue of its extra energy.

[Bert]

Have you been reading one of those lists of "stuff you didn�t know" on the internet, sonic? Most of that stuff is b0ll0cks, and stuff you didn�t want to know in the first place. A kilogram of hot water is definitely heavier than 100 grams of cold water (stating the obvious), but hot water is generally less dense than cold water except as stated above. The explanation for that, by the way, is that the crystal structure of ice is quite "open". Shortly before it freezes, water starts to take on this "open" structure, even though it is still completely liquid (as dundurn said). Please see my earier question "Why is there so much rubbish on the internet?" which I started with exactly your question about water. Another thing, when other posters have mentioned the mass of the energy added to the hot water, we are talking about incredibly small, almost undetectable, amounts of mass. It�s the Einstein equation, E=mc^2.