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Originally Posted by Yakk
I calculated how much energy it would take to turn a 10 m cube of ice into water vapour. Somewhere around 700 calories/gram.
I then divided by c^2, and got the mass of that energy. Remember, all energy has mass.
Thus, if you take a block of ice, and turn it into water vapour, the resulting water vapour will mass more (and hence be heavier) than the original block of ice, but only by a very small amount.
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That is based on the assumption that all the energy is contributed to an increase in mass, which is wrong. Energy does not get converted to mass in a simple phase change. Any energy you put into the system goes into the thermal energy in the water to increase temperature and boil it. But that energy does NOT go into a mass increase.
I still maintain that all energy does NOT have a mass. Take a 1kg block and drop it from a height of 1m. When it hits the ground it will have approximately 9.8Joules of energy, but the energy does not go into increasing the mass.
Likewise, an exothermic reaction releases energy due to a difference in Gibbs free energy. The difference in energy in the chemical bonds that make up the molecules of the reactants and products is responsible for this. But no mass is converted into energy or vice versa.
I'll agree to the statement that a given amount of energy can be associated with a given mass, according to E=mc^2. But energy does not have mass.