Chemistry
Clouds
Thermodynamics and Statistical Mechanics

Why does the temperature remain constant during melting and boiling although heat is being absorbed from the surroundings?

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2007-06-21 07:32:08

Heat is being added to ice as it melts, but that heat is going

into turning the water from a solid into a liquid (breaking

intermolecular bonds), rather than increasing the temperature. This

is also true when it boils, and that is why a boiling pot of water

won't reach a higher temperature than 100 °C.


When something changes phase from solid to liquid, or from liquid

to gas, it takes energy to break the intermolecular interactions.

These interactions between the water molecules are what make it

solid. When you have ice, these interactions are strongest, which

is why ice is hard. Then when you have water, the interactions are

not as strong, and although the water still "stays together" it is

now a liquid and moves and flows freely. Then when all the

interactions are broken, it become a gas, or steam, and now none of

the water molecules are attached to any other molecules. Whenever

it goes through a phase change like this, the energy goes into

breaking up these interactions, and so the temperature stays

constant until all the interactions are broken. Once all the ice is

melted, or all the water has turned to steam, then any added heat

will act to raise them temperature again.


See the Web Links to the left for more information about phase

changes.


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