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Why does water boil from heat?
Question Date: 2004-02-05
Answer 1:

Molecules have a tendency to interact with one-another, which causes them to stick to one-another. Heating up molecules causes them to vibrate (heat is a measure of vibration), and the more violently they vibrate, the more likely they will break free of their interactions.

Molecules of water, or anything else, at any temperature, have a likelihood of breaking away from interactions ('evaporating') and a likelihood of re-attaching ('condensing').

The chance of a molecule evaporating rises with temperature, and the chance of condensing rises with its abundance in a gas. Thus, liquids and solids maintain a 'vapor pressure' of the same substance all around them at all times, a concentration of vapor such that the rate of molecules condensing equals that of evaporation.

If, however, the temperature is high enough that the vapor pressure is above the ambient atmospheric pressure, then the vapor will expand out into its surroundings (lowering its abundance in doing so). This runaway evaporation is what we call 'boiling'.

Answer 2:

Your question why does water boil from heat is an interesting one.

What heating water really does is provide thermal energy. When water gains enough thermal energy it changes phases, from liquid to vapor. When it changes into the vapor phase, it forms pockets of water vapor in the liquid. Because of the differences of density between the liquid and vapor the vapor rises as bubbles. The movement of the bubbles then displaces the water. When enough of this is occurring you get boiling.

Why the water changes phases, I can only answer, that just sort of is the way the universe works.

Hope that helps.

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