For Jeff King: Boiling Over

>> Sunday, October 4, 2009

Jeff King asked: What makes water Boil? I know heat does, but what takes place to make the water react so violently on the surface?

This one's rather easy. Each material has three states: solid, liquid and gaseous. Generally, temperature is the key factor in which state a substance is at any one time: get it cold enough and anything will turn solid, get it warm enough and it will turn gaseous. Since the freeze point and boiling point vary from substance to substance, some are gaseous at "room temperature" (like oxygen), some are solid (like iron) and some are liquid (like water).

When you put heat into a substance, assuming you keep putting heat in indefinitely, it will eventually change state, but it take additional energy to do so. For instance, if you take an ingot of iron and heat it, it will increase in temperature proportional to its thermal properties and the heat put into the system. However, at melting point, it will go through a space where the temperature will not increase though heat continues to enter. This is is called "latent heat" and it's the heat required to change phases. Melting will occur only after this heat has been absorbed at which point the iron will melt and, again, increase temperature.

The same thing happens when one boils water. First the water must be brought to boiling temperature (100 deg C/212 deg F at sea level) and then more heat put into the system before the water changes state from liquid to gas. It is the transition from liquid to gas that causes the violence. Bubbles of gas are formed throughout the water (though likeliest at the bottom where the heat is generally coming in) and, as they are less dense than the liquid, the float to the top and are released at the surface. That surface violence is the result. Boiling water is, in theory, always exactly boiling temperature because all the heat put into it is used for phase change until you run out of water.

Contaminants can change boiling/freezing point (as salt does for water) as can air pressure. In vacuum, water will boil without additional heat. At low pressures, also, the liquid state can be bypassed entirely, a process called sublimation, where the gas is pulled off directly from the solid state.

Hope that answers your question.


  • The Mother

    I have banged the "latent heat" concept into my kids' brains since they were tiny.

    Ask them how often we watched water boil and they had to do the temperature conversions (in their heads). I'm a mean parent.

  • Jeff King

    I knew kind of what happened, but did not know exactly. So I figured I would ask I was cooking dinner for my kids (making spaghetti) and the question came to me and could not answer it satisfactorily.

  • flit

    A very good explanation, I think ... hmmm... maybe I should be asking you homework questions LOL

    How do you feel about Foucault?

  • Boris Legradic

    For even more violent boiling fun, you can try to get your water supercritical in your microwave. The phase-transition from liquid to gaseous needs a nucleation point, and clean water in a smooth glass offers none. So you can superheat your water below the boiling point (well, a bit), and when you the disturb the water it will flash into steam violently. (See for example here). Fun for the whole family, and also why you shouldn't heat water for your tea in the microwave...

    Also, as a plasma physicist I must protest against the discrimination of plasma in your post, the much-slighted fourth state of matter

  • Stephanie B

    Point taken on plasma, Boris, and I've seen those videos. But I don't want to encourage steam explosions (I'm a safety gal)!

    Mostly, in my world, we're trying to avoid the fallout from plasma (reentry, metal vapor arcing) and the like. So, I try to avoid it. Sorry about the omission.

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