>> 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.