Good question! When ice melts, it changes from a solid (ice) to a
liquid (water). In order to make something change from a solid to a
liquid, you have to heat it up. That's because when you heat something
up, you give it more energy, which makes the molecules (the tiny little
things that make up stuff) move faster and faster. When the molecules
move faster, they have a harder time holding on to each other, so the
stuff becomes less solid and more liquid. (If you were to heat it up
even more, it would become a gas.) So to melt the ice, you have to add
heat.
So then the question becomes, if the air and the water are the same
temperature, why does the water heat up the ice cube more than the air
does? The reason for that is that water is a better "conductor" for
heat-energy than air is. What that means is that the heat can travel
through the water to reach the ice more easily than it can go through
the air.
(Heat can actually move through water more than 20 times as quickly
as it can move through air! The reason that the ice in water doesn't
melt 20 times faster than the ice in the air does is because the ice in
the air is probably sitting on something - like a glass plate, and
glass is an even better conductor than water is. And metal conducts
heat even better than that!)
-Tamara
(published on 10/22/2007)
