How fast water freezes or melts depends on lots of things in the
immediate environment, such as how hot or cold the surroundings are and
how good the thermal conduction is between the water and the hot or
cold surroundings.
Water requires approximately 80 calories of heat energy per gram
in order to melt (that's calories with a little "c" -- Calories with a
big "C" are food calories and are 1000 of the little-"c" calories).
Water gives off 80 calories per gram when it freezes. So to freeze
water quickly, you need to remove this heat energy quickly. You can put
it in a very cold freezer with cold air circulating in it. To slow the
rate of freezing, you could put it in a styrofoam container. Styrofoam
will also slow down the rate of melting of water once it is frozen --
heat energy travels very slowly through styrofoam, and more quickly
through other substances. Copper is a very good conductor of heat
energy.
Another factor is the surface area of the water to be frozen or
melted. If you pour the water in a thin layer on a very cold copper
surface, it will probably freeze immediately, provided that the copper
is big enough and cold enough. If you grind up ice into a powder (or
better yet, get some snow) and sprinkle it in boiling water, it will
melt immediately. If you drop an ice cube of the same mass into the
boiling water it may last a bit longer.
Feel free to experiment with different situations to find out what makes water freeze or melt faster or slower.
Tom
(published on 10/22/2007)
