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Q & A: Freezing and melting speeds

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Most recent answer: 10/22/2007
Q:
How long does it take the water to freeze? and how long does it toke to melt?
- David (age 11)
Sandowne Elementary, Waterloo. ON Canada
A:
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)

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