That depends on the starting temperature. If the water starts at 99
degrees Celsius, it takes only 100 calories to raise the temperature up
to 100 degrees Celsius (assuming no evaporation). The amount of heat
required to change one gram of water's temperature (roughly one
millileter, but water expands when it heats up) by one degree Celsius
is one calorie. If you started at 25 degrees Celsius (a decent room
temperature), it would take 7500 calories to do the job.
100 degrees Celsius is a very special temperature for pure water,
as that's the temperature at which water boils at one atmosphere of
pressure. If you are doing the experiment at just under one atmosphere
of pressure (say in Denver), you may never be able to heat the water to
100 degrees Celsius because the boiling temperature is reduced a bit --
all the water would boil away before you got the temperature up. It
takes 540 calories per gram of additional heat to turn the water into
In addition, if you are doing this in an open pot, say, then just
plain evaporation will rob the water of some of its heat (and also some
of its water!).
If you started with ice, then it takes an additional 80 calories per gram of water to melt the ice.
(published on 10/22/2007)