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Q & A: Heat capacity of water

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Most recent answer: 10/22/2007
Q:
How much energy is required to heat 100 milliletres of water to 100 degrees celsius?
- Aimee
Great Lakes College, Australia
A:
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.

BUT:

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 vapor.

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.

Tom

(published on 10/22/2007)

Follow-Up #1: water evaporates

Q:
If water boils at 100 degree celsius, how does it evaporate at room temperature about 20 - 25 degree celsius?
- Chalryne
memphis, tn
A:
Water molecules are always flying off the liquid surface into the gas, and crashing back into the liquid from the gas. The rate of leaving the liquid increases a lot as the liquid is heated. When the boilng point is reached, more molecules leave the surface than come back in even when the gas (at standard pressure) consists of nothing but water molecules. Then even water vapor bubbles in the liquid grow, and the liquid quickly boils away. Even far below the boiling point, water molecules are leaving the surface. If the air is fairly dry (low humidity) and if there's a little breeze to mix the water vapor with the air, that evaporation rate will be bigger than the rate of water moleculs coming back into the liquid. That's what usually happens around  room temperature, 20-25C.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.