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Q & A: Hot and Cold Water

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Q:
Does hot water and cold water have the same amount of molecules? Are they the same?
- Room 15
Lusher School, St. Louis, Missouri
A:
The number of molecules in some water isn't affected by changing its temperature. But to know if you have the same number of molecules in two water samples, you need to compare their masses, not their volumes. When you heat some cool water, it expands. (When you slightly warm some ice-cold water, it contracts a little, but let's keep this simple.) So hot water takes up more space than cold water. The amount of space something takes up is called volume. If you were to have 2 cups of water, one of hot water and one of cold water with equal volumes, the cold water would have more molecules.

Hot and cold water are made of the same type of molecules. Each molecule has one oxygen and two hydrogen atoms. The difference between them is the speed of the molecules jiggling around.

Jason (w Mike)

(republished on 07/27/06)

Follow-Up #1: why wash in warm water?

Q:
I understand that cold water and warm water are pretty much the same in terms of volume and molecules etc. So why are we supposed to wash our hands with warm water instead of cold water? Whats the difference?
- Kathy (age 12)
WIlliston, Vermont, U.S.A
A:
Although cold and warm water are pretty similar, there are some ways that temperature makes a big difference. Many materials can dissolve better in hot water than in cold water. The basic reason is that it often takes some energy to pull a molecule away from other similar molecules for it to go into water. The hotter things are, the more often you get that much energy. Also, chemical processes (including dissolving) almost always happen more quickly at high temperature. Often for a molecule to go from one place to another or from one form to another it has to go through a state of higher energy, like going over a little bump. Again, the hotter things are the more often that happens.

Mike W.

(published on 03/05/11)

Follow-Up #2: water volume change on heating

Q:
why is it that molecules in a cold water is more compressed than warm water?
- jerry anne marie tabigne (age 13 years old)
maramag bukidnon mindanao, philippines
A:
When the water is hot, the molecules are rattling around faster. They don't spend as much time tightly stuck to each other, and spend more time a little bit more separated. So they take up more room.

Water is unusual, however, because when you cool it below 4C, it starts to expand when you cool it further. It happens that there is a special way that the molecules can fit nicely together (with low energy) that happens to space them a little farther apart than most ways of fitting together almost as well. When water turns to ice, the molecules line up in that special way, and the ice takes up more room than  the liquid.

Mike W.

(published on 03/05/11)

Follow-Up #3: Is hot water denser than ice?

Q:
Did I am right to say the water take more volume at high temperature (exemple: 60C), when it cold down to 5C it take less space, and after this when it freeze it retake again more volume ? If yes did it have a equal volume at some time between the freeze water volume compare when it is hot?
- Jean (age 15)
Montreal, Canada
A:
As it turns out, the density of liquid water (at one atmosphere pressure) at 100C only drops 4.2% below the density at 4C. The density of ice drops about 9% below that. So the ice takes up more volume than the liquid throughout that range.

Mike W. 

(published on 09/15/11)

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