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Q & A: Evaporation

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Most recent answer: 10/22/2007
Q:
I am doing a science fair project on evaporation. I need some background information. Can you tell me where to look on the web? Can you give me any information that would help me? Thank you. ( I am a second grader)
- Spencer
A:
Spencer -

I'm not really sure what kind of information you're looking for, but here's some of what I know about evaporation:

There are three familiar states of matter: solids, liquids, and gases. If you heat up a solid, it will melt and turn into a liquid. (Some, like dry ice, will turn into gases without becoming liquids.) If you heat up a liquid, it will evaporate and turn into a gas.
 
To understand this, start with the picture that everything is made up of little things called atoms and molecules. The hotter something is, the faster these are moving.  When the molecules are moving faster, it gets harder for them to stay stuck together. In a solid, the molecules are stuck together very tightly in a regular pattern. In a liquid, they're moving around more, so they're able to slide around and break the regular pattern. If you heat that up more, they move so fast that they just fly apart as a gas.

Some molecules will fly off the liquid surface at any temperature. If there are enough molecules of the same type in the nearby gas, they can be replaced by new molecules coming in from the gas to the liquid. Whether the liquid slowly evaporates, condenses, or stays the same depends on the balance between those rates. When the liquid gets too hot and/or the pressure gets too low, the evaporation will beat the condensation even if the nearby gas is completely made up of molecules from the liquid, not diluted by air or anything else. Then every little gas bubble grows, and the liquid boils away.

If the molecules in something tend to stick together more, then it will be harder to make them evaporate, so the boiling point will be very high. For example, the boiling point of diamond is 4200 degrees Celsius (that's really high). If the molecules don't tend to stick together very much, then they will evaporate more easily, at a lower temperature. For example nitrogen and oxygen (in air) are gases at room temperature. Water is sort of in the middle - it boils at 100 degrees Celsius.

-Tamara(small mods, mw)

(published on 10/22/2007)

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