MadSci Network: Chemistry

Re: What are some things that affect evaporation?

Date: Tue Dec 5 14:32:44 2000
Posted By: Greta Hardin, Secondary School Teacher, Science
Area of science: Chemistry
ID: 975636535.Ch

Affecting Evaporation:

Part of the reason you are having trouble finding things on evaporation is that the information you are interested in is most often found under the heading of "Vapor Pressure" or "Boiling Point." After all, evaporation is just really, really slow boiling - liquid water turning into gaseous water (water vapor). The boiling point is when ALL the liquid water will turn into gas. However, at other temperatures some other limited amount of the total water in a container will evaporate... as I am sure you are aware. How fast the water will evaporate depends on something called "vapor pressure." Functionally, this is the amount of water vapor above the liquid water. The warmer the water, the higher the vapor pressure right above the water. This is why warm water evaporates faster than cold water.

You can change the boiling point of water by adding things to it (sugar, salt, or anything else that mixes with water - rubbing alcohol, cocoa mix, and other sorts of things). All these mixtures will boil above the normal boiling point of water (around 100 C or 212 F depending how far above sea level you live, and what the weather is like). But wherever you live, and what ever the weather, these mixtures will boil at higher temperatures than plain water. As a result, they will also evaporate more slowly than plain water. (The alcohol mixture will actually do some other odd things, so you probably want to stay away from it for right now. But feel free to ask around about this situation.)

The reason this happens has do with the fact that when you mix stuff with water, the surface of the water is no longer all water. That means that the water can't evaporate at its normal rate, or boil at its normal temperature. As a result, it will evaporate more slowly, and boil at a higher temperature.

Hope this answers some of your questions, and gives you some better places to look!


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