|MadSci Network: Chemistry|
Well, actually, your desk IS evaporating but at a very slow rate. A little background on definitions: Melting is a process of going from a solid (like your desk) to a liquid. Evaporation is going from a liquid to a gas (or vapor - same thing) and sublimation is going directly from a solid to a gas without ever being a liquid. Pretty much everything can be made to melt or evaporate. Only some materials can be made to sublime. All that is needed is enough energy (which we usually apply as heat) to overcome the forces that hold the molecules of the material together. Now, when some materials are heated, they decompose before they can be heated enough to melt, evaporate, or sublime. Wood is a good example - if you heat it enough, it either starts to burn or just turns black. Other materials, such as metal can be heated until the forces holding them together in a rigid structure are overcome. Then the individual molecules can move apart and the material is seen to melt to a liquid. Additional heat gives the molecules enough energy to "fly" out of the liquid and evaporate into a gas. (In sublimation, molecules remain a solid until the gain enough energy to pass into a gas directly). The fact that you don't see solids evaporate or melt before your eyes is only due to the fact that we live in a pretty mild environment. At any time, a few molecules on your desk probably have enough energy to "fly" out of the solid into a gas but it is too few to notice. BUT, if you were sitting on the surface of the Sun, there would be so much energy that the molecules in the solid of your desk would quickly melt then evaporate, or sublime (depending on the material).
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