Boiling point

[dumbboy]

The boiling point of water is 100 deg c at sea level,so what is evaporation ??does the water boil off and vanish ??? PLEASE ,just simple answers in leymans terms,not too complicated !Im only a bloke !

[newtron]

Pure water at equilibrium with atmosphere at a specific temperature has a specific vapor pressure , meaning a proportion of the water will be present as water vapor in the air. At equilibrium , the rate at which the water is evaporating equals the rate at which water vapor is precipitating (becoming liquid water). As the temperature increases, the amount of water present as water vapor also increases . When water is at its boiling point, that means that the vapor pressure equals atmospheric pressure (all the water becomes vapor). When the water is not in equilibrium, it will evaporate untill it reaches equilibrium (untill the specifc proportion of water is present as water vapor). In an open system where water vapor is leaving the system (i.e. wind trasporting water vapor from the system), equilibrium may never be reached, resulting in evaporation of all the water. I hope this helps.

[newtron]

Sorry, I read my answer, and I think I should clarify something. The vapor pressure of water is the proportion of gases in the atmosphere in contact with the liquid water that is water vapor at equilibrium, not a proportion ot he liquid water. In other words, at a specific temperature, air can hold a specific amount of water. If liquid water is present, it will evaporate untill the air cannot hold any more water vapor.

[jake-the-peg]

Let's see if I can put this into simple terms

Think of it like this:

Boiling point is a bad word it doesn't mean the point that a liquid turns to a gas but rather the maximum temperature a liquid can get to (at atmospheric pressure) before it turns to a gas.

This is why we fry chips in oil, you can't get water hot enough, get to 100 degrees and it boils and the temperature doesn't go up any higher.

Now that doesn't mean that some of the liquid doesn't turn to gas at a lower temperature - think of all the molecules bumping around in the liquid - some are going slowly, some fast, the very fast ones can get to be fast enough to break free and "evaporate" off as a gas.

Because the faster ones escape like this the overall temperature of the remaining liquid falls. (faster is hotter). That's why evaporating sweat cools you down

There's an animation of the process here

http://www.educationusingpowerpoint.org.uk/ind ex.html?ks3science.html~mainFrame