# Vapor Pressure

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I don't get it. What exactly is it?

My questions:

1. Why is it that the greater the vapor pressure the more volatile something is?

2. Why is it that the boiling point of something is when its vapor pressure equals that of the atmospheric pressure?

Thanks!

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The vapor pressure of a liquid (simply put) indicates the tendency of the liquid's molecules to escape - the higher the vapor pressure, the more they escape (the partial pressure of the gas form of the liquid becomes higher). Usually, the escaped particles condense back into liquid.

When the vapor pressure equals atmospheric pressure, the liquid particles have enough energy to turn into a gas without condensing back.

I'm simplifying a great deal, but that's my (limited) understanding, and you can use that to work out the rest:

http://en.wikipedia.org/wiki/Vapor_pressure

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• 3 weeks later...
I don't get it. What exactly is it?

My questions:

1. Why is it that the greater the vapor pressure the more volatile something is?

2. Why is it that the boiling point of something is when its vapor pressure equals that of the atmospheric pressure?

Thanks!

This troubled me too, for a long time. So, simply imagine that all matter, whether solid, liquid, or gas, is evaporating, some slowly, some rapidly. Mercury metal, for example, evaporates slowly at room temperature; this is one reason why open mercury barometers have all but disappeared. I've seen a photograph of the mercury vapor "cloud" above a pool of mercury.

Gasoline exhibits a practical example of vapor pressure which makes it's presence noticed. A warmed can opened produces an audible "whoosh", as the higher pressure inside propels the gaseous product outward.

"Volatile" is a rather qualitative term; it depends a lot on temperature, which in turn determines the vapor pressure. imp

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Cap'N's simplified explanation is pretty much spot on. Vapor pressure is simply a measurement of the pressure exerted by the vapor of a liquid. Volatility is a measurement of how readily a liquid turns into vapor. So if you know those two things you can understand how a substance with a higher vapor pressure will be more volatile than something that isn't. The substance with the higher vapor pressure has more of itself in the vapor phase exerting a pressure on the air around it. This means that more of it is evaporating at a given temperature than something which has a lower vapor pressure.

As you raise the temperature, you always increase the vapor pressure. This is because you give more energy to the molecules and atoms at the surface of the liquid to escape from the liquid and become vapor. When something is boiling, it's vapor pressure is equal to the atmospheric pressure which means that the atmosphere is no longer able to exert a force on the liquid which makes it stay liquid. The molecules/atoms now have plenty of energy to break free from the liquid and become vapor since the atmosphere above it is no longer stopping it.

Therefore, you can make a substance boil at a lower temperature by reducing the atmospheric pressure around it. This is why the boiling point of a liquid at a higher elevation is lower than that at a lower elevation. At the high elevations, the earth's atmosphere doesn't exert as great a force on the liquid so the liquid's vapor pressure equals that of the atmosphere at a lower temperature.

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