boiling point and vapor pressure

[donaldduck]

Why is that a liquid with a high vapor pressure has a low boiling point? Also, what effect does the atmospheric pressure have on the vapor pressure and boiling point of a liquid?

---

Members don't see this ad.

 

[phltz]

As you increase the temperature of a liquid, the vapor pressure will increase. When the vapor pressure is equal to the atmospheric pressure, the liquid will boil.

Think of it like this. Suppose there was a small bubble in the liquid, not too deep. The pressure in this bubble is essentially atmospheric pressure. If the vapor pressure is more than atmospheric pressure, then the vapor in the bubble is pushing the bubble out harder than the atmospheric pressure is pushing in. Thus, the bubble expands, and rises to the surface. This is exactly what you see if you look in a pot of water on the stove. There are little bubbles on the bottom. Eventually, the water gets hot enough that these start to expand and rise to the top.

 

[SoonerLuke]

phltz is correct. The way I like to think of it is this. When vapor pressure equals atmospheric pressure, the liquid boils.... thus, a substance with a higher vapor pressure is already "closer" to equaling the atmospheric pressure, so in essence you don't have to add as much heat (lower boiling point) to get the substance to be equal to the atmospheric pressure.

 

[donaldduck]

phltz is correct. The way I like to think of it is this. When vapor pressure equals atmospheric pressure, the liquid boils.... thus, a substance with a higher vapor pressure is already "closer" to equaling the atmospheric pressure, so in essence you don't have to add as much heat (lower boiling point) to get the substance to be equal to the atmospheric pressure.

aha! thanks man that makes sense!