http://www.ck12.org Chapter 13. States of Matter
a function of temperature. To find the normal boiling point of a liquid, a horizontal line is drawn from the left at a
pressure equal to the standard pressure. The temperature at which that line intersects with the vapor pressure curve
of a liquid is the boiling point of that liquid.
FIGURE 13.13
The boiling point of a liquid also correlates to the strength of its intermolecular forces. Recall that diethyl ether
has relatively weak intermolecular forces, so the liquid has a relatively high vapor pressure at a given temperature.
The weak forces also mean that it does not require as large an input of energy to make diethyl ether boil, so it has a
relatively low normal boiling point of 34.6°C. Water, with its much stronger hydrogen bonding, has a lower vapor
pressure than diethyl ether at any given temperature, and it has a higher normal boiling point of 100°C.
As stated earlier, boiling points are affected by external pressure. At higher altitudes, the atmospheric pressure is
lower. With less pressure pushing down on the surface of the liquid, it boils at a lower temperature. This can also be
seen from the vapor pressure curves. If one draws a horizontal line at a lower vapor pressure, it intersects each curve
at a lower temperature. The boiling point of water is 100°C at sea level, where the atmospheric pressure is close to
the standard value. In Denver, CO, at 1600 m above sea level, the atmospheric pressure is only about 640 mmHg,
so water boils at about 95°C. On the summit of Mt. Everest, the atmospheric pressure is about 255 mmHg, so water
boils at only 70°C. On the other hand, water boils at temperatures greater than 100°C if the external pressure is
higher than the standard value. Pressure cookers do not allow the water vapor to escape, so the total pressure inside
the cooker increases. Since water now boils at a temperature above 100°C, the food cooks more quickly.
The effect of decreased air pressure can be demonstrated by placing a beaker of water in a vacuum chamber. At a
low enough pressure, about 20 mmHg, water will boil at room temperature!
Watch a video demonstration showing the relationship between vapor pressure and concentration at http://www.y
outube.com/watch?v=PXYorrMu0Mw.
Watch a video demonstration of the effect of changing pressure on the boiling point of water at http://www.youtu
be.com/watch?v=Cshd5MVGpfk (1:23).