13.2. Liquids http://www.ck12.org
Notice that the temperature dependence of the vapor pressure is not linear. From 0°C to 80°C, the vapor pressure of
water increases by 46.73 kPa, but it increases by another 53.99 kPa in a span of only twenty degrees from 80°C to
100°C.
Watch a video lecture about vapor pressure at http://www.khanacademy.org/science/chemistry/states-of-matter/v/
vapor-pressure.
Watch a simulation of vapor pressure at http://www.dlt.ncssm.edu/core/Chapter10-Intermolecular_Forces/Chapte
r10-Animations/VaporPressure.html.
You can participate in an online lab, theDumas Molar Mass Lab, that determines the molar mass of a volatile liquid
at http://www.youtube.com/watch?v=0UJXa9Hd88I&feature=player_embedded. The document that accompanies
this lab is found at http://www.dlt.ncssm.edu/core/Chapter7-Gas_Laws/Chapter7-Labs/Dumas_Molar_Mass_web_0
1-02.doc.
Boiling Point
As a liquid is heated, the average kinetic energy of its particles increases. The rate of evaporation increases as more
and more molecules are able to escape the liquid’s surface into the vapor phase. Eventually, a point is reached when
the molecules all throughout the liquid have enough kinetic energy to vaporize. At this point, the liquid begins to
boil. Theboiling pointis the temperature at which the vapor pressure of a liquid is equal to the external pressure.
The figure below (Figure13.12) illustrates the boiling of a liquid.
FIGURE 13.12
In the picture on the left, the liquid is below its boiling point; yet, some of the liquid evaporates. On the right, the
temperature has been increased until bubbles begin to form in the body of the liquid. When the vapor pressure inside
the bubble is equal to the external atmospheric pressure, the bubbles rise to the surface of liquid and burst. The
temperature at which this process occurs is the boiling point of the liquid.
Thenormal boiling pointof a liquidis the temperature at which the vapor pressure of the liquid is equal to the
standard atmospheric pressure of 760 mmHg. Because atmospheric pressure can change based on location, the
boiling point of a liquid changes with the external pressure. The normal boiling point is a constant because it is
defined relative to a standard pressure (760 mmHg, or 1 atm, or 101.3 kPa). The boiling points of various liquids
can be illustrated by vapor pressure curves (Figure13.13). A vapor pressure curve is a graph of vapor pressure as