14.10 A Liquid-Vapor Equilibrium Demonstration

Chemical Concept Demonstrated: Temperature's effect on the equilibrium between a liquid and its vapor

Demonstration:

The three round-bottom flasks contain bromine vapor. The first flask is at room temperature. The second flask is immersed in a ice-water bath. The third flask is immersed in a hot-water bath. Compare the intensity of the color in the flasks and their temperatures.

Observations:

    The heated flask has a thicker vapor inside of it than the flask at room temperature.  The cooled flask has a thinner vapor inside of it than the flask at room temperature.

Explanation:

    Heating the flask increases the vapor pressure of the liquid inside of it.   When the vapor presure increases, the equilibrium between the liquid and the vapor shifts to favor the vapor.  Therefore, the heated flask has more of the brown vapor floating about the inside of it relative to the other flasks.

    Cooling the flask decreases the vapor pressure of the liquid.   When the vapor presure decreases, the equilibrium between the liquid and the vapor shifts to the liquid.  Therefore, the cooled flask has less of the brown vapor floating about the inside of it relative to the other flasks.