This demonstration should accompany an introduction to the properties of pure liquids and solids. It is a good way to show that different liquids have different equilibrium vapor pressures at a specified temperature.

Two mercury-filled open ended manometers with quick disconnects on hoses
Filter flasks (250 mL, 500 mL, 1000 mL) with rubber stoppers configured with rubber septa (see figure) and quick disconnects
Several 35 mL plastic hypodermic syringes with needles
Several 100 mL beakers
100 mL samples of methyl and ethyl alcohol and water
2000 mL beaker with ice/water mixture (optional)

Figure. Apparatus for Demonstrating Liquid Vapor Pressure

Begin by filling a hypodermic syringe with 20 mL of methyl or ethyl alcohol (The choice of alcohol should depend on the temperature of the lecture hall. On a very warm day, methyl alcohol may have too high a vapor pressure and will blow off the stopper in the flask) and injecting the liquid into the apparatus. The pressure will rise rapidly to an equilibrium value. It is very useful to explain this process at the molecular level. Initially there are no alcohol molecules in the void space above the surface of the newly injected liquid. Molecules escape from the surface and enter the gas phase contributing to the increase in measured gas pressure in the system. Soon the rate at which molecules are recaptured (condense) by the liquid surface will be the same as the rate at which they evaporate. This is the idea of dynamic equilibrium in a liquid-vapor system.

Repeat the demonstration with a different liquid like water. The same rapid pressure rise will be seen, but the equilibrium vapor pressure will be different from that of methanol. Explain to your calss that this property of a liquid is as characteristic as the density, melting point, or boiling point.

Two more properties of equilibrium vapor pressure can be shown with the apparatus as it is set up. First, equilibrium vapor pressure is dependent on the temperature of the liquid. This can be shown qualitatively by dunking a filter flask containing either the methyl alcohol or the water into a container of ice. The equilibrium vapor pressure in either case will drop. Show your class what a more detailed graph of vapor pressure versus temperature looks like. This is also a good place to bring in the meaning of the boiling point of a liquid, i.e., the temperature at which the equilibrium vapor pressure equals the atmospheric pressure on the surface of the liquid.

The second property of liquid vapor equilibrium is surprising to most people, namely, that the equilibrium vapor pressure of a liquid is independent of the volume of the container as long as both liquid and vapor are present. This can be shown by repeating the vapor pressure demonstration using methyl alcohol in filter flasks of different sizes, e.g., 250 mL, 1000 mL, or even 2000 mL. The equilibrium vapor pressure will be almost (see discussion) identical in each case.

DISCUSSION

The pressure registered by methyl alcohol and water will depend on the classroom temperature when you perform the demonstration. On a particulary hot day the pressure of the methyl alcohol may be so great that the stopper in the filter flask blows out. Be sure the stoppers are inserted solidly.

With different size filter flasks the measured pressures will not be exactly identical because of the air trapped in the container. Its volume will be compressed slightly by the addition of the liquid, and for gases the pressure does depend on the volume of the container (Boyle's law). The discrepancies will be smallest if larger filter flasks are used. The origin of the small pressure differences might be an intriguing exercise for your brighter students to ponder.

HAZARDS AND DISPOSAL

Don't stab yourself with the hypodermic needles!