TOPIC: Solids and Liquids

Demo-011

Phase Diagrams: Liquid-Vapor Equilibrium

A sealed 25 mL plastic syringe can be used to show an interesting aspect of liquid-vapor equilibrium in a phase diagram.

MATERIALS

A 25 mL plastic syringe sealed at one end and mounted in a wood block (see figure below).

Distilled water and some food coloring

Approximately one foot of wire (not too thin...use 12 or 14 gauge)

Figure. Plastic syringe apparatus for vapor-liquid equilibrium demonstration.

 

Set up the syringe filled with colored water in front of your class and explain its construction. Make sure to emphasize the fact that there is no air between the syringe plunger and the surface of the water. Hold the apparatus firmly with one hand and slowly raise the syringe plunger above the liquid surface. Ask the class what gas or vapor if any occupies the space between the plunger and the water surface.

The answer is that only water vapor is present in this space. Look up or state the vapor pressure of water at the temperature of your classroom and compare this to the atmospheric pressure acting on the top of the plunger. Ask your class what will happen when you let go of the plunger.

When released, the plunger will crash back down on the surface of the liquid water, and the space occupied by vapor alone will disappear. Explain that this is because of the difference in pressure between the atomosphere and the water vapor. Also point out that the vapor pressure of a liquid does not increase when its volume is diminished. All that happens is that vapor turns to liquid.

DISCUSSION

On a water phase diagram the point representing 1 atm pressure and 25°C temperature falls in the middle of the liquid region (sse figure below). This means that under that specific pressure and temperature, the only stable form of the substance is the liquid form; vapor cannot exist at that pressure and temperature. This seems counter to common experience because we all know that air contains lots of water vapor. Where is the fallacy? The answer is that the phase diagram presumes the absenceof air or any other gas; this condition is not met in a room full of air and water vapor.

Figure. Water phase diagram showing 1 atm, 298 K point.

This demonstration reproduces the conditions implied by the phase diagram for water. If the plunger rests squarely on top of the liquid surface with no air present, only water vapor can occupy the void space when the plunger is drawn upward. You will find that this requires considerable effort to accomplish. You are working against a downward pressure of roughly one atmosphere (depends on the day's barometric pressure and your elevation) opposed only by the relatively puny vapor pressure of water at room temperature (approximately 0.05 atm). When you release the plunger, it convincingly crashes down on the liquid surface again and no trace of vapor remains visible Only liquid water can exist at 1 atm pressure and room temperature (no air present). Case closed.

HAZARDS

The author cannot imagine much in the way of hazards for this demonstration.

 

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