0.17 Phase equilibrium and intermolecular interactions  (Page 3/7)

Observation 2: vapor pressure of a liquid

Our previous observations indicate that, for a given pressure, there is a phase transition temperature for liquidand gas: below the boiling point, the liquid is the only stable phase which exists, and any gas which might exist at that point will spontaneously condenseinto liquid. Above the boiling point, the gas is the only stable phase.

However, we can also commonly observe that any liquid left in an open container will, under most conditions,eventually evaporate, even if the temperature of the liquid is well below the normal boiling point. For example, we often observe thatliquid water evaporates at temperatures well below the boiling point. This observation only seems surprising in light of thediscussion of above. Why would liquid water spontaneously evaporate if liquid is the more stable phase below the boiling point? Weclearly need to further develop our understanding of phase transitions.

The tendency of a liquid to evaporate is referred to as its volatility : a more volatile liquid evaporates more readily. To make a quantitativemeasure of liquid volatility, we slightly modify our previous cylinder-piston apparatus by adding a gauge to measure the pressureof gas inside the cylinder. ( Here is an illustration.) We begin with liquid water only in the cylinder with an appliedpressure of 1 atm at a temperature of 25 °C. We now pull back the piston by an arbitrary amount, and then we lock the piston inplace, fixing the volume trapped inside the cylinder. We might expect to have created a vacuum in the cavity above the liquidwater, and as such we might expect that the pressure inside the cylinder is small or zero.

Measuring vapor pressure

Although there was initially no gas in the container, we observe that the pressureinside the container rises to a fixed value of 23.8 torr. Clearly, the observation of pressure indicates the presence of gaseous waterinside the container, arising from evaporation of some, but not all, of the liquid water. Therefore, some of the liquid water musthave evaporated. On the other hand, a look inside the container reveals that there is still liquid water present. Since both aliquid phase and a gas phase are present at the same time, we say that the liquid water and the water vapor must be in phase equilibrium . The term equilibrium in this case indicates that neither the vapor nor the liquid spontaneously converts into theother phase. Rather, both phases are stable at equilibrium.

Very interestingly, we can repeat this measurement by pulling the piston back to any other arbitraryposition before locking it down, and, provided that there is still some liquid water present, the pressure in the container in everycase rises to the same fixed value of 23.8 torr. It does not matter what volume we have trapped inside the cylinder, nor does it matterhow much liquid water we started with. As long as there is still some liquid water present in the cylinder at equilibrium, thepressure of the vapor above that liquid is 23.8 torr at 25 °C.