Separation of components from a liquid mixture by distillation depends on the
differences in volatility of the individual components.
What makes a component volatile? The answer lies in the vapor pressure of the
component. What do we know about vapor pressure?
(1) A liquid with a higher vapor pressure will boil at a lower temperature.
(2) A liquid begins to boil when the vapor pressure of a liquid equals the pressure in
in the room. Figure 1 shows the vapor pressure of water at different temperatures.
As discussed in the animation, the vapor pressure increases with temperature,
which is why warmer water will evaporate more quickly.
The vapor pressure of a component is the equilibrium pressure exerted by molecules
leaving and entering the liquid surface. To say it in another way, vapor pressure is
a measure of the tendency of a material to change into the vapor state.
Let's use water as an example.
Suppose you place a small amount of water in a bowl in a room. If you come back in
a day, the bowl is dry. All of the water has evaporated.
How fast the water will evaporate can be measured by the vapor pressure.
As the temperature increases, the vapor pressure increases.
Consequently, warmer water will evaporate faster from the bowl.
Next, suppose we place two bowls in the room, one containing a small amount
of water and the other containing an equal amount of ethanol. If we return to the
room in a few hours, we see that some of the water remains, but all of the ethanol is
gone. This happens because the vapor pressure of ethanol is higher than water and
so it evaporates more quickly. This is what we mean when we say that ethanol is more
volatile than water. In order to understand how distillation works, we also need the
concept of equilibrium. The animation illustrates the concept of equilibrium using
the example of evaporating water. The equilibrium condition is a function of
temperature. In the animation, hotter air can hold more water vapor than colder air.
Consequently, the relative humidity can change with just a change in temperature,
even if the amount of water in the air remains the same.
This explains why you may see dew on the grass on a chilly morning.
Let's return to our example of water in a bowl inside a closed container.
If we place a large bowl of water in the container, will it continue to evaporate?
Not necessarily. Within the pool of water, many molecules are continually escaping
from the liquid into the container air, while some water vapor molecules return
to the liquid. At some point equal numbers are escaping and returning.
This is the condition of equilibrium.
At this equilibrium condition, we usually say that the air in the container is saturated.
If we add any more water vapor to the saturated air, the water will condense
back to the liquid state until equilibrium is again reached.
This is what happens in the atmosphere when it rains.
Relative humidity, which is often called just the humidity, is the measure of how
saturated the air is with water vapor, expressed as a percent.
At 100% relative humidity, the air is saturated.
Next, we need to explore the concept of equilibrium with a mixture.
In a mixture, of the vapor pressure of each component is different than for the
pure substance. In a mixture, the component with the higher vapor pressure
is called the light component, and the component with the lower vapor pressure is
called the heavy component. The lighter component will have a greater tendency to
evaporate because of the higher vapor pressure. Let's use our example of a liquid
mixture of water and ethanol.
If we place a bowl of the mixture inside a closed container, molecules of both water
and ethanol will enter the vapor because of the vapor pressure of each component.
At equilibrium, how much of each component will be in the vapor mixture?
The answer depends on the relative vapor pressure.
Since the vapor pressure of ethanol is higher than water, ethanol is more volatile
than the water. Therefore, ethanol molecules will want to escape from the
liquid in a higher ratio than water. At equilibrium, the concentration of ethanol
in the vapor will be higher than the concentration of ethanol in the liquid
mixture. Since the vapor has a higher concentration of ethanol, the liquid must
have a lower concentration than before any of the ethanol evaporated.
As a result, the liquid has become less concentrated in ethanol.
The vapor pressure of each component in the mixture will change with the
concentration of each component. This means that the composition of the
vapor mixture in equilibrium with the liquid mixture will change depending on the liquid
composition. We can measure the vapor composition in equilibrium for a number of
different liquid mixture compositions. The result is the equilibrium curve.
The equilibrium curve for water and ethanol is shown for different ethanol
concentrations. If the fraction of ethanol molecules is zero in the mixture, the liquid
is pure water, and the fraction of ethanol in the vapor is zero.
If the fraction of ethanol molecules is 100% in the mixture, the liquid is pure ethanol and
the fraction of ethanol in the vapor is also 100%. For all of the mixtures in between,
the equilibrium curve tells us what the vapor concentration is. For the example on the
previous page, this curve will tell us what the vapor concentration of ethanol is.
Recall that the liquid mixture is 25% ethanol and 75% water.
If we find this point on the curve we see that the concentration of ethanol in the vapor
is 56%. The water concentration is just the difference between the ethanol
concentration and 100% since the two concentrations must add up to 100.
So the concentration of the water in the vapor is 44%.