Is it possible to distil a mixture of solids and liquids

In a distillation, a liquid is boiled in the "distilling flask," then the vapors travel to another section of the apparatus where they come into contact with a cool surface. The vapors condense on this cool surface, and the condensed liquid called the "distillate" drips into a reservoir separated from the original liquid.

In the simplest terms, a distillation involves boiling a liquid, then condensing the gas and collecting the liquid elsewhere. Put your pot on the stove. Procedure Measure out and pour one cup of the fruit juice into a glass. Have a look at its color and take a small sip to taste it. Is it very sweet? How does the color look; is it very intense? Keep the rest of the juice for comparison at the end.

Pour an extra cup of colored fruit juice in the bottom of the pot. Your small ceramic plate or ceramic coffee cup will now be standing in the juice. Together with your adult helper, turn on the stove to medium heat and bring the juice to a boil. It should be a moderate rather than a rolling boil. Can you see the steam developing once your juice starts boiling? Now place the cover on the pot, upside down, so that the tip of the sloping lid is facing toward the bowl placed inside the pot.

What happens to the steam once you close the lid? Put ice in the cover of the pot. You might have to replace the ice in the lid as it melts. If you use a transparent lid, can you see droplets forming on the inward-facing side of the lid? Where do they come from and what happens to the droplets? Allow the juice to boil for 20 to 30 minutes, making sure some juice always remains in the bottom of the pot. Do you see any changes in the amount of juice inside the pot?

After 20 to 30 minutes, turn the burner off. Allow the pot to cool for a few minutes. Put on oven mitts and carefully remove the cover from the pot. What do you notice about the empty bowl that you placed under the lid? Still wearing hot mitts, lift the bowl off the small ceramic plate or coffee cup and set it down on a heat-resistant surface. Remove the small plate or coffee cup. Looking at the remaining juice in the pot, is there more or less juice left than the amount you poured in?

After it cools, pour the remaining juice from the pot into a glass. Did the juice change during boiling? What is different? Pour the cooled distillate the condensed steam , which is now the liquid inside the small bowl, into a glass.

How does the distillate look? Now take the glass from the beginning with the original juice, and place it next to the remaining juice and distillate. Compare their appearances. How do they differ? Did you expect these results? Why do you think the juice changed the way it did? How much fruit juice is left compared with what you poured into the pot? Let the liquids cool to room temperature. Because you used clean kitchen utensils and edible fruit juice in this experiment, go ahead and take a sip of each of the solutions.

How do the three different liquids compare in taste? At this temperature and pressure, the water would begin to boil and would continue to do so.

It is not possible to achieve a vapor pressure greater than 1 atmosphere in a container left open to the atmosphere. Of course, if we put a lid on the container, the vapor pressure of water or any other substance for that matter would continue to. Elevation of the boiling point with increase in external pressure is the principle behind the use of a pressure cooker.

Elevation of the boiling point with an increase in external pressure, while important in cooking and sterilizing food or utensils, is less important in distillation. However, it illustrates an important principle that is used in the distillation of many materials. If the boiling point of water is increased when the external pressure is increased, then decreasing the external pressure should decrease the boiling point. While this is not particularly important for the purification of water, this principle is used in the process of freeze drying, an important commercial process.

In addition, many compounds cannot be distilled at atmospheric pressure because their boiling points are so high. At their normal boiling points, the compounds decompose. Some of these materials can be distilled under reduced pressure however, because the required temperature to boil the substance can be lowered significantly. A nomograph is a useful device that can be used to estimate the boiling point of a liquid under reduced pressure under any conditions provide either the normal boiling point or the boiling.

Figure 2. A nomograph used to estimate boiling points at reduced pressures. To use, place a straight edge on two of the three known properties and read out the third. Column c is in mm of mercury. An atmosphere is also equivalent to To use the nomograph given the normal boiling point, simply place a straight edge at on the temperature in the central column of the nomograph b. Rotating the straight edge about this temperature will afford the expected boiling point for any number of external pressures.

Simply read the temperature and the corresponding pressure from where the straight edge intersects the first and third columns. Using the nomograph in Figure 2 and this temperature for reference, rotating the straight edge about this temperature will afford a continuous range of expected boiling points and the required external pressures necessary to achieve the desired boiling point.

Although all of us have brought water to a boil many times, some of us may have not realized that the temperature of pure boiling water does not change as it distills.

This is why vigorous boiling does not cook food any faster than a slow gentle boil. The observation that the boiling point of a pure material does not change during the course of distillation is an important property of a pure material. The boiling point and boiling point range have been used as criteria in confirming both the identity and purity of a substance.

Of course, additional criteria must also be satisfied before the identity and purity of the liquid are known with certainty. You will use both of these properties later in the semester to identity an unknown liquid. Occasionally, mixtures of liquids called azeotropes can be encountered that mimic the boiling behavior of pure liquids. These mixtures when present at specific concentrations usually distill at a constant boiling temperature and can not be separated by distillation.

The azeotropic composition sometimes boils lower the than boiling point of its components and sometimes higher. Mixtures of these substances at compositions other than those given above behave as mixtures. Returning to our discussion of boiling water, if we were making a syrup by the addition of sugar to boiling water, we would find that the boiling point of the syrup would increase as the syrup begins to thicken and the sugar concentration becomes significant.

Unlike pure materials, the boiling point of an impure liquid will change and this change is a reflection of the change in the composition of the liquid. In fact it is this dependence of boiling point on composition that forms the basis of using distillation for purifying liquids.

We will begin our discussion of distillation by introducing Raoult's Law, which treats liquids in a simple and ideal, but extremely useful manner.

Figure 3. The apparatus used in a simple distillation. Note the position of the thermometer bulb in the distillation head and the arrangement of the flow of the cooling water.

This relationship as defined is capable of describing the boiling point behavior of compound A in a mixture of compounds under a variety of different circumstances. Although this equation treats mixtures of compounds in an oversimplified fashion and is not applicable to azeotropic compositions, it does give a good representation of the behavior of many mixtures.

Let's first consider a binary system 2 components in which only one of the two components is appreciably volatile. Raoult's law states that the observed vapor pressure of water is simply equal to the product of the mole fraction of the water present and the vapor pressure of pure water at the temperature of the mixture. Once the sugar-water mixture starts to boil, and continues to boil, we know that the observed vapor pressure of the water must equal one atmosphere.

Water is the only component that is distilling. Since the mole fraction of water in a mixture of sugar-water must be less than 1, in order for the observed vapor pressure of water to equal one atmosphere, must be greater than one atmosphere.

As the distillation of water continues, the mole fraction of the water continues to decrease thereby causing the temperature of the mixture to increase. Remember, heat is constantly being added. If at some point the composition of the solution becomes saturated with regards to sugar and the sugar begins to crystallize out of solution, the composition of the solution will become constant; removal of any additional water will simply result in the deposit of more sugar crystals.

During the course of the distillation, the water vapor which distilled was initially at the temperature of the solution. Suspending a thermometer above this solution will record the temperature of the escaping vapor.

Cooling below this temperature will cause most of the vapor to condense to a liquid. This is why the distillate is frequently chilled in an ice bath during the distillation. The distillation of a volatile material from non-volatile is one practical use of distillation which works very well. However, often there may be other components present that although they may differ in relative volatility, are nevertheless volatile themselves.

Let's now consider the two component system you will be using in the distillations you will perform in the laboratory, cyclohexane and methylcyclohexane. The vapor pressures of these two materials in pure form are given in Table 1. As you can see from this table, although cyclohexane is more volatile than methylcyclohexane, the difference in volatility between the two at a given temperature is not very great.

This means that both materials will contribute substantially to the total vapor pressure exhibited by the solution if the distillation is carried out at 1 atmosphere. The total pressure, P T , exerted by the solution against the atmosphere according to Dalton's Law of partial pressures, equation 2, is simply the sum of the observed vapor pressures of cyclohexane, , and methylcyclohexane, :.

As before, boiling will occur when the total pressure, P T , equals an atmosphere. However since we have two components contributing to the total pressure, we need to determine the relative contributions of each. Again we can use Raoult's Law but we need more information about the system before we can do so. In particular we need to know the composition of the solution of cyclohexane and methylcyclohexane. For ease of calculation, let's assume that our original solution has equal molar amounts of the two components.