What happens if you distilled to dryness

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. What we would like to determine is whether it would be possible to separate cyclohexane from methylcyclohexane by distillation. By separation, we would like to determine if it would be possible to end up with two receiver flasks at the end of the experiment that would contain mainly cyclohexane in one and mainly methylcyclohexane in the other.

It is clear that at some point we will need to intervene in this. Table 1. Vapor pressures of cyclohexane and methyl cyclohexane as a function of temperature. Otherwise, if we were to collect the entire contents of the original distilling flask, called the pot, into one receiver flask, we would end up with the same composition as we started. Initially the mole fractions of both cyclohexane and methylcyclohexane are 0. From Raoult's Law equation 1 , Dalton's Law equation 2 and the information in Table 1, we can estimate that boiling will occur at approximately K when the total pressure of the two components equals one atmosphere or The first thing that we should note is that the initial boiling point is higher than the lowest boiling component and lower than the highest boiling component.

Next, we should inquire about the composition of the vapor. Is the composition of the vapor the same as the initial composition of the pot or is it enriched in the more volatile component? If the composition of the vapor is the same as that of the original mixture, then distillation will not be successful in separating the two components.

However, we should ask, "What is the composition of the vapor? First we note that:. If the total vapor can be treated as an ideal gas, then according to Dalton's Law, so can each of the components. Since the two components are in thermal contact and are distilling together, we can expect them to be at the same temperature.

We don't necessarily know the volume of the container, but since it is assumed that the volumes of the molecules are very small in comparison to the total volume the gas occupies, whatever the value of V, it is the same for both components. This means we can establish the following ratio:. If we use the experimental values found in Table 1, we conclude that the composition of the vapor is 1. This simple treatment allows us to understand the principles behind distillation.

However it is important to point out that distillation is far more complex than our simple calculation indicates. For example, we just calculated the composition of the vapor as soon as the solution begins to boil and we have correctly determined that the vapor will be enriched in the more volatile component. This means that as the distillation proceeds, the pot will be enriched in the less volatile component.

Since the composition of the pot will change from the initial mole ratio and become enriched in the less volatile component; the new composition in the pot will introduce changes in the composition of the vapor. The composition of the vapor will also change from the initial ratio we just calculated to a new ratio to reflect the new composition of the pot. The consequences of these changes are that the temperature of both the pot and the distillate will slowly increase from the initial value to a value approaching the boiling point and composition of the less volatile component.

If we are interested in separating our mixture into components, we are left with the task of deciding how much material to collect in each receiver and how many receivers to use. Obviously this will depend on the quality of separation we are interested in achieving. Generally, the more receivers we use, the less material we will have in each. It is possible to combine fractions that differ very little in composition but this requires us to analyze each mixture.

While it is possible to do this, in general, we really want to end with three receivers, one each enriched in the two components of our mixture and a third that contains a composition close to the initial composition. It is difficult to describe how much material to collect in each receiver since the volume collected will depend on the differences in the boiling points of the components. Each fraction collected can be analyzed and those with compositions similar to the initial composition can be combined.

The main fractions collected can then be fractionated a second time if necessary. The experiment we have just discussed is called a simple distillation.

It is an experiment that involves a single equilibration between the liquid and vapor. This distillation is referred to as involving one theoretical plate.

As you will see, it is possible to design more efficient distillation columns that provide separations on the basis of many theoretical plates. Before discussing these columns and the advantages offered by such fractionating columns, it is important to understand the basis of the advantages offered by columns with many theoretical plates.

The following is a simplified discussion of the process just described involving a column with more than one theoretical plate. We have just seen that starting with a composition of , cyclohexane: methylcyclohexane, the composition of the vapor was enriched in the more volatile component. Suppose we were to collect and condense the vapor and then allow the resulting liquid to reach equilibrium with its vapor.

The properties of liquid 2 will differ from the original composition in two ways. First, since the composition of liquid 2 is higher in cyclohexane than the initial one; the temperature at which liquid 2 will boil will be lower than before what is the approximate boiling point of a 1. In addition, the composition of the vapor, vapor 2, in equilibrium with liquid 2 will again be enriched in the more volatile component.

This is exactly what happened in the first equilibration first theoretical plate and this process will be repeated with each new equilibration. If this process is repeated many times, the vapor will approach the composition of the most volatile component, in this case pure cyclohexane, and the liquid in the pot will begin to approach the composition of the less volatile component, methylcyclohexane. In order for this distillation to be successful, it is important to allow the condensed liquid which is enriched in the less volatile component relative to its vapor, to return to the pot.

In a fractional distillation, the best separation is achieved when the system is kept as close to equilibrium as possible. This means that the cyclohexane should be removed from the distillation apparatus very slowly.

Most fractional distillation apparati are designed in such a way as to permit control of the amount of distillate that is removed from the system. Initially the apparatus is set up for total reflux, i. Once the distillation system reaches equilibrium, a reflux to takeoff ratio of about is often used about 1 out of every drops reaching the condenser is collected in the receiver.

This fraction of distillate will be primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated.

Figure 1. Distillation apparatus. A distillation flask with a thermometer is placed in a heating mantle and is connected to a condenser. Figure 2. The tubes on the condenser are attached to a water source, with the water flowing in the low end and flowing out the high end of the condenser. The condensed vapor drips into the collection receiver.

Figure 3. The thermometer is inserted in the distillation flask through a hole in the cork stopper. The arm of the flask is inserted through a hole in the stopper of the condenser. Make sure these stoppers are airtight, or the vapor will escape.

Figure 5. The collection receiver The vapors condense and drip from the condenser into the flask. Simple distillation is effective only when separating a volatile liquid from a nonvolatile substance or when separating two liquids that differ in boiling point by 50 degrees or more. It may be helpful to insulate the distilling flask and three-way adapter to better retain heat and allow the sample to remain in the gas phase longer. To insulate a portion of the distillation, wrap the parts prior to the condenser with glass wool Figure 5.

A small gap can be left in the insulation in order to "peek in" on activity inside the apparatus. Glass wool has an appearance similar to cotton, but unlike cotton is not flammable so is useful as an insulating material when an apparatus is to be heated. Glass wool comes in two forms: a fibrous form and a cottony form. The fibrous form Figure 5. This type of glass wool should not be manipulated with bare hands, but only when wearing thick gloves. The more cotton-like glass wool Figure 5.

If glass wool is unavailable, aluminum foil can be used alone to insulate a portion of the distillation. It will not insulate well if the foil is wrapped too tightly to the glass, but works well if a small pocket of air is allowed between the foil and glass.

If vapor is noticed escaping out of the vacuum adapter like a tea kettle, the condenser is not doing a good enough job of trapping the gas Figure 5. Reasons for this may be that you have forgotten to turn on the water in the condenser, the water stream is too weak, or the heating is too vigorous. However, many compounds are somewhat susceptible to peroxide formation, including compounds with allylic hydrogen atoms e.

Lisa Nichols Butte Community College. Complete text is available online. Condenser Hoses The condenser is an intricate piece of glassware, and allows for cold water to circulate through the distillation apparatus. Simple Distillation Procedure An assembled simple distillation apparatus is shown in Figure 5. Assemble the Apparatus: To visualize the assembly of the apparatus, it may be helpful to first lay out the glassware on the benchtop before assembling the parts Figure 5.

Pour the liquid to be distilled into a round bottomed flask, trying to avoid pouring liquid on the ground glass joint.

If liquid drops onto the joint, wipe it off with a KimWipe. Alternatively use a funnel to be sure no liquid ends up on the joint, which can sometimes cause the joint to freeze. The flask should ideally be between one-third to one-half full of the liquid to be distilled. If the flask is more than half full, it will be difficult to control the boil. If less than a third full, the recovery may be compromised, as there is a quantity of vapor required to fill the flask that will not distill over this is called the " holdup volume ", and later condenses when the flask is cooled.

Add a few boiling stones or magnetic stir bar to the solution to prevent bumping during heating. Use a metal extension clamp to secure the round bottomed flask containing the sample to the ring stand or latticework at least 4 inches above the benchtop to leave room for the heat source.

The clamp should securely hold the joint below the glass protrusion on the flask Figure 5. Attach a three-way adapter or "distilling adapter" to the round bottomed flask Figure 5. Then delicately insert a thermometer into the hole of the rubber fitting. Safety note: While inserting the thermometer, position your hands near the joint Figure 5. A prepared thermometer adapter with inserted thermometer is shown in Figure 5.

Connect the thermometer adapter to the three-way adapter, securing the joint with a plastic clip sometimes called a " Keck clip ", the yellow clip in Figure 5. The clip is directional, and if it doesn't easily snap on it is probably upside down. Check that the clip is not broken, and if it is, replace it. A plastic clip should not be used to connect the round bottomed flask to the three-way adapter Figure 5.

First, this is one of the hottest parts of the apparatus, and could cause the plastic to melt especially if the compound has a high boiling point or a strong heat source is used. Secondly, a plastic clip in this location interferes with a secure grip by the metal extension clamp. A secure clamp on the flask is necessary in order to maintain the integrity of the system while lowering the heat source at the end of the distillation.

Adjust the thermometer so the bulb is just below the arm of the adapter Figure 5. If the bulb is positioned too high, it will not register the correct temperature of the vapors as they make their turn toward the condenser.

Prepare the condenser: inspect two rubber hoses and cut off any cracked sections in the rubber with scissors. Wet the ends of two hoses using the faucet or by dipping into a beaker of water, then twist the wet ends of the two arms on the condenser. The hoses should fit onto the condenser arms higher than one centimeter or else water pressure may cause them to pop off. Use another clamp to secure the condenser to the ring stand or latticework not so tight or it may crack , and position the condenser at roughly the location it will eventually be, with a slight downward angle Figure 5.

Connect the condenser to the rest of the apparatus Figure 5. While keeping the clamp's arm connected to the condenser, slightly loosen the clamp's attachment to the ring stand or latticework so that it can rotate in all directions. Then wiggle the condenser toward the three-way adapter, holding onto the clamp near the ring stand. When the two joints connect in a perfect match, secure them with a plastic clip must not be broken! Adjust the clamps and height of the condenser so that the thermometer is perfectly vertical in the apparatus.

A tilted distillation apparatus sometimes refluxes instead of distilling where gas condenses and drips back into the distilling flask.