Distillation is a powerful method that can separate the fractions of a homogenous mixture (i.e., mixed or blended well together) of liquids. The method exploits the difference between the fractions' boiling temperatures. Simple distillations, by definition, are said to have one theoretical plate. A theoretical plate is one vaporization-condensation cycle. In a simple distillation, the mixture is heated to reflux, and the vapors travel up the glassware until they hit the cold reflux condenser, the vapors re-condense and are collected in a collection flask – one vaporization-condensation cycle. 

Why do students struggle to understand fractional distillation?

1. Lack of basic knowledge

Basic knowledge is important to understand every topic. Distillation is a simple process, but the knowledge behind it is somehow tricky. Students may confuse about why we use the distillation process, and the answer is almost all liquids evaporate upon giving temperature. So the answer is; we use a simple distillation process to purify solid impurities from liquid chemicals. We can also separate chemicals with low boiling points from those with high ones through simple distillation. We use fractional distillation if we have liquid impurities in our liquid sample. When performing a fractional distillation, they must be clear about the boiling points of the liquids.

2. The boiling points

Several liquid chemicals are in use, so students face difficulty remembering the boiling points of individual solvents. They don't know which solvent is more volatile, so you must have a good list of solvents and boiling points. Provide this list to the students or make a poster and hang it on the lab wall. This is a simple step, but it makes the work easy. 

The 2nd thing is; if there is a solvent has an impurity, you must tell the students about which kind of impurity it may contain. This knowledge comes from the basics of chemistry. If there is methanol as we know that methanol absorbs moisture. So the impurity must be water; we can separate this through fractional distillation. Water has a relatively higher boiling point than methanol, so when we heat the mixture, methanol evaporates first and condenses at a different point in a fractional distillation chamber.

3. It’s conceptually challenging.

Theoretical imagination of the distillation is complicated, as we use chemicals of nearly the same boiling points. This may confuse the students on how they will separate from each other as the boiling point in nearly the same. Regarding the fractionating column, it's difficult to imagine how it collects solvents separately. So video graphics is a better option when you don't have the apparatus. You can use your laptop to show the images or draw the apparatus, which is more time-consuming. 

5 interesting ways of exploring fractional distillation

1. Review the basics of distillation

Following are some important topics to know before going to the main topic.

• Boiling point: The boiling point is the temperature at which the vapor pressure of the liquid becomes equal to the pressure of the gas above it.
• Volatility: It describes how easily a compound converts into gas or vapor. The more volatile, the more quickly it will evaporate.
• Condensation: When any gas trunks into liquid, the process is called condensation.
• Simple distillation: In the simple distillation process, when you heat the mixture, the most volatile liquid will evaporate first and leave the distilling flask. This vapor then passes through a condenser before being collected in the receiving flask at the end.
• Fractional distillation: If the mixture includes components with similar boiling points, simple distillation is not enough to separate the substances. In such cases, we do fractional distillation by adding a fractionating column between the distilling flask and the distilling head. The fractionating column makes the mixture go through multiple cycles of evaporation and condensation before entering the condenser.

2. Explain fractional distillation

If the mixture includes components with similar boiling points, simple distillation is not enough to separate the substances. In such cases, we do fractional distillation by adding a fractionating column between the distilling flask and the distilling head. The fractionating column makes the mixture go through multiple cycles of evaporation and condensation before entering the condenser.

Figure 1: Image of how the fractional distillation apparatus is set up (Image source Labster theory).

Theoretical plates

When vapor moves up through a fractionating column, it goes through multiple cycles of evaporation and condensation. These cycles happen because of the many indentations or surfaces inside the column. The vapors temporarily condense on these surfaces before vaporizing again due to the heat from the heating mantle. Every cycle of evaporation and condensation is called a theoretical plate. With each cycle, the amount of the lower boiling point component in the distillate increases.

The number of theoretical plates needed is equivalent to the number of simple distillation cycles we would have to do to reach the same separation. Using theoretical plates is a more practical way of performing more than one cycle of distillation without using large amounts of equipment and space in the lab.

3. Briefly explain the fractional distillation apparatus

• Heating mantle: The basic function of the heating mantle is to provide heat to the sample.
• Distilling flask: The distilling flask is generally the largest round-bottomed flask, which houses your crude, unpurified mixture. It’s placed over a heating source
• Distilling head: The distilling head, known as the Y-arm, connects the fractionating column to the condenser.
• Fractionating column: Fractionating columns have multiple levels each being further away from the heating source. When moving up the column, the temperature, therefore, decreases. As vapors travel up the column, they cool and condense at more than one level. Some vapors will condense at the bottom range of their boiling point. Other vapors of the same compound will condense at the top range of their boiling point. This ensures that all the components of a compound will vaporize and condense at their full boiling point range, so the separation is high yielding.
• Condenser: There are several types of condensers, but their main mission is to condense the purified vapors escaping the fractionating column. 
• Receiving flask: The receiving flask is the ending point for all liquids that are trickling down from the condenser.

4. Explain the Similarities and differences between simple and fractional distillation

Fractional distillation can look daunting at first, but it shares many commonalities with simple distillation and only a few subtle, but key differences. Both apparatuses have a distilling flask that houses the mixture to be purified. They both use heat to vaporize compounds and read the boiling point of vaporization at the distilling head, or Y-arm, using a thermometer. As the vapor climbs up the glassware, the column cools, which encourages condensation of the vapor into the condenser. This is where the liquid cools further and falls with gravity to be collected into the receiving flask.

All the differences of fractional distillation to simple distillation are due to the former separating compounds with similar boiling points, that simple distillation cannot. To do this, the vaporized compounds must not enter the distilling head directly from the distilling flask but pass through a fractionating column first. The bevels in the column encourage the condensation of compounds at specific levels with descending local temperatures. This encourages a compound to vaporize and condense at its full boiling point range which means the separation is higher yielding. Also, since we are separating compounds with similar boiling points, the receiving flask is often multi-pronged to allow the collection of different liquids in tandem.

5. Use virtual lab simulations

With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment. In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.

In Labster’s interactive Fractional Distillation: Separate a liquid mixture into its fractions Virtual Lab, students will learn how to turn a mix of toluene and cyclohexane into its pure components through fractional distillation. Together with Dr. One, you will overcome the limitations of a simple distillation apparatus by adding a fractionating column. When the components of a homogeneous liquid mixture have similar boiling points, you cannot separate them entirely by simple distillation. You will, therefore, use a secret weapon: a fractionating column! When vapors move up through this column, they go through multiple cycles of evaporation and condensation. With each cycle, the vapors get increasingly rich in one component, which means that you can collect a pure product at the end of the apparatus.