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. Simple distillation is only an effective technique to purify a mixture of liquids if the liquids have very different boiling points (typically a difference of >70 °C).
Figure 1: Distillation apparatus. 1. Thermometer held by a distilling head; 2. Distillation flask; 3. Heating mantle; 4. Condenser; 5. Cooling water out; 6. Cooling water in; 7. Receiving flask. (Image source Labster theory)
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 boiling points 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.
There are several liquid chemicals in use, so students face difficulty in 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 along with the 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 having 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.
Theoretical imagination of the distillation apparatus is complicated, as we use the same names like sample flask or receiving flask. This may confuse the students whether they are identical or different. 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.
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.
The evaporating system consists of a heating mantle to provide heat, a distilling or sample flask for the sample, and a distilling head that connects the evaporating system with the condensation column.
The basic function of the heating mental is to provide heat to the sample. It has a controlled heating system, and most heating mantles also have the function of stirring. In the distillation process, we put the sample flask with a round bottom on the heating mental and heat the mixture.
The distilling flask is generally the largest round-bottomed flask, which houses your crude, unpurified mixture. It’s placed over a heating source that heats the mixture until the relevant components vaporize and leave the flask. We can reduce the pressure in the distilling flask in order to heat the mixture more costly and time-effectively; however, this puts us at the risk of flash boiling the mixture. If the pressure inside the flask is significantly lower than the compound’s vapor pressure, the compound can rapidly volatilize, especially when heated in the flask. We, therefore, can add small solids packed with air, known as boiling chips, to prevent this.
The distilling head, known as the Y-arm, connects the fractionating column to the condenser. The Y-arm is where the boiling point of the fractionating level is read and measured by a thermometer. The placement of the thermometer is crucial for an accurate boiling point reading. If it is too low, you’ll not be reading the temperature before the component has condensed, and the reading will be too warm. Too high, and the temperature reading will be after the mixture has condensed and so too cold. The thermometer bulb should be placed just below the point where the Y-arm stretches out to the condenser, as shown in the figure below.
Figure 3: A distilling head with a thermometer. The thermometer bulb is just below the point where the Y-arm stretches out to meet the condenser (Image source Labster theory).
The condensation column involves the condensin column in simple distillation and the fractionating column in fractional distillation. Most simple or fractionating columns have a water inlet and an outlet for the water flow. This water flow cools down the column, making condensation easy and fast.
There are several types of fractionating columns, and they relate to theoretical plates. All fractionation columns are placed between the distilling flask and the distilling head, which connects the fractionating column to the condenser. 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.
Figure 4: Image of fractionating column (Image source Labster theory).
There are several types of condensers, but their main mission is to condense the purified vapors escaping the fractionating column.
All condensers work the same in simple and fractional distillation. A glass tube is surrounded above and below by a water jacket. The bottom half of the jacket is connected to a cold tap, allowing cold water to flow through and cool the bottom half of the condenser tube. The cold water must come in from the bottom, which is the coldest end, to avoid thermal shock to the glassware. The top of the jacket is where this water flows out to make space for new, cold water to enter the bottom. The boiling point of the vapors entering the condenser is measured at the top of the condenser, where the distilling head and condenser meet. The mouth is where the exit of the condensed liquid, due to gravity, into the receiving flask.
The receiving system consists of a receiving flask that collects the condensed liquid.
The receiving flask is the ending point for all liquids that are trickling down from the condenser. There are various types of collection and receiving flasks depending on the mixture and type of distillation.
We can benefit from using a cow-type receiving flask when collecting multiple fractions in relatively quick succession. This flask has multiple protrusions, like the udders on a cow, that can be spun around to quickly collect small amounts of different liquids- without needing to remove and replace the glassware.
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 Exploring a Distillation Apparatus Virtual Lab, students will get a closer look into the distillation apparatus by studying the setup of the distillation system and learning about the purpose of each laboratory instrument, including glassware such as the distilling flask, collection flask, and condenser, temperature measurement through a heating mantle and thermometer, and finally the vacuum apparatus. Learn about the basic principle of distillation and what type of mixtures you can apply this simple distillation technique to. And why it is important to choose the suitable solvent and the best ways to recrystallize your solid.
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