Distillation is a laboratory method for separating liquids by using differences in boiling point. It matters because many useful mixtures, such as alcohol and water, crude oil, and contaminated solvents, contain liquids that are mixed at the molecular level. Simple distillation works best when the liquids have very different boiling points or when separating a liquid from dissolved solids.
Fractional distillation is used when the boiling points are close together and a sharper separation is needed.
In both methods, the mixture is heated so the more volatile component vaporizes first, then the vapor is cooled and condensed back into a liquid. Simple distillation sends vapor directly from the boiling flask to the condenser, so it gives only one main vaporization and condensation step. Fractional distillation adds a fractionating column, where vapor repeatedly condenses and re-evaporates on surfaces inside the column.
These repeated steps enrich the vapor in the lower-boiling component before it reaches the condenser.
Understanding Chemistry: Simple and Fractional Distillation
A liquid mixture does not produce vapor with the same composition as the liquid left in the flask. Molecules that escape more easily are more common in the vapor. This is why collecting vapor can change the composition of a sample.
As heating continues, the liquid remaining in the flask becomes richer in the less volatile substance. Its boiling behavior then changes over time. A pure liquid usually gives a nearly steady temperature during collection.
A mixture often gives a temperature that gradually rises, or shows separate temperature regions. These temperature changes help chemists decide when to change the receiving container.
A fractionating column works by creating reflux. Some rising vapor cools on the column packing and turns back into liquid. That liquid flows downward while new vapor rises through it.
At every contact, the vapor becomes richer in the component that vaporizes more readily, while the returning liquid becomes richer in the other component. Glass beads, metal packing, or shaped glass ridges provide many contact surfaces. A tall, well packed column can make a cleaner separation, but it slows the process.
If the mixture is heated too strongly, vapor rushes through the column without enough contact. This can reduce the quality of the collected fraction.
Good apparatus setup affects both results and safety. The thermometer bulb should sit at the entrance to the condenser, where it measures the vapor that is actually being collected. If it is placed too low or too high, the reading may not represent that vapor.
Cooling water enters the condenser at the lower connection and leaves from the upper connection. This keeps the water jacket full and gives efficient cooling. The apparatus must never be sealed completely.
Vapor pressure can build rapidly in a closed system, creating a serious risk of breakage. Boiling chips or a stirrer help prevent sudden violent boiling, called bumping.
Distillation has limits that students should recognize. Some liquids form an azeotrope, a mixture that boils with a fixed composition. Ordinary distillation cannot separate such a mixture beyond that composition.
Ethanol and water are a familiar example. Pressure matters too. Lowering pressure lowers boiling temperatures, which is useful for liquids that decompose when strongly heated.
This method is called vacuum distillation and is used in laboratories and industry. Oil refineries use large fractional columns to sort crude oil into useful ranges such as gases, petrol, kerosene, and heavier oils.
In experiments, pay attention to temperature, collection volume, heating rate, and the appearance of each fraction. Together, these observations show whether a separation is working well.
Key Facts
- Distillation separates substances based on boiling point and volatility.
- Simple distillation is most effective when boiling points differ by about 25 °C or more.
- Fractional distillation is used for liquids with close boiling points, often less than 25 °C apart.
- Boiling occurs when vapor pressure equals external pressure.
- Temperature at the still head is close to the boiling point of the vapor reaching the condenser.
- Greater surface area in the fractionating column gives more vaporization and condensation cycles, improving separation.
Vocabulary
- Distillation
- A separation technique that heats a mixture to form vapor and then cools the vapor to collect purified liquid.
- Boiling point
- The temperature at which a liquid's vapor pressure equals the surrounding pressure and the liquid boils.
- Volatile
- A volatile substance evaporates easily and usually has a relatively low boiling point.
- Condenser
- A glass tube cooled by water that turns hot vapor back into liquid.
- Fractionating column
- A column packed with surfaces that cause repeated condensation and vaporization to improve separation of close-boiling liquids.
Common Mistakes to Avoid
- Heating the mixture too strongly, because fast boiling can carry droplets of the original mixture into the distillate and reduce purity.
- Using simple distillation for liquids with close boiling points, because one vaporization step usually cannot separate them well.
- Putting the thermometer bulb too low or too high, because it must measure the vapor entering the condenser rather than the liquid in the flask or cooler air above it.
- Running condenser water in the wrong direction, because water should enter at the lower port and leave at the upper port to keep the jacket full and cooling efficiently.
Practice Questions
- 1 A mixture contains liquid A with boiling point 56 °C and liquid B with boiling point 118 °C. Which distillation method should give a good separation, and what component should distill first?
- 2 Ethanol boils at 78 °C and water boils at 100 °C. Their boiling points differ by 22 °C. Would simple or fractional distillation be more appropriate for improving the ethanol content of the distillate?
- 3 Explain why adding a fractionating column improves the separation of two liquids whose boiling points are close together.