Distillation is a basic chemical laboratory operation used to separate and purify liquid mixtures based on differences in boiling points. In organic chemistry experiments and basic chemistry experiments, it is used to purify liquid products after a reaction or to isolate a specific component from a mixture.
In a report on a distillation experiment, simply writing that “a liquid distilled over” is not sufficient. It is important to discuss the boiling point, the temperature range of each fraction, the amount distilled, the quality of the separation, comparison with literature values, and possible sources of error.
This article explains the results that should be examined in a distillation experiment, how to interpret boiling points and fractions, causes of poor separation efficiency, and discussion examples that can be used in reports.
Note: This article is a reference for discussing results obtained in chemistry experiments at universities and similar institutions. For actual experimental procedures and safety precautions, always follow your university’s laboratory manual and the instructions of your instructor or teaching assistant (TA).
- What Is Distillation?
- Results to Examine in a Distillation Experiment
- What Can Be Learned from the Boiling Point?
- What Is a Fraction?
- Difference Between Simple Distillation and Fractional Distillation
- Main Causes of Poor Separation Efficiency
- Discussion When Heating Was Too Strong
- Discussion When Heating Was Too Weak
- Error Due to Thermometer Position
- Discussion When Cooling Was Insufficient
- Discussion When Bumping Occurred
- Reasons Why the Boiling Point Differs from the Literature Value
- What Can Be Learned from a Distillation Temperature Graph?
- Causes of Low Recovery
- When Distillation Can Be Considered Successful
- When Distillation Is Considered Insufficient
- How to Write Improvements
- Difference Between a Superficial Discussion and a Good Discussion
- Example Expressions That Can Be Used in Reports
- Points to Check When Discussing a Distillation Experiment
- Summary
What Is Distillation?
Distillation is an operation in which a liquid is heated to form vapor, and the vapor is then cooled and recovered as a liquid again. When the components in a liquid mixture have different boiling points, the lower-boiling component tends to evaporate first, making it possible to use distillation for separation and purification.
For example, when a mixture containing a low-boiling component and a high-boiling component is heated, vapor containing a larger proportion of the low-boiling component is produced first. The liquid obtained by cooling and collecting this vapor in a condenser is called the distillate.
In a distillation experiment, the temperature range and amount of the distilled liquid are recorded, and the components present in larger proportions in each fraction are discussed.
Results to Examine in a Distillation Experiment
For the results of a distillation experiment, organize the temperature at which distillation began, temperature changes during distillation, the amount of liquid recovered, and the temperature range of each fraction. From this information, determine how effectively the desired substance was separated.
Main Items to Include in the Results
- Temperature at which distillation began
- Temperature range during distillation
- Amount recovered for each fraction
- Color and odor of the distillate
- Amount and condition of the residue
- Comparison with the literature value or theoretical boiling point
- Graph of temperature changes
- Whether the separation was sufficient
Example of Writing the Results:
Distillation began at approximately 76°C, and the main fraction was obtained at 78–80°C. The volume of distillate obtained within this temperature range was 12.5 mL. After that, the temperature gradually increased, and the amount of distillate decreased above 85°C. The temperature range of the main fraction was close to the literature value, although some variation was observed in the distillation temperature.
What Can Be Learned from the Boiling Point?
The temperature measured in a distillation experiment is the temperature of the vapor being distilled. When a pure substance is distilling steadily, the temperature remains relatively constant near the boiling point of that substance.
In contrast, for mixtures or samples containing impurities, the temperature may not remain constant and may change gradually. This is because the composition of the vapor being distilled changes over time.
| Temperature Change | Possible Interpretation |
|---|---|
| Constant near the boiling point | A relatively pure component may be distilling |
| Broad temperature range | Multiple components may be distilling together |
| Temperature gradually increases | The proportion of the low-boiling component may be decreasing while that of the high-boiling component increases |
| Lower or higher than the literature value | Possible effects of thermometer position, atmospheric pressure, impurities, or apparatus conditions |
Discussion Example:
The main fraction was obtained at 78–80°C, and the temperature remained within a relatively narrow range. This suggests that a fraction containing a large proportion of the desired component was obtained within this temperature range. On the other hand, because the temperature was not completely constant, a small amount of impurities or other components may have been present.
What Is a Fraction?
A fraction is a portion of distillate collected within a certain temperature range. In distillation, the composition of the distillate changes depending on the distillation temperature, so the liquid may be collected separately according to temperature range.
Fractions obtained at lower temperatures tend to contain a larger proportion of low-boiling components, while fractions obtained at higher temperatures tend to contain a larger proportion of high-boiling components. However, complete separation into a single component does not necessarily occur, and other components may be mixed into each fraction.
| Fraction | Characteristics | Discussion Point |
|---|---|---|
| Initial fraction | Liquid that distills first | May contain low-boiling components or impurities |
| Main fraction | Liquid obtained near the boiling point of the desired substance | Considered to contain a large proportion of the desired component |
| Late fraction | Liquid obtained after the temperature has increased | May contain high-boiling components or impurities |
Discussion Example:
Because distillation was observed in the initial fraction at a temperature lower than the boiling point of the desired substance, the fraction may have contained low-boiling impurities or volatile components. In contrast, because the main fraction was obtained within a temperature range close to the literature boiling point of the desired substance, it is considered to contain a relatively large amount of the desired component.
Difference Between Simple Distillation and Fractional Distillation
Distillation includes simple distillation and fractional distillation. Simple distillation is used to separate liquid mixtures with large differences in boiling points or to separate volatile and nonvolatile components. Fractional distillation is used when better separation is desired for liquid mixtures whose boiling points are close to each other.
| Method | Characteristics | Suitable Situation |
|---|---|---|
| Simple distillation | Relatively simple apparatus | Separation of mixtures with large differences in boiling points |
| Fractional distillation | Uses a fractionating column to improve separation efficiency | Separation of liquid mixtures with small differences in boiling points |
When components with similar boiling points are separated by simple distillation, multiple components are more likely to be present in the same fraction. Therefore, in a report, you can discuss whether the distillation method used was appropriate for the properties of the sample.
Discussion Example:
The distillation temperature extended over a broad range, suggesting that other components may have been present in the main fraction. This is considered to be because the boiling points of the components in the mixture were close, making sufficient separation difficult by simple distillation. To obtain higher separation efficiency, fractional distillation using a fractionating column is considered effective.
Main Causes of Poor Separation Efficiency
Causes of poor separation efficiency in distillation include heating rate, thermometer position, insufficient cooling, the tightness of the apparatus, boiling conditions, and differences in boiling points between components. In a report, select causes that match your experimental conditions and observations.
| Cause | What Happens | Effect on Results |
|---|---|---|
| Heating is too strong | Rapid evaporation occurs | Multiple components are more likely to distill together |
| Thermometer position is inappropriate | Vapor temperature cannot be measured correctly | Boiling point deviates from the literature value |
| Insufficient cooling | Vapor does not completely condense | Amount of distillate decreases |
| Vapor leaks from the apparatus | Components are lost | Recovery decreases |
| Small difference in boiling points | Vapor compositions are similar | Separation becomes insufficient |
| Bumping occurs | Liquid suddenly splashes upward | Impurities are more likely to enter the distillate |
Discussion When Heating Was Too Strong
In distillation, the heating rate greatly affects separation efficiency. If heating is too strong, the liquid boils rapidly, and not only the low-boiling component but also the high-boiling component tends to move into the vapor phase at the same time. As a result, the purity of the fraction may decrease.
Discussion Example:
One possible reason why the distillation temperature did not stabilize and distillation occurred over a broad temperature range is that the heating was too strong. When heating is rapid, the liquid boils vigorously, making both low-boiling and high-boiling components more likely to distill simultaneously. Therefore, other components may have entered the main fraction, reducing the separation efficiency.
Discussion When Heating Was Too Weak
If heating is too weak, distillation may take too long or the distillation rate may not stabilize. In addition, the vapor may not reach the condenser sufficiently, resulting in a smaller amount of distillate.
Discussion Example:
One possible reason why the amount of distillate was small is that the heating was weak and the desired component did not evaporate sufficiently. In distillation, vapor must reach the condenser and condense before it can be recovered as distillate. If heating was insufficient, the amount of vapor would have been small, reducing the amount of distillate that could be recovered.
Error Due to Thermometer Position
The temperature measured during distillation is not the temperature of the liquid but the temperature of the vapor being distilled. If the thermometer bulb is not positioned appropriately, the vapor temperature cannot be measured correctly.
If the thermometer bulb is positioned too high, the vapor may not contact it sufficiently, causing it to indicate a temperature lower than the actual value. Conversely, if the position is inappropriate and the thermometer is affected by the liquid or part of the apparatus, it may indicate a value different from the vapor temperature.
Discussion Example:
One possible reason why the measured distillation temperature differed from the literature value is that the thermometer was not positioned appropriately. In distillation, it is necessary to measure the temperature of the vapor being distilled, but if the thermometer bulb was not sufficiently exposed to the vapor, it may have indicated a temperature lower than the actual boiling point. Therefore, the measured boiling point may not have agreed with the literature value.
Discussion When Cooling Was Insufficient
In distillation, the generated vapor is cooled in a condenser and recovered as a liquid. If cooling is insufficient, some of the vapor may fail to condense and may not be recovered as distillate.
Discussion Example:
One possible reason why the amount of recovered distillate was small is that cooling in the condenser was insufficient. If the vapor was not sufficiently cooled, some of it may not have condensed and may have been lost. As a result, the amount of distillate actually recovered decreased, and the recovery is considered to have been reduced.
Discussion When Bumping Occurred
Bumping is a phenomenon in which a liquid suddenly boils and splashes upward. When bumping occurs, not only vapor but also liquid droplets and impurities may move toward the distillate side and enter the collected liquid.
Discussion Example:
If bumping occurred during distillation, not only vapor but also small droplets of liquid may have moved toward the distillate side. As a result, high-boiling components or impurities may have entered the distillate, reducing the purity of the fraction. In addition, the temperature change may have become unstable, possibly affecting the measurement of the boiling point.
Reasons Why the Boiling Point Differs from the Literature Value
It is not unusual for a measured boiling point to differ from the literature value. Differences in boiling point can be related to experimental procedures, apparatus conditions, sample purity, atmospheric pressure, and other factors.
| Cause | Possible Effect |
|---|---|
| Impurity contamination | Boiling point range broadens and the temperature becomes less constant |
| Incorrect thermometer position | Indicates a value different from the actual vapor temperature |
| Heating is too strong | Temperature rises rapidly and fractions are more likely to mix |
| Difference in atmospheric pressure | Measured value differs from the standard boiling point |
| Heat loss from the apparatus | Vapor temperature becomes difficult to stabilize |
Discussion Example:
Possible reasons why the measured boiling point differed from the literature value include impurities in the sample, the position of the thermometer, and the heating rate. In particular, because the temperature did not remain constant and gradually increased, the distillate may not have consisted of a single component but may have contained multiple components. In addition, if the atmospheric pressure during the experiment differed from standard conditions, a difference may also occur between the literature boiling point and the measured value.
What Can Be Learned from a Distillation Temperature Graph?
In a distillation experiment, temperature may be recorded against the amount distilled or elapsed time and plotted as a graph. By examining the graph of temperature changes, it becomes easier to consider the temperature range in which the main fraction was obtained, whether the temperature was stable, and whether the separation was sufficient.
| Graph Feature | Possible Interpretation |
|---|---|
| Plateau at a constant temperature | The main component may have distilled steadily near that temperature |
| Temperature continuously increases | Multiple components may be distilling while mixed together |
| Rapid temperature increase | The low-boiling component may have decreased and the high-boiling component may have begun to distill |
| Large temperature fluctuations | Possible effects of unstable heating, bumping, or thermometer position |
Discussion Example:
In the graph of distillation temperature, the temperature changed only slightly around 78–80°C, and the main fraction was obtained within this range. This suggests that the fraction obtained within this temperature range contained a large proportion of the desired component. On the other hand, because the temperature gradually increased afterward, the low-boiling component may have decreased while the proportion of high-boiling components increased.
Causes of Low Recovery
In distillation, the entire sample is not necessarily recovered as distillate. Recovery may decrease because liquid remains inside the apparatus, vapor is not completely condensed, components escape through connections in the apparatus, or some liquid remains as residue.
Recovery (%) = Amount of recovered distillate ÷ Amount of sample before distillation × 100
Discussion Example:
Possible reasons why the recovery of the distillate was low include liquid remaining inside the distillation apparatus and insufficient cooling that prevented some vapor from condensing. In addition, if there were gaps at the connections of the apparatus, volatile components may have escaped to the outside, reducing the amount recovered.
When Distillation Can Be Considered Successful
Distillation can be considered relatively successful when the temperature range of the main fraction is close to the boiling point of the desired substance, the temperature is stable, and a sufficient amount of the fraction is obtained. Furthermore, if the initial fraction, main fraction, and late fraction are appropriately separated, the purity of the desired substance is more likely to be higher.
Discussion Example:
The main fraction was obtained within a temperature range close to the literature value, and the temperature change during distillation was relatively small. This suggests that a fraction containing a large proportion of the desired component was separated. In addition, collecting the initial and late fractions separately may have reduced the contamination of the main fraction by low-boiling impurities and high-boiling components to some extent.
When Distillation Is Considered Insufficient
Distillation may have been insufficient when the distillation temperature range is broad, the temperature does not become constant, the properties of the individual fractions are not clearly separated, the recovery is low, or the measured temperature differs greatly from the boiling point of the desired substance.
Discussion Example:
Because the distillation temperature extended over a broad range and did not show a clear constant temperature, multiple components may have been present in the distillate. Particularly if the heating was too strong, low-boiling and high-boiling components would be more likely to distill simultaneously, reducing separation efficiency. Therefore, the desired component is considered not to have been completely separated in this experiment.
How to Write Improvements
In the discussion of a distillation experiment, including improvements as well as causes of error makes the report easier to organize. It is important to write improvements in relation to the problems that actually occurred in your experiment.
Improvements for Increasing Separation Efficiency
- Avoid rapid heating and keep the distillation rate stable.
- Position the thermometer appropriately.
- Use fractional distillation when the difference in boiling points is small.
- Separate the initial fraction, main fraction, and late fraction appropriately.
- Avoid bumping and maintain stable boiling.
Improvements for Increasing Recovery
- Provide sufficient cooling to ensure that the vapor condenses.
- Prevent vapor from leaking from the connections in the apparatus.
- Take into account the amount of liquid remaining inside the distillation apparatus.
- Avoid losses caused by excessive heating.
Example of Writing Improvements:
To improve separation efficiency, it is important to avoid rapid heating and maintain a constant distillation rate. In addition, placing the thermometer bulb appropriately in the path of the vapor makes it possible to measure the distillation temperature more accurately. For mixtures with small differences in boiling points, separation by simple distillation tends to be insufficient, so using a fractionating column is considered effective for improving separation efficiency.
Difference Between a Superficial Discussion and a Good Discussion
In a distillation experiment, writing only that “the temperature was different” or “the separation did not work” makes the discussion superficial. A more convincing discussion connects the boiling point, fractions, apparatus conditions, and heating method.
| Superficial Discussion | Good Discussion |
|---|---|
| The boiling point differed from the literature value. | Possible reasons why the measured boiling point differed from the literature value include inappropriate positioning of the thermometer and impurities in the sample. If the thermometer bulb was not sufficiently exposed to the vapor, it may have indicated a temperature lower than the actual vapor temperature. |
| The separation did not work well. | Because the distillation temperature extended over a broad range, multiple components may have distilled together. If the heating was too strong, low-boiling and high-boiling components would be more likely to distill simultaneously, reducing separation efficiency. |
| The amount recovered was small. | Possible reasons why the amount of distillate recovered was small include insufficient cooling that prevented some vapor from condensing and liquid remaining inside the apparatus. These factors may have reduced the amount of liquid actually collected in the receiving vessel. |
Example Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of a distillation experiment. Adjust the necessary parts according to your own experimental results.
- Because the main fraction was obtained within a temperature range close to the literature value, it is considered to contain a large proportion of the desired component.
- Because the distillation temperature did not remain constant, multiple components may have distilled together.
- One possible reason for the broad temperature range is that the heating rate was too fast.
- If the thermometer was not positioned appropriately, the vapor temperature could not be measured accurately, causing the boiling point to differ from the literature value.
- If cooling was insufficient, some of the vapor may not have condensed, reducing the amount recovered.
- If droplets moved toward the distillate side due to bumping, impurities may have entered the fraction.
- Because components with similar boiling points were separated by simple distillation, the separation may have been insufficient.
- To improve separation efficiency, it is important to heat gently and maintain a constant distillation rate.
- Separating and collecting the initial and late fractions can improve the purity of the main fraction.
Points to Check When Discussing a Distillation Experiment
Before writing the report, checking the following points can make it easier to write the discussion.
- At what temperature did distillation begin?
- Within what temperature range was the main fraction obtained?
- Did the temperature remain constant or gradually increase?
- How did the measured value compare with the literature boiling point?
- How much of each fraction was obtained?
- Were the initial, main, and late fractions separated?
- Was the heating too strong?
- Was the thermometer positioned appropriately?
- Was cooling sufficient?
- Did bumping or liquid carryover occur?
- Was the sample suitable for sufficient separation by simple distillation?
- Was the main problem separation efficiency or recovery?
Summary
Distillation is a basic operation used to separate and purify liquid mixtures based on differences in boiling points. In a report on a distillation experiment, the discussion is based on the distillation temperature, amount of each fraction, comparison with literature boiling points, temperature changes, and separation efficiency.
If the temperature remains stable near the boiling point, a relatively pure component may be distilling. In contrast, if the temperature range is broad or the temperature continuously increases, multiple components may be distilling together.
Possible causes of poor separation efficiency include heating strength, thermometer position, insufficient cooling, bumping, and small differences in boiling points. In a report, rather than simply writing that “the distillation did not work well,” explain specifically which conditions affected the boiling point, fractions, and recovered amount and how they affected them.
