Why is sand added to the heating mantle during simple distillation?

Sand is added to the heating mantle to enable more homogeneous heating of the round-bottom flask. This ensures even temperature distribution throughout the mixture, preventing hot spots that could cause bumping or uneven vaporization. Uniform heating is critical for accurate boiling point measurements and consistent separation of components.

What does the temperature change during distillation tell you about vapor composition?

Temperature changes indicate shifts in vapor composition. Early distillation at lower temperatures means the vapor is rich in the lower-boiling component. As temperature rises significantly, the higher-boiling component increases in the vapor. When temperature stabilizes near the higher boiling point, the vapor becomes predominantly that component.

How do you use a boiling point diagram to determine mixture composition during distillation?

Record the temperature at each collection point and locate it on the dew point curve of the boiling point diagram. The corresponding x-axis value shows the mole percent composition of the vapor at that temperature. This allows you to track how the composition of the distillate changes as distillation progresses throughout the experiment.

What is the correct water flow rate for the condenser during distillation?

Turn on the water flow slowly to avoid excessive pressure that could dislodge tubing from the condenser. The water inlet should connect to the port farthest from the distilling head, with the outlet near the distilling head. Proper flow ensures efficient cooling without equipment damage.

Why should you stop heating before the round-bottom flask becomes dry?

Heating the mixture until the flask is dry may cause an explosion due to dangerous pressure buildup and thermal decomposition of residual material. Stop distillation when approximately 0.5 mL of liquid remains, appearing as a thin layer. This critical safety precaution protects you and your equipment.

How do you calculate the volume of each component distilled from a mixture?

Use the dew point curve to determine vapor composition at each collection interval. Apply each percentage to the volume of liquid distilled since the previous measurement. Summing these incremental volumes gives the total amount of each component recovered in the distillate from your original mixture.

What does the thermometer bulb position indicate about distillation accuracy?

The thermometer bulb must be positioned below the bend of the distilling head to accurately measure the boiling point of the vapor being condensed. This placement ensures the bulb contacts the hottest vapor, providing reliable temperature readings that reflect the composition of the distillate being collected.

Simple Distillation

Concepts Instructor Prep Student Protocol

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As a liquid is heated, it gains energy until the increased disorder of the gas phase outweighs the intermolecular interactions in the liquid phase. Once enough molecules are in the gas phase, they escape the liquid in the form of bubbles. This effect, called boiling, occurs when the total vapor pressure of the substance is equal to the atmospheric pressure.

Vapor pressure is the pressure of the vapor in equilibrium with its condensed phase, and it varies with temperature. In a mixture of liquids, each component has its own vapor pressure, which we call its partial pressure. The total vapor pressure of the mixture is equal to the sum of the partial pressures. If the liquids are miscible, meaning that they always form a homogeneous solution, the partial pressure of each component is the vapor pressure of the pure compound at that temperature times its mole fraction in the liquid.

The temperature at which the first bubble of vapor starts to form in a liquid mixture is called the bubble point. For a pure liquid, both the bubble point and the temperature at which vapor starts to condense, or the dew point, are the same as the boiling point. However, for a mixture of two miscible liquids, both the bubble point and the dew point will be between the boiling points of the components.

When the mixture first boils, the vapor is rich with the compound with the lower boiling point, or the more volatile compound. This increases the proportion of the compound with the higher boiling point, or the less volatile compound, in the liquid mixture.

Distillation is a separation technique that takes advantage of this phenomenon. In a simple distillation, a homogeneous liquid mixture is boiled. The rising vapor then enters the inner chamber of a water-cooled condenser. The vapor condenses to a liquid, called the distillate, which is then collected in a separate vessel.

As the boiling continues, the compositions of the liquid and the vapor change as the more volatile component is removed. So, if we collect the distillate in small fractions, we'll see that each fraction contains the compounds in a different molar ratio.

As the proportion of the less volatile component in the liquid mixture increases, so do the bubble point and dew point. Plotting the mixture's bubble and dew points versus the mole fractions of the components makes a boiling point diagram. Once we have this diagram, we can use the dew point curve to determine the composition of the vapor at a given temperature.

In this lab, you will set up and perform the simple distillation of a mixture of cyclohexane and toluene and record the temperature of the vapor throughout the experiment. You'll then use the published boiling point diagram for cyclohexane and toluene to determine the composition of the vapor, allowing you to estimate the composition of the liquid mixture throughout the distillation.

Print Procedure Steps

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Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

Simple Distillation
In this lab, you will use simple distillation to separate a mixture of cyclohexane and toluene.
- Before you start the lab, put on the appropriate personal protective equipment, including a lab coat, safety goggles, and gloves. This experiment must be conducted in a hood.
- Assemble the distillation apparatus. Refer to the example set up that has been prepared by your instructor.
- Then, set up the two stands next to each other in the hood. Place the magnetic stirrer on the base of one of the stands.
- Set the heating mantle on top of the magnetic stirrer, and then plug it into the temperature controller.
- Add 5 – 10 mL of sand to the heating mantle. This will allow more homogeneous heating of the round-bottom flask. Clamp the round-bottom flask to the stand above the heating mantle.
- Now, obtain cyclohexane and toluene from your instructor. Measure 2 mL of cyclohexane using a volumetric pipette and dispense it into the round-bottom flask.
- Then, use a clean volumetric pipette to measure 2 mL of toluene and add it to the flask.
- Carefully insert a thermometer into an adapter by gently pushing and twisting it through the top.
- Lightly grease the adapter with vacuum grease and insert it into the distilling head. Make sure that the thermometer bulb is below the bend of the distilling head. Then, lightly grease the joints of the distilling head.
- Add a stir bar to the round-bottom flask and insert the distilling head into the round-bottom flask.
- Now, clamp the condenser to the second stand with it oriented diagonally toward the side joint of the distilling head.
- Connect one piece of tubing to the tap water faucet in the hood, and attach the other end to the inlet port of the condenser, which is located farthest from the distilling head.
- Attach the second tubing to the outlet port, which is located closest to the distilling head, and place the other end of the tubing into the drain.
- Connect the condenser to the distilling head. Then, lightly grease the joint of the condenser and attach the connecting tube. Place a 10-mL graduated cylinder below the outlet of the tube.
- Gently lower the setup until the round-bottom flask is seated in the sand in the heating mantle.
- Double-check all of the connections on your setup to make sure that they are tight and oriented correctly. Secure each joint with a plastic clip.
- Now, slowly turn on the flow of water to the condenser. Note: Be careful not to turn on the water too quickly. If the water pressure is high, the tubing may pop off the condenser.
- Turn the magnetic stirrer to the lowest setting, and set the temperature controller to heat the mantle to just above 90 °C.
- Once bubbles start forming in the liquid mixture, adjust the heat until the distillation rate is about 2 drops/min. You should also see drops forming on the thermometer bulb.
- Record the temperature after every 2 drops that fall into the graduated cylinder or every minute.
  Table 1: Distillation of toluene and cyclohexane
  
  Time (min) Distilled volume (mL) Temperature (°C)
  1
  2
  3
  4
  5
  6
  7
  8
  9
  10
  11
  12
  13
  14
  15
  Click Here to download Table 1
- When there is about 0.5 mL of the mixture remaining in the flask, which will look like a thin layer of liquid, turn off the heat and gently lift the glassware setup from the heating mantle. Note: Never heat the mixture until the flask is dry as this may cause an explosion.
- Remove the stir plate with the heating mantle and set it aside.
- Then, turn off the water to the condenser and allow the setup to cool completely. When it is cool, carefully disassemble the glassware.
- Dispose of all solutions in the non-halogenated organic waste container. Clean your glassware with acetone followed by detergent and deionized water.
Results
- Plot the dew point temperature versus the distilled volume during the experiment. We see that drops start to fall into the collection vessel at ~ 81 °C, which is the boiling point of cyclohexane. As the distillation progresses, the temperature increases to almost 111 °C, which is the boiling point of toluene.
- Early in the distillation, the vapor is rich in cyclohexane. As the second milliliter is distilled, the temperature increases by ~ 20 °C, which tells us that the amount of toluene in the vapor is increasing significantly. The temperature stabilizes as it approaches 111 °C, which tells us that the vapor is rich in toluene towards the end of the distillation.
- Look at a boiling point diagram for cyclohexane and toluene, with the mole percent of cyclohexane and toluene on the x-axis and the temperature on the y-axis. The solid line denotes the bubble point, and the dashed line denotes the dew point.
- Use the dew point curve to determine the composition of the vapor throughout the distillation. For example, after 0.5 mL has been distilled, the condensing vapor is about 84 °C. This corresponds to 95% cyclohexane on the dew point curve.
- Once you know the composition of the vapor at each point, estimate the composition of the boiling mixture throughout the distillation. At room temperature, cyclohexane and toluene have nearly the same number of moles per milliliter, so think of the percentages in terms of volume.
- To determine how much of each liquid was distilled in each step, apply each percentage to the volume of liquid distilled since the previous measurement. This tells us that we distilled 1.9 mL of cyclohexane and 1.1 mL of toluene from our mixture of 2 mL of each liquid. Thus, our distillate was 63% cyclohexane and 37% toluene, and our remaining liquid mixture was 10% cyclohexane and 90% toluene.