# Question: A substance has a vapor pressure of 50 mmHg at the melting point (100C). Describe how you would experimentally sublime that substance.

My answer: I would put the substance in a sidearm test tube apparatus, put ice in the smaller tube and apply pressure and apply. There would then be no pressure and the material to be sublimated would do so as heat is applied.

Sound reasonable?

Thanks from Sheryl

Since a liquid phase exists at 50 mm Hg, you are going to have to go to a lower pressure to observe sublimation. When you go to a low enough pressure, you will get sublimation at a lower temperature than 100 C.

You are going to have to use a vacuum pump to get a pressure below 50 mm Hg. As you pump out the vapor, the liquid will cool and eventually solidify, unless heat is being added. Let the material reach equilbrium at a pressure and temperature where a solid phase exists. If you stabilze the temperature there, you can turn off the vacuum pumping. Then add heat slowly and observe sublimation.

http://www.av8n.com/physics/melt-sublimate.htm

## Your answer is partially correct. To experimentally sublime a substance with a vapor pressure of 50 mmHg at its melting point, you would need to create a system with a controlled pressure and temperature environment. Here are the step-by-step instructions for the process:

1. Set up a vacuum sublimation apparatus: This typically consists of a round-bottomed flask with a sidearm, a condenser, and a receiving flask. The sidearm should have a valve or stopcock to control the pressure in the system.

2. Place the substance in the round-bottomed flask: Ensure that the substance is clean and pure, free from any impurities.

3. Connect the apparatus to a vacuum pump: Attach the vacuum pump to the system and start evacuating the air from the apparatus. This will lower the pressure inside the flask.

4. Gradually lower the pressure: Slowly open the valve or stopcock on the sidearm to decrease the pressure in the system. As the pressure decreases, the substance will start to sublimate.

5. Maintain a controlled temperature: If the substance sublimes at a temperature lower than 100°C, you may need to cool the flask using ice or a cold bath. This helps to create a lower temperature environment for sublimation.

6. Observe and collect the sublimed material: As the substance sublimes, it will condense in the condenser and collect in the receiving flask. This collected material is the sublimed product.

7. Record and analyze the results: Measure the mass of the sublimed material collected and examine its properties to determine its purity and identity.

Remember to follow proper safety precautions while conducting any experiment, including wearing appropriate personal protective equipment and working in a well-ventilated area if necessary.

## Your explanation is partly correct, but there are some additional steps and considerations to take into account when experimentally subliming a substance. Here is a more detailed explanation:

1. Start by setting up a vacuum apparatus, which typically consists of a round-bottom flask connected to a sidearm test tube or a cold finger apparatus. Make sure all the connections are airtight to create a vacuum.

2. Place the substance you want to sublime in the round-bottom flask. This could be done by accurately weighing a sample of the substance and placing it in a sample holder or directly in the flask.

3. Close off the vacuum apparatus and begin pumping out the air using a vacuum pump. This will gradually decrease the pressure inside the flask.

4. Monitor the pressure using a vacuum gauge. Keep reducing the pressure until you reach a pressure below 50 mmHg but above the triple point of the substance (the temperature and pressure at which all three phases - solid, liquid, and gas - can coexist).

5. Once you have reached the desired pressure, stabilize the temperature of the flask at a point below the substance's melting point but above its sublimation point. This can be done using a thermostated bath or heat source.

6. Allow the substance to equilibrate at this temperature and pressure until it solidifies. Sublimation will not occur until the substance is in a solid state.

7. Once the substance has solidified, you can then start heating the flask slowly. This can be done using a Bunsen burner, a heating mantle, or any other heat source. As heat is applied, the solid substance will directly convert into vapor, bypassing the liquid phase. This is the process known as sublimation.

8. Observe the sublimation process and record any changes in appearance or physical properties, such as the formation of a vapor or changes in color or shape.

9. If necessary, collect the sublimed vapor by using a cooled surface, such as a cold finger apparatus. The vapors will condense on the cold surface and can be collected for further analysis.

It's important to note that the specific experimental setup and temperature range may vary depending on the properties and characteristics of the substance being studied. It is always recommended to refer to applicable literature or consult with experts before conducting any experimental procedures.