Thursday
November 27, 2014

Homework Help: Subliming a substance

Posted by Sheryl on Monday, November 13, 2006 at 11:28pm.

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.

For further reading I suggest
http://www.av8n.com/physics/melt-sublimate.htm

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