Wednesday
October 22, 2014

Homework Help: Sublimes?

Posted by Sheryl on Sunday, October 1, 2006 at 8:56pm.

Question: An unknown compound gave a melting point of 230C. When the molten liquid solidified, the melting point was redetermined and found to be 131C. Give a possible explanation for this discrepancy.

My answer: The compound sublimed.


How would sublimation work to do this?


Doesn't sublimation send it right from melting point to boiling point? So when they heated it to melting, they didn't catch it until it hit boiling point. Then they solidified it and got the temp as soon as it got liquid.

Sheryl


When a substance sublimes, it goes from the solid state directly to the vapor state without going through the liquid state. And it will solidify from the vapor sate directly to the solid state without passing through the liquid state. So there is no boiling point. The literature will quote a sublimation temperature.


How about if it was in a capillary tube? That might not explain why it was so high initially.

Maybe the thermometer was broken. Any hints?

Sheryl


The only thing that comes to mind is that it melted at 230 C, rearranged into another compound, cooled and solidified. The new pure comound melted at the lower temperature.


Thanks very much. I had so much Organic work this weekend that I couldn't believe it but I think I'm finished for the night.

Sheryl


Could it form after melting into the solid of another isomer? Often isomers have vastly different mp...
as an example, the para, ortho, and meta forms of dichlorobenzene come to mind.


Sounds like that is the answer.

Thanks from Sheryl

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