Melting Point Determinations

Introduction

Solid substances melt at a temperature known as the melting point. One of many physical properties that characterize solids, the melting point can be used to suggest or confirm the identity of an unknown solid. Be warned however, that many solids have similar melting point temperatures.

Impurities in the solid sample lower the expected melting point of the solid. The melting point depression phenomenon can be used to your advantage. Suppose you believe you have identified an unknown solid and have measured its melting point temperature. You can test your belief by mixing a small amount of your unknown with the substance you believe your unknown to be. If you are right, there will be little or no change to the melting point you observe for the mixture you have just prepared. However, if your identification is wrong, you will have mixed two different substances together and the observed melting point will be much different from the unknown all by itself. TRY IT!!

In the laboratory, a solid substance very rarely melts at a specific temperature. Rather, the solid will be observed to begin to melt at one temperature and finish melting at a greater temperature. These two temperatures define the melting point range. Pure substances exhibit very narrow melting point ranges while impure (and possibly improperly prepared) solids melt over a broad range.

In your notebook and for all lab reports be sure to report and clearly label the MELTING POINT RANGE for all melting point determinations!

Procedure

Obtaining a melting point requires a procedure that is initially time consuming but after practice is really quite easy! Here are the steps:

• Load a capillary with a small amount of the solid sample
• Pack the sample into the capillary by dropping the capillary from great heights
• Insert the packed capillary into a Mel-Temp apparatus
• Turn on the Mel-Temp apparatus and turn up the dial to heat at the correct rate.
• Watch carefully through the eyepiece and record the temperatures when the sample starts to melt and is finished melting.