Purity and Purification of Solids--Melting Points

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Melting points of different mixtures of naphthalene and biphenyl were examined in this lab. Samples of these various mixtures were collected and tested by different individuals in the lab in order to find the eutectic point of biphenyl. Unknown substances were then tested using the mixture melting point method in order to determine their identities.

The melting point of a solid is the temperature where the substance changes from a solid to a liquid state. Using known substances and determining their melting points is a way to establish the purity of and to identify a sample. This lab demonstrates the temperature range where certain compositions of solid naphthalene and biphenyl begin to melt and completely liquefy. Pure substances will have what is called a sharp melting point. This means that the temperature where the solid begins to melt and completely melts does not differ by more than 5℃. Substances that have a mixed composition, or are less pure, are said to have a broad melting point and their melting range usually differs by more than 5℃. Eutectic points are an example of an exception to this rule due to the fact that while the solid is not pure it maintains a sharp melting point. Because many materials have equal or comparable melting ranges, determining melting points alone is not a suitable method for identifying a substance. Using the mixture melting point method is a reasonable way to substantiate the identity of an unknown substance. This method is performed by using two known chemicals as a standard for comparison and to mix equally with the unknown sample. Finding the melting points of the unknown and the two mixtures should provide acceptable data to establish the identity of the substance.

The substances used in this lab to establish the eutectic point (through finding melting points) and to identify an unknown...
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