1. Give the two differences that are typically observed when comparing the melti

Question

1.  Give the two differences that are typically observed when comparing the melting behavior of an impure compound with that of a pure sample of the same compound.

2.  In attempting to identify an unknown carboxylic acid, you determine its melting point as 140-141 oC.  Tables list three common carboxylic acids with melting points of 140 oC; 2-chlorobenzoic acid, 3-nitrobenzoic acid, and suberic acid.  Assuming you have pure samples of these three carboxylic acids, and that the unknown carboxylic acid is one of these three, explain how you could identify the unknown compound using only melting points.  Describe exactly what you would do and expect to observe.

Explanation / Answer

1.The temperature at which a solid melts is known as the melting point (MP) of that substance. The
melting point is a physical property of a solid and can be used to help identify a substance. In
practice, a solid usually melts over a range of temperatures rather than at one specific
temperature. For this reason it is more useful to speak of a melting point range. Although the
term "melting point" is usually used, what is meant is "melting point range". If the compound
melts over a very narrow range, it can usually be assumed that the compound is relatively pure.
Conversely, compounds that melt over a wide range are assumed to be relatively impure.

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