Joseph K. Myers
2-10-04
The melting point is the phase change from solid to liquid. The effect of impurities can change the melting point, or extend the melting point over several degrees of temperature.
The eutectic point is the average of the compound melting point that is formed by the mixture of materials.
In this experiment, the theory will be applied by observing the characteristics of melting points from differing ratios of two compounds.
There are none for this experiment.
Benzoic acid and Napthalene are used singly or in combination, so they are they only reagents.
| Compound | Molecular weight | Melting point (C) | Boiling point | Density |
|---|---|---|---|---|
| Benzoic acid | 122.122 | 122.35 | 249.2 | 1.2659 |
| Napthalene | 128.171 | 80.26 | 217.5 | 1.0253 |
The flowchart consists of the following table, which is completed easily by following the same process for each sample. The results of the melting points are filled into the table from during the experiment.
| Sample # | Benzoic acid | Napthalene | M.p. (C) |
|---|---|---|---|
| 1) | 100% | 0% | 123 |
| 2) | 98 | 2 | 122.5 |
| 3) | 95 | 5 | 109 |
| 4) | 90 | 10 | 122-125 |
The following instructions complete the flowchart of the experiment:
After a sample is prepared, the capillary melting point apparatus is used in order to find the temperature at which the sample melts (with sufficient time for the apparatus to cool between samples).
At less pure compositions of compounds, the melting occurs in a less precise fashion. Often a small amount of liquid will form at the top of the main sample, which remains solid until a slightly higher temperature.
The literature value of the melting points of the compounds is not known for the percentages used in the experiment, so there are no further results. The literature values of pure forms of both reagents are recorded within the table of reagents.
There is an additional source of error if some small portion of liquid solvent remains in an otherwise dry compound. This will be able to dissolve a percentage of the main compound, giving the appearance of partial melting before the true melting point is reached.
It such a case it might be better, when a sample is needed rapidly before proper drying, to glaze it from the top of a sample (which is likely to be very dry) with some instrument suitable for the purpose.
It is difficult to gather homogenous elements of a sample; this may result in differing temperature zones because of effective surface area of separated pieces of material.
1. Gilbert JC, Martin SF. Experimental Organic Chemistry. 2002. 3rd Edition. p107-115.
If it is unkind to recommend an expensive, newer book, there is also an expensive, older book:
2. Beyer H, Walter W. Organic Chemistry. 1997. Translated 22nd Edition. p3.