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MELTING POINTS AS CHARACTERISTIC PROPERTIES

The melting point of a substance is defined as the temperature at which the solid and liquid form of the substance can co-exist. E.g., for water, under normal atmospheric pressure, at temperatures below 0oC, the solid, ice, is the stable form for water. At temperatures above 0oC (and less than 100oC under normal atmospheric pressure), the liquid form of water is the stable form. At exactly 0oC, the addition or removal of heat can change the relative amount of ice and liquid water, but both are stable. The melting point can also be characterized as a freezing point.

The melting point of a substance under prescribed conditions of pressure (usually, 1 atm) is a characteristic property of that substance. Indeed, the Handbook of Chemistry and Physics indexes its extensive list of the properties of organic compounds by melting points.

For a sample of a known substance, the melting point provides a second useful type of information. You should be familiar with the phenomenon of freezing point depression - the lowering of the melting point of one substance when a second substance is dissolved in the liquid form of the first substance. The use of salt to melt ice at temperatures at which ice is the stable form of water is one practical application of this phenomenon. The use of salt/ice mixtures to freeze the liquid mixtures that constitute ice cream when frozen is another. Based on this principle, we expect that the melting point of a substance will be lower if it is impure than if it is pure. The deviation of a melting temperature of a sample of a substance from the accepted melting point for that substance can be an indication of the existence of an impurity. The extent to which freezing points are depressed by impurities is not very large. For water, dissolving one mole of a substance in 1kg of water causes a depression of only a little less than 2 oC.

In the laboratory, we can turn this logic around to our benefit- even when we deal with pure but unknown substances. For example, suppose in a laboratory exercise, a student identifies an unknown to be malic acid. The melting point of malic acid is known to be 100oC. About eight other organic compounds have melting points of 100 +/- 0.5 oC listed in the Handbook of Chemistry and Physics. Suppose we have an pure, authentic sample of malic acid. We measure the melting point of our tentatively identified unknown with a result of 99 +/- 1oC. We measure the melting point of the pure malic acid to be 100 +/- 1oC. The identification of the unknown is consistent with malic acid but it could also be any of the other 8 materials with a similar melting point. Suppose we next measure the melting point of an intimate mixture of the unknown and pure malic acid. If the mixture melts substantially below 100 oC, we have strong evidence that the two substances are different and what we are observing is the freezing point depression of both of the components of the mixture. If, on the other hand, the result is near 100 oC, we have strong evidence that the two substances are the same. The effectiveness of this technique depends on the relative solubilities of the substances in each other.

The above procedure is called the determination of a mixed melting point.

 

MEASURING MELTING POINTS

 

We use a melting point apparatus to measure the melting points of a number of materials in the Introductory Chemistry Laboratory Course. A picture of the apparatus is shown below with its components labeled. The samples are placed on a glass cover slip which fits into the holder

MP apparatus

Holder Close-up

A close-up of the holder

The sample is placed on a glass slip

   

The magnifier is adjusted so the sample can be observed as it is heated

   

While the melting point of a substance is defined to be a temperature with arbitrarily high precision, their experimental determination can result in quantities with considerably less precision. For example,

  • if the rate at which heat is introduced into the sample is too fast, the sample may heat unevenly producing local melting and overheating of a part of the sample before the entire sample has melted. This will result in the appearance of a range of melting temperatures rather than a well defined temperature.
  • if the substance has a particularly low ability to conduct heat, the sample may be heated unevenly.
  • If the sample is a powder of non-uniform sized particles, the melting can agan produce a range of temperatures
  • If the sample is not pure, the presence of substances other than the one of interest will affect the melting point as described earlier -- particularly if the impurity is not uniformly mixed with the desired substance.

Finally, some substances simply do not have a well defined melting point. This may be because the substance does not have a well defined composition (such as a sample of a polymer in which the polymer molecules are not of uniform molecular mass), or the material is in a thermodynamically metastable state (such as a glass or a supercooled liquid).

   

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Last Update: RFS 2015-07-03