Pure substances melt "sharply", i.e. within a very narrow temperature range; contaminated substances melt in a wide and lower range. A melting range of 1 to 2 degrees is not unusual for an organic substance.
Something that has been forgotten by modern analytical machinery is the mixture melting point for the determination of identity: If an unknown substance is suspected to be a particular substance, mix a sample of both substances and determine the melting point of the mixture. If it does not change, the sample and reference substance are identical. If it drops, they are different.
If the substance does not melt even when heated to the limit, the melting point cannot be determined with this device. But it is still possible to say that the melting point is > XXX °C which should also be noted in the laboratory notebook and later in the log.
If, when determining the melting point, your substance somehow always decreases and, instead, deposits appear at the upper end of the capillary or you know that your substance sublimates from the outset, you need to melt the melting point tubes after introducing the substance. The resulting internal pressure during heating prevents sublimation.
Not all substances can be heated to the melting point without decomposing, but decompose beforehand. Signs of decomposition are:
Decomposition points are not sharp and cannot be precisely reproduced. Greater deviations from existing literature values can therefore be regarded as tolerable here. Decomposition points are specified with a trailing "Z". (e.g. “278 °C (Z)”)
Source: Freie Universität Berlin - Institute of Chemistry and Biochemistry
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