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Melting points

Updated Thursday, 3rd August 2006

What can we tell by comparing melting points?

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Carrots Copyrighted  image Icon Copyright: BBC

In 1888 Friedrich Reinitzer was typical of many ‘gentlemen scientists’ of his time. He was working during a period when science still hadn’t grappled with or delivered a adequate account of the nature of atoms and how they combined to produce molecules. Without such a fundamental system of explanation, many of the phenomena of the natural world must have seemed disorganised and perhaps even chaotic.

He will have spent many hours in systematic observation of natural phenomena. Investigating the colour of natural objects seemed quite a common practice amongst these scientists, and during 1888 Reinitzer entered into an investigation of the carrot. He was interested in one compound in particular found in the carrot; cholesteryl benzoate. His interest in this compound was possibly related to an interest in explaining the characteristic colour of the carrot.

As part of his observations Reinitzer would have collected some standard sets of data. Obviously, there were only a limited range of observations, standard tests and experiments which could be made in a lab of a hundred years ago. However, one of the most important observations concerns that of melting since most (but as we know now - not all!) pure substances which melt at all have a unique melting point.

Now Reinitzer had had problems with the isolating the benzoate compound from the raw carrot in his own lab. He had approached a manufacturer to provide a more pure sample. One of the major surprises was that not only did his home made sample have multiple melting points - but so did that of the external and apparently pure source. So why was melting point so important and why should there have been only one?

A melting point is a singular, measurable, precise point on the thermometer scale where the substance turns from a solid to a liquid. The existence of melting points was a very important observation for scientists to make at the time, not least because it was a practical and relatively simple test, it could be done with a fairly simple set of equipment.

The melting point test was also one amongst many tests which helped to begin to identify a substance. Many substances were logged by their broad appearance, their colour, their reactions with other standard compounds and of course melting point would be another key defining index - recorded, published and disseminated within the scientific community as a defining characteristic of a substance.

However, a unique melting point also indicated something else. It indicated that the substance being examined was pure. If a substance was a mixture of different compounds then there would be no single melting point. Each of the components in the mixture would have its own melting point - and as the temperature was raised it would appear that the mix as a whole would melt over a range of temperatures.

Now one reasonable question to ask is why should a pure substance have only one measurable melting point? Think about a container with a mixture of pure ice and pure liquid water. If you start to heat it and measure the temperature change what would you see? You might assume that the temperature of the liquid component of the mix would begin to rise in advance of the temperature of the solid components.

So what you might not see a single point but a temperature range over which it melted. However, (and you could test this) if mixture of liquid and solid substance at its freezing point is heated something rather surprising happens. The temperature of the mix doesn’t change at all - but remains fixed.

Why? The heat energy is absorbed not in increasing the temperature of the mix but in providing energy to change the state of the solid. It’s as if one thing (changing state) takes priority over another (changing temperature). Until one process is complete - the other can’t happen.

Melting points tests are an important initial crude means of identifcation and reasonably accurate measure of purity and are often still carried out in labs today.

This article was originally published in 1999





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