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TGF1: Melting behaviour of mixtures
Notes on the video
Once you reach the section where there are no captions, you may prefer to use the slider to speed up the video. Students should notice that:

while the wax shows a definite start to melting, there is no such obvious change point for the lump of chocolate: chocolate does not seem to have an exact melting point;
at the end of the video, the wax has melted completely, but the chocolate is ‘gooey’ rather than being ‘runny.

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Notes on the use of the term ‘substance’

In 'Stuff and Substance' the word ‘substance’ is used with a precise meaning.
Some books use the word ‘substance’ to mean any kind of stuff: wax and chocolate would both be called substances. This misses the important distinction between a pure sample and a mixture. ‘Material’ is another word that is often used without a careful definition.

Books also often talk about ‘pure substances’, but in the way we use the term, a substance is a substance, and cannot be pure or not pure: being pure refers to the sample of stuff - is the sample just one substance or a mixture of two or more substances?

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