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.
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?
