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Crystalline Substance Shrinks When Frozen
Name: Martha
Status: educator
Grade: 6-8
Location: MD
Country: USA
Date: Summer 2011
Question:
I can demonstrate to students that water expands when it
freezes, by putting water in a glass jar, marking the volume,
freezing it, etc. My question is this: Is there a liquid (readily
available, freezable with a normal freezer) that I could use to
demonstrate that substances other than water are more dense in the
solid than the liquid form? (I would like to be able to show that
the substance "shrinks" when frozen.)
Replies:
This is easy to demonstrate, and you do not even need a freezer. All
you need is a small package of ordinary paraffin wax, used for sealing
jars of home made jams, jellies etc. It is usually sold at larger
grocery stores.
Paraffin wax usually freezes (depending on the exact type) at around
60°C to 80°C. Heating it over a pot of boiling water will melt it, then
when you remove it from the heat, it will gradually "freeze" to a solid
as it cools, and in doing so, it will very visibly shrink.
Regards,
Bob Wilson.
Martha,
Water is actually the exception here in that it is less dense (larger)
in the solid form than in the liquid form. Almost every other liquid
will be the other way with the liquid taking up more volume than the
solid. Anything not water based will likely work though the larger the
difference in the density the larger the change will be that you see.
Two demonstrations we do here are vegetable oil (where 5 mL in a 10 mL
grad. cylinder with shrink to about 4 mL) and wax. For the wax example
we heat it until a liquid and then pour into a test tube. As it cools
you can see it shrink and voids form in the solid.
Brad Sieve
There are other substances that expand when frozen, but I don't know of
any simple, cheap candidates.
vince Calder
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