8th Grade Science – Professor Shai
Freezing-Melting Point Experiment
Section 3.2 – Page 41
Introduction
We
studied density
in detail as a characteristic property of matter.
Another such identifying property is the
freezing-melting point of a substance.
The freezing-melting point of a substance is the temperature at
which it
changes from a liquid to a solid (freezing) or vice versa from a solid
to a
liquid (melting). It is neat that these
two temperatures are usually the same!
Moreover, the change in state from solid to liquid requires heat. The heat does not change the
temperature the substance. Instead, the
heat is used exclusively to melt the substance that for the most part
remains
at a constant temperature throughout the melting process.
Once all the substance is melted, the heat
again raises the temperature of the substance until it starts to boil,
but
that’s our next lab.
The
freezing-melting point of water is well-known to you;
it is 32 degrees F and zero C.
In
this
experiment, you will measure the melting-freezing point of two unknown
substances. (If you really care, the
substances are called butylated hydroxy toluene (BHT), and
tert-octylphenol
(TOP)). The two substances have
different freezing-melting points, both well above room temperature. So at room temperature, both substances are
solids.
I will
give you
some random amount between 7 and 13 grams of one of the substances. Recall that characteristic properties are
independent of the amount of the substance being measured.
You will measure the freezing-melting point
of your substance and produce a graph of temperature versus time.
It
easier to
notice when something completely freezes than when it completely melts. Therefore, we look for freezing rather than
melting. In order to do this, we heat up the substance until it
is a liquid, and we plot the graph as it cools down and turns into a
solid. We heat the substance using a double-boiler. That is, we put the substance in a test tube
inside a beaker with water and we heat the water. This
technique is used so that we heat the substance gently and
evenly and do not burn anything. It is
a common technique used in cooking delicate items like chocolate sauce,
yum!