Regulars Exhibition chemistry
Microwaveable solvents
Declan Fleming shows you how to capture your students’
imaginations with spectacular demonstrations
I was once approached by
a confused student who
had produced copper oxide by
decomposing copper carbonate.
They had written their name on
a label and stuck it on the tube of
copper oxide. A few seconds later,
their name vanished.
After some questioning, it
turns out they had been using
an ‘erasable’ pen that becomes
colourless when the dye is
heated by rubbing it. Their tube
had not completely cooled
before they stuck on the label.
I was able to make their name
reappear by sticking the tube in
a freezer for a few minutes. This
gave me an idea for an extension
to a demonstration I do with
microwaves and solvents.
In science lessons the
microwave oven usually only
serves to help calculate the
speed of light or show the effect
of microwaves on a light bulb.
However it can also be used to
illustrate concepts in chemistry.
A common experiment
involves holding an
electrostatically charged rod
or balloon near to the streams
from burettes filled with a variety
of solvents. This demonstrates
how polar the solvent is. You can
perform a similar experiment
to show how the polarity of a
solvent affects how efficiently
a microwave oven can heat it.
This effect can be exploited
by chemists allowing for more
efficient heating of reactants in
solution – reactions can often
be far quicker and more selective
under microwave conditions.
Kit
icrowave oven
M
2 x 50 cm3 beakers
20 cm3 paraffin (also known
as kerosene, harmful)
20 cm3 water
Sticky labels
Thermochromic ‘erasable’
pen (eg Pilot ‘FriXion’) or
thermochromic mugs.
Using paraffin to demonstrate how the polarity of a solvent affects how efficiently a microwave oven can heat it.
See how to perform this demonstration at http://rsc.li/EiC513ec
Preparation
The idea here is to show that
the water will be heated in the
oven but the paraffin will not.
The heating time will depend on
the exact quantity of liquid used
and the power rating of your
microwave. Somewhere
between 30 seconds and 1 minute
will be enough. Practise this in
advance using insulating gloves
and anti-bumping granules. Aim
to reach a temperature above
60°C for the water, but well
below boiling, as microwave
heating carries with it the risk of
superheating and the associated
potential of scalding as the liquid
boils over following nucleation.
In the past I have placed the
two beakers in the microwave
and then passed them around
for the class to feel, but thermo­
chromic inks provide a visual clue
to what is happening too. The
thermochromic ink will disappear
when heated so I labelled the
beakers ‘not hot water’ and ‘not
hot paraffin’ respectively. The
words are written in normal ink
apart from the ‘not’, which is
written in thermochromic ink.
In front of the class
Place approximately 20 cm3 of
each liquid into their respective
beakers and ask the students
what they expect to happen.
Many will suggest that it would
be dangerous to put a fuel like
paraffin into a microwave oven
and may even predict that there
will be an explosion.
After heating for the time you
have determined, the water should
have reached above 60°C, high
enough to switch the ink to its
colourless form, and its label will
read ‘hot water’. The other beaker
will still read ‘not hot paraffin’.
Having first carefully tested
the temperature of the beaker
yourself, you can then offer them
both for students to feel.
Safety
hile in theory any non-polar solvent will work,
W
paraffin is particularly suitable for use in schools
as it has a higher flashpoint
Alternative method
The beakers can be replaced by
thermochromic mugs where a
pattern appears when they are
hot. Check the mug is microwave
safe before you use it!
Teaching goal
In order for microwave ovens to
heat a substance, the substance
must contain particles that are
free to move (eg in a fluid). If these
particles have some anisotropy
of charge, their orientation
can be affected by an applied
electromagnetic field.
Polar water molecules will
try to align themselves with the
oscillating field in a microwave.
However the resistance caused by
their intermolecular forces means
they are always ‘playing catch-up’.
The resulting collisions as they
stumble about lead to heating.
Molecules that are not polar will
not experience this force, and as
such the paraffin will remain cool.
Gavin Whittaker has written an
excellent article suggesting other
ways in which microwaves can be
used in chemistry lessons.1
Reference
1 G Whittaker, School Science Review, March
2004, 85, 87 (http://bit.ly/11JtiVf)
Education in Chemistry | September 2013 | www.rsc.org/eic | 13