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The Salt in the Soup
The following wouldn't be a investigation for an Aztec archaeologist, but rather
for a chef or baker: Suppose there is a crystalline, white substance in a bowl in
the kitchen. Is it salt or sugar? One easy way to answer that question would be
to taste it. But what if you didn't know if it was even edible? And if you were to
solve the problem by tasting, you would also deprive yourself of aesthetic pleasure
for your eyes. Under the microscope, crystals are astonishingly regular and beauti-
ful structures — and sugar and salt can indeed be differentiated on the basis of
the shape of their crystals! While crystals that are as regular as the sodium chloride
crystals in the prepared slide are not so simple to produce, interesting results can
be achieved with very little effort nonetheless.
Growing Microcrystals
In order to grow microscopically small crystals directly on the slide, you will
need:
• a slide
• the pipette and water
• the dissecting needle for stirring
• common salt, sugar, mineral (inorganic) fertilizer (such as fertilizer for potted
or garden plants). When using mineral fertilizer, heed the warnings on the
packaging and have an adult help you!
Respectively place about three grains of salt, sugar, and fertilizer on the middle
of a slide. You can already check now whether the grains can be differentiated
with a microscope. This time, you can even choose whether you would like to
work with the magnifying glass (macro-view with incident light) or the micro-
scope (with transmitted light). Try both — you'll see that both are worthwhile!
Now drip two or three drops of water onto each slide, right onto the grains. If
you stir gently with the dissecting needle, they will dissolve more quickly. Now
you'll have to wait until the water has evaporated again. Depending on the
temperature, that may take several hours. Once all of the water has evapo-
rated, you will be able to see a white crust with the naked eye where the clear
water drops had been. Under the microscope, however, the crust turns out to
be a "crystal garden." With some luck, you'll find some larger crystals that are
as nicely formed as the ones in the prepared slide.
Another tip: The slower the water evaporates, the greater the possibility of
beautiful crystals forming. So it's not a good idea to help the evaporation
along using a blow drier or radiator!
Common salt forms regular, rectangular crystals.
Sugar often crystallized in irregular shapes. Star-
like structures are visible here.
Did You Know?
Mineral water got its name because
it contains minerals — salts, to be
precise. You can allow one drop
each of tap water and mineral
water to dry next to each other on a
slide and have a look at the differ-
ence under the microscope.
Sea salt recovery in Portugal. As the water evapo-
rates, fine salt crystals are left behind. Image:
Marisol
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