ARTICLES
Snot, Snow, Milk – Chemistry!
By Chris Warren
A precipitation reaction between a group I carbonate and a group
II chloride is much more complicated than you might initially
think. The reaction between 2 molL -1 potassium carbonate and
2 molL -1 calcium chloride has always been my go-to reaction
when I want to pique the interest of even the most disengaged
student. However, the downsides of this experiment are cleaning
the carbonate that adheres to the glassware after the experiment,
and the large amount of reagent required.
When the students “massage” the bag to mix the reactants, a gel
begins to form in the bag after a few seconds. With more mixing,
the gel begins to thicken and become a brighter white colour.
Still more mixing and the students are left with a white solid - all
the liquid component has disappeared.
Gel
Baggy science to the rescue! Performing the reaction in a small
zip-lock bag (Jones, 2019) reduces these problems but has an
additional advantage: students just love the tactile nature of the
products of this reaction.
Prepare a 2 M solution of anhydrous potassium carbonate by
• dissolving 138 grams in 500 mL of deionised water, and prepare
a 2 M solution of anhydrous calcium chloride by dissolving
110 grams in 500 mL of deionised water. This is sufficient for
20 experiments.
• Students add 25 mL of each solution to a plastic bag and zip
it shut.
• Mix the solutions together in the bag for about 3-4 minutes by
hand.
Hazard Analysis
Yet more mixing and the solid suddenly turns into a milky white
suspension. Left overnight, the precipitate settles out leaving
behind a clear solution on top of a fine white precipitate of calcite
(CaCO 3 ). This could then be filtered, dried and weighed.
At the concentrations described above, both calcium chloride
and potassium carbonate have a the GHS signal word “Warning”.
They are harmful if swallowed and can cause eye and/or skin
irritation. The products are designated as non-hazardous. It is
the responsibility of teachers supervising this experiment to
carry out an appropriate risk assessment.
Precipitate
Initial appearance
The chemistry of this reaction is not straightforward. Carbonate
ions also cause water molecules to dissociate and produce
hydroxide ions which remain present until the final stage. The gel
is mostly a hydrated calcium carbonate CaCO 3 .H 2 O, but there
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SCIENCE EDUCATIONAL NEWS VOL 68 NO 4