ARTICLES
Snot, Snow, Milk – Chemistry! (continued)
their experience allows them to recognise that it doesn’t follow
the normal reaction path observed in precipitation.
will be some calcium hydroxide present. The white solid is a solid
solution of potassium chloride in the hydrated calcium carbonate
solid. The precipitate is vaterite (a meta-stable form of CaCO 3 )
which will convert to calcite over a few hours when in contact with
water (Declet et al, 2015; Rodriguez-Blanco, et al 2010).
To illustrate some unusual data you can obtain from this
experiment, I performed the experiment in a coffee-cup calorimeter
(using 50 mL of each solution) and stirred with a PASCO digital
thermometer, monitoring the data using the SPARKVue app.
The net reaction can be simplified to:
K 2 CO 3 (aq) + CaCl 2 (aq)
→
CaCO 3 (s) + 2 KCl(aq)
As well as being visually engaging, the feel of the gel and solid
forming is appealing to students. You will get exclamations such
as “Yuk- it's snot”, “Have we just made snow?” and “Is it milk?”
The initial gel formation is exothermic, but may not be noticed.
The drop in temperature when the water of hydration is released
is noticeable to the hands.
To extend the sensory nature, you could also first prepare the
calcium chloride solution in the bag by combining 4.4 g of
CaCl 2 solid with 25 mL of water, before adding the prepared
potassium carbonate. Both the solids dissolve exothermically
(General Chemistry, 2019) – but the calcium chloride is much
more noticeable and will reach ~ 40-50°C (CAUTION!).
CaCl 2 (s)
K 2 CO 3 (s)
→
→
Ca 2+ (aq) + 2 Cl − (aq) ∆H = −81.3 kJmol -1
2 K + (aq) + CO 3 −2 (aq) ∆H = −30.9 kJmol -1
The formation of the gel involves two exothermic steps, whilst the
release of the waters of hydration is endothermic. Note that if the
calorimeter is scratched or not cleaned properly, you may not
observe two distinct exothermic steps.
While I mainly use this experiment in Stage 5 as a way of
engaging students as an introduction to precipitation reactions,
other applications of this experiment are plenty:
It also lends itself to a good project for a Stage 4/5 Science Fair
and is fertile ground for a Year 11 Chemistry Depth Study. The
chemicals are relatively safe to use for Years 7-12 with standard
handling procedures.
• You can use it when studying mixtures in Stage 4 to give visual
impact to the terms solution, gel, suspension and precipitate;
• Filter and dry the precipitate formed (Stage 4); • Track changes
phenolphthalein;
in
pH
using
universal
indicator
or
• Weigh the bag at each stage to prove the Law of Conservation
of Mass (Stage 5); • Add food colouring to one of the solutions and observe
changes;
• Use it as a discussion point that chemical equations
representing the overall reaction may not capture the reaction
mechanism and intermediates (Stage 5 or 6); • Examine the crystals produced at different stages under a
microscope;
• Compare the reaction with and without agitation;
• Increase the volumes to 50 or 100 mL and perform the reaction
in a polystyrene cup while stirring with a temperature probe to
measure the enthalpy changes associated with each step in
the reaction, using q = mc∆T (Stage 6). This may also lead
to a discussion about what value should be used for heat
capacity;
• Make a time-lapse video;
• Use different combinations of group 1 carbonates and group
2 chlorides: e.g. Na 2 CO 3 and MgCl 2 to look for trends and
patterns in the precipitate;
• Try using different temperature solutions to see if the solid
phase always appears or compare % yield;
• Calculate the entropy/enthalpy changes for the overall reaction
from second-hand data (Stage 6).
• Try altering the concentration of one or more of the reactants
to find the concentration at which the solid solution no longer
forms;
Interestingly, I find that older students get more intrigued by the
experiment than younger students. I’m not sure why, but maybe
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SCIENCE EDUCATIONAL NEWS VOL 68 NO 4