Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium hydroxide : NaOH
sulphuric acid : H2SO4
hydrochloric acid : HCl
calcium nitrate : Ca(NO3)2.4H2O
ammonium iodate : NH4IO3
Class:
------
elem=Ca,I
precipitation
Summary:
--------
Calcium iodate is very sparingly soluble in water, but it can form strongly
oversaturated solutions and may take a long time to crystallize.
Calcium iodate also is quite stable, it must be heated strongly in order to
decompose it.
Description:
------------
Prepare a solution of ammonium iodate. Do this by taking a spatula full of
solid ammonium iodate (appr. 300 mg), add a few ml of water, and then gently
heating this solution.
Prepare a solution of calcium nitrate. Take approximately the same amount (by
the eye, appr. same volume) of solid and add a few ml of water. The solid
quickly dissolves and gives a clear solution.
Mix the two solutions (both of which are moderately concentrated, somewhere
between 5% and 10% by weight): When the solutions are mixed, then no
precipitate is formed. The resulting liquid is clear and colorless. It is luke
warm, due to the heating of the initial solution of ammonium iodate.
Set the liquid aside: Even after tens of minutes and cooling down to room
temperature there still is no precipitate.
Take a look one day later: Quite a large amount of needle-like crystals (length
is appr. 4 mm, width is less than 1 mm) has settled at the bottom.
Take the solid material out of the test tube and put on a filter paper, such
that nearly all of the liquid is absorbed by the paper. Then allow to dry in a
warm place: A nice dry and non-hygroscopic white material is obtained,
consisting of many little needles.
Heat some of the dry material: Even with very strong heating (the glass test
tube started to glow orange a little bit) there only is weak decomposition.
Iodine is released and the solid becomes somewhat darker (pale yellow/brown).
Very small crystals of iodine settle at the glass of the test tube at cooler
places.
Allow to cool down and add concentrated hydrochloric acid: The liquid becomes
yellow and some chlorine gas is produced (smell!). All of the solid dissolves
and the liquid becomes clear. The fine crystals of iodine also quickly dissolve
when the yellow liquid is rinsed over them.
Add a small amount of dilute sulphuric acid: There is no visible change, the
liquid remains yellow and clear.
Dropwise add a solution of NaOH: Each time when a drop of NaOH is added, a
white precipitate is formed, which quickly redissolves on shaking. However,
after adding a certain amount of NaOH the precipitate does not redissolve
anymore and the yellow color nearly disappears.
-------------------------------------------------------
Based on these observations, one can only conclude that the white solid must be
Ca(IO3)2. This solid only is sparingly soluble in water, but solutions of it
can be strongly over-saturated. Calcium iodate is quite stable, much more so
than the starting material ammonium iodate.