Date: Fall 2010
We made crystals from non iodized salt
and iodized salt to compare differences for my sons
science project due soon. The iodized salt made
perfect cubes the size of a pencil eraser, while
the non iodized made cubes much smaller and
"snowflake" looking crystals. My question is
why the difference if the iodine is an
impurity that should drop to the bottom
of the solution? Should they not be even due
to the impurities separating?
Iodized salt will contain a small proportion of
sodium iodate or sodium iodide, along with the
normal sodium chloride. In any case, the iodine
(in whatever form) will be soluble in the solution
so it will not be an "impurity that should drop
to the bottom of the solution". It does not contain
any elemental iodine, although this could form in tiny
amounts by the oxidation of the iodide ion, or
reduction of the iodate.
As you evaporate your solution to form crystals
the sodium chloride even though it has a higher solubility)
will certainly reach its solubility limit first, as it is
such a large proportion of the total solute, so the
crystals will be sodium chloride for both the normal and
The amount of iodide or iodate involved is so small that I
find it hard to believe that it will affect the crystal
formation of the sodium chloride. But crystal formation is
famously fickle, so I could be wrong. However, it is much
more likely that other factors are influencing the crystal
formation pattern. I would suggest that you repeat the study,
paying particular attention to controlling all the variables
that could affect crystal formation: initial concentration
of salt solution, volume of solution, surface area,
conditions and time of evaporation etc etc.
If you can repeat these results under controlled conditions,
they would be an interesting result! It is possible that the
iodide or iodate ion is influencing the crystal structure
(it only takes very small amounts of impurities to change
pure corundum to sapphire or ruby, for example...)
I understand your confusion; the thinking is correct as
far as it goes. Impurities should mess up a crystal if
they are included (not make it grow bigger/neater/faster),
and do nothing if they aren't included.
A small point: impurities not included in the growing
crystal may either precipitate elsewhere
(drop to the bottom), or remain dissolved.
In principle, remaining dissolved is what happens first,
and later if the concentration builds up precipitation
may happen ( and may interfere with the crystal too).
There is an in-between behavior called "habit-modifier".
Some substances that do not fit into the crystal
nonetheless hang around temporarily on the growing surface
and are cloying enough to have effects, such as:
- changing the relative growth rates in various crystal
- changing the crystal shape by changing the balance
between growth-rates of the directions of facets vs.
edges vs. points.
- suppressing the nucleations that mess up the crystal,
- assisting the kinds of nucleations that must occur
regularly for the crystal to keep growing fast
(such as starting a new row along a growth-step
on a surface, or starting a new layer on a
Both surfactants such as soaps and partly-interchangeable ions
(such as iodide vs. chloride in this case) can have such effects.
Of course, to be fair, you should make sure that your
salt-without-iodide has high purity, i.e., is equivalent
to the salt-with-iodide minus the iodine. It would have to
come from a chemical company with specs. Adding your own pinch
of iodide to this pure sodium chloride would make the fairest
Any consumer table-salt will often have sodium silicates in it,
to cover each grain with a hard, water-resistant thin skin,
to make it less easily glued to its neighbors by humidity.
This silicate stuff definitely settles to the bottom in a
crystal growth experiment, if you're lucky. Otherwise, it
may mess up and force small messy crystals such as the
snowflakes you mentioned. If you dissolve some salt in more
water than it needs, and the water is still faintly cloudy
instead of perfectly clear, that's probably the silicates.
Silicates can be dissolved in basic solutions,
so if you add a pellet of sodium hydroxide (such as drain-cleaning lye)
and the cloudiness disappears, I would take that as
a sign that the cloudiness was in fact silicates.
Not sure I would actually grow crystals with that batch,
but you could try it and see.
To make sure there are no silicates in your salt,
it needs to be bought from a chemical company
(or pharmacy drug counter), or else you need to crystallize it
once and wash the crystals with rubbing alcohol,
before dissolving it again to grow the crystals you really intend.
It is work and time, but crystal substances are distinctly
cleaner and grow different crystals the second and third time around.
I would have expected this experiment to work more or less
equally well with both kinds of salt, so I am really not sure.
Generally the things that determines the size and quality of the
crystals grown are concentration, temperature, cooling rate,
and the availability of nucleation centers. These include scratches
on the inside of the flask, dissolved impurities in the water,
and even vorticies created by stirring in highly
Usually to get big crystals you want slow cooling and heterogeneous
nucleation, i.e., scratches in the flask, a piece of string, etc. For sure,
if the crystals form too suddenly from a supersaturated solution
they will be small and flaky.
The iodine in iodized salt is not in the form of pure I2, so no I2
goes to the bottom of the beaker as an impurity. Different
manufacturers use different sources of iodine but in the US only
sodium iodide and cuprous iodide are allowed. Sodium iodide is
by far the most common. All four of these are pretty soluble and will
be in small enough concentration that I would be surprised if they
interfered with the experiment. They should instead stay in the liquid
above the crystals (chemists call this the "supernatant liquid").
According to the American Salt Institute's web site,
US manufacturers use 0.006% to 0.01% KI by weight in
iodized salts. Also, either of your two salts may also have a small
amount of fillers and/or anti-caking agents like dextrose and other
In the end, I recommend approaching this with the scientific
method. Try different salts from different manufacturers. Also,
you might try switching flasks and trying again with the same two salts.
Best, Dr. Topper
Iodine is added to salt only as an extra added health benefit.
The human thyroid gland needs a supply of iodine to properly function,
So salt manufacturers "iodize" their salt to provide this benefit.
Here is an online article that tells you all you would ever want to know
Refer to section 9 of this article for the biological role of iodine and the
So you ran a crystal experiment and one ended up with cubes and the other
ended up in a snow flake texture.
I do not think the cause is the presence of iodine.
Neither do I believe that the iodine should drop to the bottom of the
solution because there just is not that much iodine added to the salt.
(Iodine solubility in water is discussed in section 8.1 of the Wiki article)
One way to test for the presence of iodine is to rub the substance under
test on a potato and if that place on the potato turns dark purple then you
have confirmed the presence of iodine. (See the "Analytical chemistry and
bioanalysis" section of the wiki article. Sec 8.6) You probably have a lot
of impurities in your salt samples as well.
I would run the experiment again and be extra sure that the parameters are
all the same.
Update: June 2012