When iodine is mixed with both baking powder and
cornstarch, it produced similar results: a dark purple, almost
black color. My resources tell me that iodine with baking powder is
a physical change, while iodine with cornstarch is a chemical
change. What is the difference between the reactions?
I am not sure about the accuracy of your sources. Baking powder contains
sodium carbonate, some acid (usually cream of tartar) and a drying agent
(usually starch). Cornstarch, of course, also has starch. When iodine is
mixed and interacts with starch, I3- ions will get trapped within the starch
forming a complex that ranges in color from brown (for very dilute
solutions) to deep blue. Since this color is very strong, it only takes a
little bit of this complex to form in order to perceive the dark color.
Thus, both baking powder and cornstarch -which have starch- will turn dark
because of the complex formation.
I thought for a while that you might have meant baking soda - which is pure
sodium carbonate - in which case, it may be possible to form sodium iodide.
However, sodium iodide in solution is yellow and not black. So you can't
Thus, the change is the same whether iodine is added to baking powder or
cornstarch - it is the complex formation of I3- in starch.
Greg (Roberto Gregorius)
This is an interesting situation, in that the observations support the
chemistry, yet the documentation may not support the observations. Possibly
a misprint, where the original experiment had Baking Soda, in lieu of Baking
Baking Powder typically consists of a dry mixture of Baking Soda (NaHCO3, a
base), Cream of Tartar (KHC4H4O6, an acid) and Filler (usually corn
starch). By adding the iodine solution to Corn Starch, or Baking powder, it
is not surprising that you observe a similar change. In either case, the
change is a chemical change or chemical reaction of the starch reacting with
As observed, when starch is mixed with iodine in aqueous solution, a colored
starch/iodine complex is formed. This complex is utilized to indicate redox
titrations that involve iodine because the color change is so pronounced. It
can also be used as a general redox indicator. When there is excess
oxidizing agent present, the complex is blue, when there is excess reducing
agent, the complex breaks up into iodine and iodide and the color
The difference in a "physical change" opposed to a "chemical change" is to
a large extent a matter of definition. Sometimes, like in the iodine
reaction with baking soda compared to iodine with starch is fairly clear
cut, but that is not always the case. The dark purple color when iodine and
some starches are combined is due to a complex formed between the two. In
the case of mixing iodine and baking soda, no obvious chemical change
occurs, so it is referred to as a "physical change". However, there are
many chemical processes in which the terms are blurred, and distinguishing
a change as "chemical" versus "physical" becomes arbitrary. There are many
chemical reactions that occur without any obvious changes that are easily
observable, and there are many physical changes, for example melting of
ice, that can be considered to be chemical even though that is not the
usual way of thinking about that process.
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Update: June 2012