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Name: Brandon
Status: educator
Grade: 6-8
Location: WI
Country: N/A
Date: 1/24/2006


Question:
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?


Replies:
Brandon,

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 mean that.

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)


Good Day,

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 Powder.

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 the iodine.

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 disappears.

Thanks,

Fred Boeheim


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.

Vince Calder



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