Ask A Scientist Chemistry Archive

name         Jennifer Lynn H.
status       educator
age          20s

Question -   I need an UNCOMPLICATED explanation for my sixth grade
students.  We have finished a lab about chemical and physical
changes.  Can you explain the difference between these two
reactions?  After adding iodine solution to baking powder, the powder
slowly turned very black, similar to wet ash.  The same thing was
observed when adding iodine solution to cornstarch.  My resources say
that the iodine with baking powder was a physical change, and the iodine
with cornstarch was a chemical change.  I am wondering if they are both
actually chemical changes, or is my information correct?  If so, what is
the difference between the two reactions that we are not seeing?
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Iodine reacts with starch to make a dark blue complex.

Most baking powder contains starch as an extender/standardizer.  Baking
powders are made with different formulations, so starch is added in varying
proportions to give them all the same rising power per teaspoon.  Most
likely, the starch in your baking powder reacted with the iodine, just like
the plain starch did.  No difference.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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I am not aware of any chemical reaction between dry baking powder (sodium
bicarbonate) and iodine. The black color probably results from the physical
absorption on I2 onto the surface of the NaHCO3. If there were moisture
present some HI might form which could react with the NaHCO3 to produce CO2,
but that is a stretch!!

The reaction of I2 with starch (actually not all starches are equally
effective) beta amylose is the actual starch component that is reacting to
form an intensely colored blue complex believed to be I5(-1) -- probably a
complex of I3(-1) and I2. However, I have not found any reference that gives
strong evidence for the structure of the complex.

I do not know your resource for a physical vs. chemical change, but I would
have selected different examples, where the contrast is clearer and the
chemistry is simpler.

Vince Calder
===========================================================
This inquiry revisited. If you do a search using the string "chemical
physical change" on the search engine www.google.com
you will find numerous websites with a wide variety of examples to choose
from.

Vince Calder
===========================================================
Jennifer,

In both instances, you were observing a chemical change because in both
situations, new substances were formed. Consider the following -- probably
more than you wanted to know.

Every substance has its own set of characteristic properties which serve to
identify it and determine its usefulness. When chemical changes occur, new
substances with different properties are formed. For example: When a lump of
coal is burned, it is transformed into carbon dioxide, water, ash, and many
other new substances that were not present in the original piece of coal.
During the combustion process, they were formed by a rearrangement of the
atoms and molecules that made up the coal.

In a carefully performed, controlled chemical change, where precautions are
taken to ensure that no reactants or products are lost; mass is always
conserved.  This means that in chemical processes, matter can be changed in
form and function, but the quantity of matter undergoing the change can
never be altered or destroyed. In a sense, all matter is immortal; its atoms
can be disassembled and reassembled into an endless variety of structures,
but the atoms themselves cannot be annihilated in a chemical change. The
atoms of your body have been around forever; it is just that they have been
arranged to build "you" just this one time.

The chemical immortality of matter has prompted chemists to accord its
permanence the stature of a chemical law. It is called the Law of
Conservation of Mass.

Some examples of physical changes are cutting a substance, grinding it,
melting it, bending it, dissolving it in an inert solvent. In all these
instances, the form of the substance was changed but not the properties of
the substance. For example: Cutting a nail in half certainly changed the
form of the nail. Still, its composition (iron) remained unchanged. Intact
or cut in two, the nail would still be attracted to a magnet. Thus, a
physical change. However, if you were to dissolve the nail in acid, a
chemical reaction between the iron and acid would occur. If a magnet were
dipped in the resulting solution, no particles of iron would accumulate on
it.

Nearly always, chemical changes result in physical changes as well. However,
the reverse is rarely true. As distinct from chemical changes, physical
changes, on the other hand, do not involve the formation of new substances.
In a physical change, atoms, molecules, and formula units of a substance may
be rearranged, but the bonds that hold the atoms in their original molecules
or formula units are, for the most part, not broken.

How to decide:  Is it a chemical or physical change? Here is a simple
question you might ask yourself to help you with the decision:

Has the process produced new substances with properties that are different
from those of the original material?

If your answer is yes, the process is an example of a chemical change.

Regards,
ProfHoff 418
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