Re: what happens when corn starch mixed in water is heated?

Hi Vivianne.

You've identified a chemistry question in your kitchen, where there is a 
lot of chemistry occurring every time you cook.

Corn Starch is something called a "polymer".  You can think of the box of 
corn starch as containing a huge number (millions of billions of billions) 
of things called molecules.  Each molecule is made up of the atoms 
carbon,oxygen and hydrogen.  An atom can be thought of as the building 
blocks of all molecules, and molecules can be thought of as the 
smallest things that have properties that you can notice.  For example, 
table salt is a molecule (it has taste for example) made up of the elements 
sodium (a metal) and chlorine (a poisonous gas)...the resulting molecule 
has new properties over those of the atoms.  If this is confusing, don't 
worry too much, as it isn't critical to understanding the rest of this 
response.

Imagine the starch molecules as long sticky strings.  When you add them to 
cold water, they don't dissolve very well.  If you add salt to cold water, 
you'll see that it does dissolve pretty well, and after adding enough salt, 
you saturate the solution (that is, add so much that no more will 
dissolve).  If you use hot water, more starch can dissolve (solubility is 
the scientific word for how much material can dissolve in a particular 
liquid, and solubilities generally increase as temperature goes up).  When 
the starch dissolves, it is free to tangle up with the other "sticky 
strings".  Tangled sticky strings make the solution of water/starch thicker 
(scientifically called more viscous) because the solutions resist motion 
(this is why things like oil are "thicker").  Salt molecules are smaller 
and are more like marbles than strings.  As a result, a solution of salt 
doesn't get thicker.  

I hope this answers your question!
Please feel free to email me at weibel@chemistry.chem.utah.edu if you have 
any further questions.

Best Regards,
Mike