| MadSci Network: Chemistry |
Dear Scott,
The chemical reaction that makes Benedict’s solution turn from blue to red is a reduction of the Cu2+ ion by the various sugars to Cu2O which then precipitates. Cuprous Oxide is a reddish brown powder which is mostly insoluble in water. As larger amounts of the oxide are formed, the blue of the aqueous ion Cu2+(H2O)4 is decreased forming nice color sequence. When you have finally used up all of the Cu2+ ions, the solution will look a dirty red brown from the copper oxide suspended in the sugar, base, and leftover reagents.
You might want to look at the MadSci archives, particularly this question,
and
this one, where the color sequence of the reaction is described. Chemical
oxidation and reduction have a number of websites that can be tapped for
information. Some of them are:
http://www.bced.gov.bc.ca/irp/chem1112/ch1222.htm
http://cator.hsc.edu/~mollusk/ChemArt/redox.html
http://erkki.kennesaw.edu/~lcombs/genchem4/ge00010.htm
A trip to the public library, looking up REDOX (REDuction and Oxidation) or either of the two terms should provide a plentiful supply of information. Freshman college texts should also be in the library and are sure to have more information that you can assimilate.
As far as your chemistry research project, remember that many things other than sugars can cause Benedict’s solution to change colors. Also, many materials taste sweet that are neither sugars nor reducing materials (Nutrasweet® for example). One idea might be to search the web for information provided by the supplier of the toothpaste. A very good document to have is the Material Data Safety Sheet (MSDS) that suppliers have to make available. That can describe the general composition of the material, without giving away trade secrets.
Good Luck and I hope this helps you out.
Dr. Michael M. Gallagher
Senior Research Chemist
J.R. Simplot Co.
Try the links in the MadSci Library for more information on Chemistry.