Kids, did you know that the rate of a chemical reaction can be affected by the physical size of the reactants?   When decreasing the size of particles that weigh a certain amount, you will increase the number of particles.  Here you will test the hypothesis that smaller particle size can increase the rate of a reaction because the surface area of the reactant has been increased.  Before beginning, make a prediction that a powdered form of a tablet will react X times faster than a whole tablet (guess at X).

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

You will need: 3 clear glasses, 3 Alka-Seltzer® tablets (original formula effervescent), a mortar and pestle, and a stopwatch. Here is the procedure:

1. Whole Tablet
   a. Fill a clear glass with exactly 8 oz. of room temperature or lukewarm water.
   b. Drop 1 whole Alka-Seltzer tablet into the water. Measure and record the time to dissolve.
2. Tablet Broken into ~8 Pieces
   a. Place 1 Alka-Seltzer tablet onto a sheet of paper and break into approximately 8 pieces of about equal size.
   b. Fill a clear glass with exactly 8 oz. of room temperature or lukewarm water.
   c. Slide broken tablet into the water from the sheet. Measure and record the time to dissolve.
3. Powdered Tablet
   a. Place 1 Alka-Seltzer tablet into mortar and grind to a fine powder.
   b. Transfer powder into a clear cup. (Note: It's important to have the powder in the cup before adding water.)
   c. Add 8 oz. of water to the glass. Measure and record the time to dissolve.

From your measurements, determine whether the rate of reaction increased or decreased as the particle size decreased, and by how much. Was is 2 times faster, or more? 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
March 2007

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Reference:
http://www.alka-seltzer.com/asp/student_experiments_2.html