Can you Change the Speed of a Reaction by Changing the Particle Size of the Reactants?

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Updated on May 06, 2014

Grade Level: 5th - 7th; Type: Chemistry


The goal of this experiment is to study a chemical reaction and evaluate whether the size of the reactants determines the speed of the reaction.

Research Questions:

  • What is a chemical reactant? What is a chemical reaction? What is a reaction rate?
  • How does the size of a particle affect the reaction rate?

Among other things, Alka Seltzer contains citric acid and sodium bicarbonate. When dropped into water, the sodium bicarbonate molecule splits into sodium and bicarbonate, and the citric acid releases a hydrogen ion. The bicarbonate molecule reacts with hydrogen ions to form carbon dioxide. This reaction can be expressed as: 3HCO-3 + 3 H+ Here, the citric acid and sodium bicarbonate are reactants, the hydrogen ion and bicarbonate molecule are intermediaries, and the water and carbon dioxide are products. The speed of this reaction can be accelerated by breaking up the reactants so that more surface area is exposed. Since there is more surface area available to react, the reaction occurs faster.


  • One package Alka Selzer
  • Four transparent beakers
  • Water
  • Serrated knife
  • Pestle and mortar (if available)
  • Stopwatch

Experimental Procedure

  1. Pour one cup of water into a transparent beaker. Repeat three times so that you have a total of four beakers, each full of water. Measure the temperature of the water. Once the temperature of the water is at room temperature, you can proceed.
  2. Drop one Alka Seltzer into a first beaker of water. Start your stopwatch. How long did it take the Alka Seltzer to dissolve?
  3. Break one Alka Seltzer in half. This may be easier if you score the surface of the tablet with a serrated knife first. Drop the halves into the second beaker of water and start the stopwatch. Note how long it takes to dissolve.
  4. Break one Alka Seltzer into quarters, scoring the tablet if necessary. Drop the quarters into a third beaker and measure how long it took to dissolve.
  5. Break the Alka Seltzer into many crumbly pieces, using a pestle and mortar if necessary. Add the crumbly bits to the fourth beaker and measure how long it took to dissolve.
  6. Graph your results. Your y-axis should reflect the relative size of the reactants so you can use the numbers 0.125, 0.25, 0.5 and 1.0 to show reactant size. The x-axis can show time in seconds.

Terms/Concepts: Reactants, Intermediaries, Ions, Products, Particle Size, Surface Area, Reaction Rate



Moore, John T. Chemical Essentials for Dummies. For Dummies Press (2010. This text should not be confused with the other Chemistry for Dummies books, which may be a little too advanced.

Gonick, Larry. The Cartoon Guide to Chemistry. Collins, (2005)

Cy Ashley Webb is a science writer. In addition to having worked as a bench scientist and patent agent, she judges science fairs in the San Francisco bay area. She loves working with kids and inspiring them to explore the world through science.

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