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Big Pieces or Small Pieces: Which React Faster?

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Some chemical reactions occur explosively fast, others may occur almost imperceptibly s-l-o-w-l-y. This project explores what effect the particle size of the reactants has on the speed of a chemical reaction: production of carbon dioxide gas by an Alka-Seltzer® tablet.


Areas of Science
Time Required
Short (2-5 days)
Material Availability
Readily available
Very Low (under $20)
Adult supervision recommended when working with hot water solutions

Andrew Olson, Ph.D., Science Buddies



The goal of this project is to measure the effect of reactant particle size on the rate of a chemical reaction.


You may have seen a television commercial for Alka-Seltzer tablets, or heard one of their advertising slogans: "Plop, plop, fizz, fizz, oh what a relief it is!®" When you drop the tablets in water, they make a lot of bubbles, like an extra-fizzy soda. And like a soda, the bubbles are carbon dioxide gas (CO2). However, with Alka-Seltzer®, the CO2 is produced by a chemical reaction that occurs when the tablets dissolve in water.

The main ingredients of Alka-Seltzer tablets are aspirin, citric acid, and sodium bicarbonate (NaHCO3). When sodium bicarbonate dissolves in water, it dissociates (splits apart) into sodium (Na+) and bicarbonate (HCO3) ions. The bicarbonate reacts with hydrogen ions (H+) from the citric acid to form carbon dioxide and water. The reaction is described by the following chemical equation:

Chemical equation for bicarbonate and hydrogen reacting to form water and carbon dioxide

So how does particle size come into this? In order for the reaction shown above to take place, the ingredients in the tablet first have to dissolve. The table has a large surface area, so this step should be pretty fast, right? What effect do you think particle size will have on the speed of the bicarbonate reaction? You can find out for yourself by plopping prepared Alka-Seltzer tablets (whole tablets, halved tablets, quartered tablets, and powdered tablets) into water at the same temperature, and timing how long it takes for the chemical reaction to go to completion.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:



Materials and Equipment

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Experimental Procedure

  1. Do your background research and make sure that you are familiar with the terms, concepts, and questions, above.
  2. In this experiment, you will be measuring the time it takes for one Alka-Seltzer tablet to react completely in water. You will investigate how the reaction time changes as you vary the particle size of the reactants.
  3. You will use the same glass for repeated trials, so it is convenient to mark the desired water level.
    1. Use the measuring cup to add 8 ounces (236 mL) of water to the glass. (If you're using metric volume units, rounding up to 250 mL is fine.)
    2. Use a piece of masking tape on the outside of the glass to mark the water level. Place the tape with its top edge even with the water level in the glass.
    3. Now you can use the masking tape to fill the glass to the right level for each trial.
  4. For observing the reaction, you will use the same volume of water at the same starting temperature. The only variable that you should change is the particle size of the tablets. You will use four different particle sizes for the Alka-Seltzer tablets, as shown in Figure 1, below:
    1. A whole tablet
    2. A tablet broken in half
    3. A tablet broken in quarters
    4. A tablet ground into powder. To do this, fold a single tablet to be ground inside a clean piece of paper. Place the folded paper on a piece of scrap wood, and use the hammer to firmly pound the tablet about ten times. Stop immediately if the paper shows signs of tearing: you don't want to lose any of the powder. You can also use the back of a metal spoon to carefully crush your tablet into a powder once it is wrapped in paper.
    Alka-seltzer tablet broken in four different ways

    Figure 1. For this experiment you will test a whole tablet, a tablet broken in half, a tablet broken in quarters, and a tablet ground into powder.

  5. Here is how to measure the reaction time:
    1. Fill the glass with water to the level of the masking tape.
    2. Measure the temperature of the water, and record it in your lab notebook. Each trial should be carried out at the same temperature, so adjust the water temperature (by adding warm or cold water) as necessary.
    3. Remove the thermometer. (It's not a good idea to use the thermometer as a stirring rod. It might break.)
    4. Have your helper get ready with the stop watch, while you get ready with an Alka-Seltzer, as shown in Figure 2, below. Have your helper count one-two-three. On three, the helper starts the stop watch and you drop the tablet (or tablet pieces) into the water.
    5. You will immediately see bubbles of CO2 streaming out from the tablet.
    6. Stir the water gently and steadily. Use the same stirring method and speed for all of your experimental trials. The tablet will gradually disintegrate. Watch for all of the solid white material from the tablet to disappear.
    7. When the solid material has completely disappeared and the bubbles have stopped forming, say "Stop!" to have your helper stop the stopwatch.
    8. Record the reaction time in your lab notebook.
    9. Tip: be careful when opening the packets and handling the Alka-Seltzer tablets. The tablets are thin and brittle, so they break easily. You need to have four whole tablets for this experiment.

    Alka-Seltzer tablet being placed in a glass of water

    Figure 2. Make sure your helper is ready to to time the reaction while you get ready to drop the Alka-Seltzer in the water.

  6. For each of the four particle sizes, you should repeat the experiment three times, for a total of 12 trials. You can organize your data in a table like the one below.
    Particle Size Temperature
    Reaction Time
    Average Reaction Time
    Trial #1 Trial #2 Trial #3
    Whole Tablet          
    Tablet Broken in Half          
    Tablet Broken in Quarters          
    Powdered Tablet          
  7. Calculate the average reaction time for each of the four particle sizes.
  8. Make a bar graph showing the average reaction time, in seconds, (y-axis) vs. particle size (x-axis).
  9. How does reaction time change with particle size?
icon scientific method

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  • More advanced students should also calculate the standard deviation of the reaction times for each temperature.
    • Use the standard deviation to add error bars to your graph.
    • For example, say that the average reaction time for one particle size was 45 seconds, and the standard deviation was 5.2 seconds (these are made-up numbers). You would graph the bar for the data point at 45 seconds, and then draw short vertical bars above and below the top of the bar. Each vertical bar would have a length equivalent to 5.2 seconds.
    • Error bars give your audience a measure of the variance in your data.
  • Does changing the particle size of the reactants have as big an effect as changing the temperature of the water? For an experiment that investigates the effect of temperature on the speed of the reaction, see the Science Buddies project Plop, Plop, Fizz Fast: The Effect of Temperature on Reaction Time.
  • In this experiment you observed the reaction mixture and watched as the tablets disappeared and formed gas bubbles. For more advanced versions of this experiment, you can build a simple apparatus so that you can measure the volume of the gas produced over time. Because you will be able to collect data at multiple time points, you get information about how the reaction rate changes over time. For more advanced versions of measuring the reaction rate, see the Science Buddies projects:


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MLA Style

Science Buddies Staff. "Big Pieces or Small Pieces: Which React Faster?" Science Buddies, 20 Nov. 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p028/chemistry/big-pieces-or-small-pieces-which-react-faster. Accessed 6 June 2023.

APA Style

Science Buddies Staff. (2020, November 20). Big Pieces or Small Pieces: Which React Faster? Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p028/chemistry/big-pieces-or-small-pieces-which-react-faster

Last edit date: 2020-11-20
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