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Can You Change the Rate of a Chemical Reaction by Changing the Particle Size of the Reactants?

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The ingredients in Alka-Seltzer® tablets undergo a chemical reaction that produces carbon dioxide gas as soon as the tablets hit water. Do you think you can cause the tablets to produce gas faster by breaking them into smaller pieces before dropping them in water? Find out for yourself with this project.


Areas of Science
Time Required
Short (2-5 days)
Material Availability
A kit is available from our partner Home Science Tools. See the Materials section for details.
Low ($20 - $50)
Andrew Olson, PhD, Science Buddies
Edited by Svenja Lohner, PhD, Science Buddies
Alka Seltzer® is a registered trademark of Bayer HealthCare LLC.

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The goal of this project is to measure the effect of the particle size of the reactants 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 (Figure 1).

An Alka-Seltzer tablet dissolves in a glass of water
Figure 1. Alka-Seltzer tablet dissolving 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 tablet 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 measuring the volume of carbon dioxide gas collected at regular intervals after the reaction starts.

Terms and Concepts

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



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Experimental Procedure

Setting Up the Gas Collection Apparatus

  1. Remove the small red cap from one of the squeeze bottles. Then connect the tubing to the tip opening, as shown in Figure 2. Make sure that you have a tight fit.

    A plastic tube connects to the nozzle of a squeeze-bottle
    Figure 2. Tube connected to the bottle opening.

  2. You will be collecting carbon dioxide from the Alka-Seltzer® chemical reaction by displacing water trapped in an inverted graduated cylinder. Here's how to set it up:
    1. Fill your plastic dishpan (or bucket) about one-third full with water.
    2. Fill the 250-mL graduated cylinder with water.
      1. If your dishpan is deep enough, fill the graduated cylinder by tipping it on its side inside the dishpan. Allow any bubbles to escape by tilting the cylinder up slightly, while keeping it under water. Keeping the opening of the cylinder under water, turn it upside down and attach it to the side of the dishpan with packing tape (or have your helper hold it in place).
      2. If your dishpan is not deep enough, fill the graduated cylinder completely using the faucet and cover the top tightly with plastic wrap. Quickly invert the cylinder and place the opening in the dishpan, beneath the surface of the water. Remove the plastic wrap. Attach the cylinder to the side of the tub with packing tape (or have your helper hold it in place).
    3. The graduated cylinder should now be upside down, full of water and with its opening under the surface of the water in the dishpan. Place the free end of the tubing from the plastic bottle inside the graduated cylinder. Your apparatus is now ready to trap carbon dioxide from the Alka-Seltzer® chemical reaction (see Figure 3).

      An upside down cylinder is used to trap and collect gas

      A graduated cylinder is placed upside-down in a tub of water and a plastic tube enters the cylinder from underwater. The plastic tube is connected to the nozzle of a squeeze bottle. Air from the squeeze bottle will be funneled into the upside-down cylinder and will be trapped by the water below.

      Figure 3. Picture of the inverted graduated cylinder gas collection apparatus.

    4. You can test your gas collection apparatus by removing the tube from the bottle top and blowing gently into the tube. The bubbles you create should be captured inside the cylinder. (You will need to reconnect the tube to the bottle and re-fill the cylinder before starting your experiment.)

Running the Experiment

  1. In this experiment, you will be measuring the reaction rate for the production of carbon dioxide gas from a single Alka-Seltzer® tablet.
    1. You will measure the volume of gas produced at 10-second time intervals after the reaction begins.
    2. You will investigate how the reaction rate changes as you vary the particle size of the reactants.
  2. You can use the same plastic bottle for repeated trials, so it is convenient to mark the desired water level.
    1. Fill the bottle with about 120 ml (4 oz.) water.
    2. You do want to use the same amount of water for each trial. Use a permanent marker to mark the water level on the outside of the bottle.
  3. For measuring the reaction rate, you will use the same volume of water at the same starting temperature. You will use four different particle sizes for the Alka-Seltzer® tablets:
    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 the back of a large metal spoon 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.
  4. Here is how to measure the reaction rate:
    1. Fill the bottle with water up to the level of your marker line.
    2. Measure the temperature of the water, and record it in your lab notebook.
    3. Remove the thermometer.
    4. Have your helper get ready with the stop watch, while you get ready with an Alka-Seltzer® tablet. Hold the tablet in one hand and the bottle cap (with tubing attached) in the other hand.
    5. Have your helper count one-two-three. On three, the helper starts timing and you drop the tablet (or tablet pieces) into the water in the bottle.
    6. Quickly cap the bottle tightly using the cap with the tubing attached. You will immediately see bubbles of CO2 streaming out from the tablet.
    7. Using the hand that you don't use for writing, swirl the bottle gently, keeping the bottom of the bottle flat on the table top.
    8. Every ten seconds, your helper should call out "Time!" You should immediately read the carbon dioxide volume in the graduated cylinder and write it down in your lab notebook. Prepare a table to keep your data organized.

      Particle Size
      Trial # Volume of CO2 after reaction begins (times in seconds)
      10 20 30 40 50 60 70 80 90 100 110 120

    9. Continue recording the volume of gas at 10-second intervals until the volume is no longer changing. At this point, the reaction is complete.
    10. Tip: be careful when opening the packets and handling the Alka-Seltzer® tablets. The tablets are thin and brittle, so they break easily. Remember that you need to have three whole tablets for this experiment.
  5. For each of the four particle sizes, you should repeat the experiment three times, for a total of 12 trials.

Analyzing Your Data

  1. For each particle size, calculate the average volume of gas at each time point for the four trials (see the table below)

    Particle Size
    Trial # Volume of CO2 after reaction begins (times in seconds)
    10 20 30 40 50 60 70 80 90 100 110 120
    whole tablet 1                        
    whole tablet 2                        
    whole tablet 3                        
    Average -                        

  2. Make a graph of the volume of CO2, in mL, (y-axis) vs. time after the reaction begins, in seconds (x-axis).
    1. You can include the data from all four particle sizes on one graph.
    2. Use a different symbol and color for each particle size.
    3. Remember to include a legend that identifies the particle size associated with each symbol.
  3. More advanced students should also calculate the standard deviation of the reaction times for each particle size.
    1. Use the standard deviation to add error bars to your graph.
    2. For example, say that the average volume for two half-tablets 30 seconds after the reaction began was 45 mL, and the standard deviation was 5.2 mL (these are made-up numbers). You would graph the symbol for the data point at 45 mL, and then draw short vertical bars above and below the symbol. Each vertical bar would have a length equivalent to 5.2 mL.
    3. Error bars give your audience a measure of the variance in your data.
  4. How does reaction rate change with particle size?
icon scientific method

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MLA Style

Science Buddies Staff. "Can You Change the Rate of a Chemical Reaction by Changing the Particle Size of the Reactants?" Science Buddies, 17 Apr. 2023, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p030/chemistry/reactant-size-changes-chemical-reaction-rate. Accessed 7 June 2023.

APA Style

Science Buddies Staff. (2023, April 17). Can You Change the Rate of a Chemical Reaction by Changing the Particle Size of the Reactants? Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p030/chemistry/reactant-size-changes-chemical-reaction-rate

Last edit date: 2023-04-17
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