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How Fast Does an Alka-Seltzer® Tablet Make Gas?

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This is a straightforward, fun project to measure the rate of the chemical reaction that occurs when Alka-Seltzer® tablets are plopped into water. You will track the volume of carbon dioxide gas produced at regular intervals after the reaction begins. How does changing the temperature of the water affect the production of gas?


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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Measure the effect of temperature 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 dissolving 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
The compounds on the left-hand side of the equation (bicarbonate ions and hydrogen ions) are called the reactants, and the compounds on the right-hand side of the reaction (water and carbon dioxide) are called the products.

So how does temperature come into this? In order for the reaction shown above to occur, the bicarbonate ions have to come into contact with the hydrogen ions. Molecules in a solution are in constant motion, and are constantly colliding with one another. The hydrogen and bicarbonate ions must collide at the right angle and with enough energy for the reaction to occur. The temperature of a solution is a measure of the average motion (kinetic energy) of the molecules in the solution. The higher the temperature, the faster the molecules are moving.

What effect do you think temperature will have on the speed of the bicarbonate reaction? In this project you can find out for yourself. You will build a simple apparatus to collect the carbon dioxide gas produced by the chemical reaction. Then you will plop Alka-Seltzer tablets into water at different temperatures, and measure the volume of carbon dioxide gas collected at different time points.

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 with water temperature.
  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 three different starting temperatures: hot tap water, cold tap water, and ice water.
    1. For the hot and cold tap water, run the water until the temperature stabilizes. Fill the bottle with water up to the level of the marker line.
    2. For ice water, put some ice cubes in the bottle and then add cold tap water up to the level of the marker line. Stir for a minute or two so that the temperature equilibrates.
  4. Here is how to measure the reaction rate:
    1. Fill the bottle with water up to the level of the 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 into the water.
    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 like the water temp table to keep your data organized.

      Water temp.
      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. If some of the tablets are whole, and some are broken into many pieces, the separate trials will not be a fair test.
  5. For each of the three temperatures, you should repeat the experiment four times, for a total of 12 trials.

Analyzing Your Data

  1. For each water temperature, calculate the average volume of gas at each time point for the four trials (see the example table below):

    Water temp.
    Trial # Volume of CO2 after reaction begins (times in seconds)
    10 20 30 40 50 60 70 80 90 100 110 120
    17°C 1                        
    17°C 2                        
    17°C 3                        
    17°C 4                        
    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 three temperatures on one graph.
    2. Use a different symbol and color for each temperature.
    3. Remember to include a legend that identifies the temperature associated with each symbol.
  3. More advanced students should also calculate the standard deviation of the reaction times for each temperature.
    1. Use the standard deviation to add error bars to your graph.
    2. For example, say that the average volume for 17°C water 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 temperature?
icon scientific method

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MLA Style

Science Buddies Staff. "How Fast Does an Alka-Seltzer® Tablet Make Gas?" Science Buddies, 15 Apr. 2023, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p026/chemistry/how-fast-does-an-alka-seltzer-tablet-make-gas. Accessed 21 Sep. 2023.

APA Style

Science Buddies Staff. (2023, April 15). How Fast Does an Alka-Seltzer® Tablet Make Gas? Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p026/chemistry/how-fast-does-an-alka-seltzer-tablet-make-gas

Last edit date: 2023-04-15
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