Summary
Introduction
Have you ever wondered why bubbles form when an Alka-Seltzer tablet is put in water? If you’ve ever tried it, you’ve seen that the tablet fizzes furiously when dropped into water. The moment the tablet starts dissolving, a chemical reaction occurs that releases carbon dioxide gas. This is what the bubbles are. Some factors can change how quickly the carbon dioxide gas is made, and consequently affect how furiously the tablet fizzes. In this activity you’ll explore whether you can make an Alka-Seltzer tablet fizz faster or slower by changing the temperature of the water. How does the water’s temperature affect the reaction?
Background
Alka-Seltzer is a medical drug that works as a pain reliever and an antacid (antacids help neutralize stomach acidity, such as heartburn). The pain reliever used is aspirin and the antacid used is baking soda, or sodium bicarbonate. To take the tablets, they’re fully dissolved in water, where they famously undergo a chemical reaction that produces lots of carbon dioxide bubbles. Why is this? As the tablets dissolve, the sodium bicarbonate splits apart to form sodium and bicarbonate ions. The bicarbonate ions react with hydrogen ions (from citric acid, another ingredient in the tablets) to form carbon dioxide gas (and water). This is how the bubbles are made.
How is temperature related to this reaction? For the reaction to occur, the bicarbonate ions must come into contact with the hydrogen ions in just the right way. The probability of the bicarbonate and hydrogen ions doing this is affected by temperature: the higher the temperature, the faster the molecules move; the lower the temperature, the slower they move. (The temperature of a solution is a measure of the average motion and energy of the molecules in the solution.)
Materials
- Two identical jars, drinking glasses, clear plastic cups, bottles, or vases
- Enough ice cubes to fill one of the jars halfway
- Cold tap water
- Spoon
- Hot tap water
- Two Alka-Seltzer tablets
- Timer or clock that shows seconds
- Optional: Helper
Preparation
- Fill one of the jars halfway with ice cubes. Add cold tap water to about an inch from the rim. Stir the ice cubes in the jar for about a minute so that the temperature equilibrates. Right before you start the activity, use a spoon to remove the ice cubes.
- Add hot tap water to the second, empty jar till about an inch from the rim. Be careful when handling the hot water.
- Continue with the procedure immediately after preparing the jars (so that the water in the jars is still very hot or very cold).
Instructions
- Drop an Alka-Seltzer tablet in the jar with hot water. Time how long it takes for the tablet to disappear. You may want to have a helper time the reaction. How long does it take the tablet to disappear? How vigorous are the bubbles?
- Drop an Alka-Seltzer tablet in the jar with the ice-cold water. Again time how long it takes the tablet to disappear. How long does it take the tablet to disappear in the colder water?
- Do you notice other differences in how the reaction happens in the colder water versus in the hotter water?
- Why do you think you got the results that you did?
Extra: Test Alka-Seltzer tablets in a wider range of temperatures, and then graph the time it takes a tablet to dissolve in water at each temperature (check with a thermometer). What temperature change is required to increase the reaction time by a factor of two? What about decreasing the reaction time by a factor of two?
Extra: Compare whole Alka-Seltzer tablets to pieces of Alka-Seltzer tablets. If there is a greater surface area (i.e., a tablet is broken up into more pieces), does the same amount of tablet result in the reaction happening faster or slower?
Extra: You could turn this activity into a homemade lava lamp! To do this, you will want to use a tall jar or empty clear plastic 1-liter or 2-liter bottle, fill it with about 2 inches of water, add 5 drops of food coloring, and then fill it at least three-quarters full with vegetable oil. You could repeat this activity using your homemade lava lamp at a cold and hot temperature. (You’ll need to devise a safe way to heat or cool each container.) You will also want to use one-quarter of an Alka-Seltzer tablet at a time (instead of a whole tablet). How does the bicarbonate reaction look in the homemade lava lamp?
Observations and Results
Did the Alka-Seltzer tablet dissolve much faster in the hot jar compared to the cold one? Were there a lot more bubbles produced initially in the hot water compared to in the cold water?
When the Alka-Seltzer tablet was added to the hot water, the tablet should have quickly dissolved, taking around 20 to 30 seconds to do so, depending on the exact temperature. When the tablet was added to the ice-cold water, it should have taken much longer to dissolve, with most of the tablet dissolving after about two to three minutes, but with some bubbles (from the tablet’s remains) still apparent after six minutes or longer. In the hot water, the tablet should have more vigorously produced bubbles than in the cold water. The higher the temperature, the faster the molecules move, and the more likely it is that the bicarbonate will contact hydrogen in just the right way for the chemical reaction to occur and produce carbon dioxide bubbles.
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Additional Resources
- Chemical Reactions, from Rader's Chem4Kids.com
- Factors affecting the Speed-Rates of Chemical Reactions, from Doc Brown’s Science
- Rates of Reaction Menu, from Chemguide
- Plop, Plop, Fizz Fast: The Effect of Temperature on Reaction Time, from Science Buddies
