Key concepts
Chemical reactions

Have you ever wondered why bubbles form when an Alka-Seltzer tablet is dropped into water? If you've ever tried it, you've seen that the tablet fizzes furiously. The moment the tablet starts dissolving a chemical reaction occurs that releases carbon dioxide gas. This is what comprises the bubbles. Some factors can change how quickly the carbon dioxide gas is produced, which 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 water’s temperature. How does this affect the reaction?

Alka-Seltzer is a medication that works as a pain reliever and an antacid. (Antacids help neutralize stomach acidity, which can cause heartburn.) The pain reliever used is aspirin and the antacid used is baking soda, or sodium bicarbonate. The tablets also include other ingredients, such as citric acid (a weak acid that adds flavor—as well as provides important hydrogen ions, which will come into play as you shall soon see).

To take the tablets, they're fully dissolved in water, where they famously undergo a chemical reaction that produces lots of carbon dioxide bubbles—or fizz. 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 the citric acid 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 its molecules’ average motion and energy.) Can you guess whether fast-moving molecules or slow-moving ones will speed the reaction time?

• Two identical jars (You can also use drinking glasses, clear plastic cups, bottles or vases.)
• Spoon
• Enough ice cubes to fill one of the jars halfway
• Cold tap water
• Hot tap water
• Two Alka-Seltzer tablets
• Timer or clock that shows seconds
• Optional: helper

• 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 evens out. Right before you start the activity use a spoon to remove the cubes.
• Add hot tap water to the second, empty jar until it is 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 cold or very hot).

• Drop an Alka-Seltzer tablet into 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 into the jar with the ice-cold water (after having removed the ice cubes with a spoon). 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 versus in the hotter water?
Why do you think you got the results you did?
Extra: Test Alka-Seltzer tablets in a wider range of temperatures, and then draw a graph showing 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 (make it as twice as fast)? 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 (that is, a tablet is broken up into more pieces to expose more surface), does the same amount of tablet result in the reaction happening faster or slower?
Extra: You can turn this activity into a homemade lava lamp! To do this, you will use an empty container, such as a tall jar or clear plastic one- or two-liter bottle. Fill it with about two inches of water, add five drops of food coloring and then fill it at least three quarters full with vegetable oil before adding one quarter of an Alka-Seltzer tablet. You could repeat this activity using your homemade lava lamp at colder and warmer temperatures. (Because it contains oil, you should have an adult help you devise a safe way to warm or cool the contents of each container.) 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 water compared to the cold? Were there a lot more bubbles produced initially in the hot compared with the cold water?

After the Alka-Seltzer tablet was added to the hot water the tablet should have quickly dissolved, taking some 20 to 30 seconds to do so, depending on the exact temperature. After the tablet was added to the ice-cold water it should have taken much longer to dissolve, with most of the tablet disappearing after about two to three minutes, but with some bubbles 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.

More to explore
Chemical Reactions, from Rader's
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

This activity brought to you in partnership with Science Buddies