Alka-Seltzer is made of aspirin, citric acid, and sodium bicarbonate (NaHCO3).
When sodium bicarbonate dissolves in water, it splits up to make sodium (Na+) and bicarbonate (HCO3−) ions.
NaHCO3 (Sodium bicarbonate) → Na+(Sodium ion) + HCO3− (Bicarbonate Ion)
Then, the bicarbonate reacts with hydrogen ions (H+) from the citric acid to form carbon dioxide and water:
H+ (Hydrogen Ion) + HCO3− (Bicarbonate Ion) → CO2 (Carbon dioxide) + H2O (Water)
The Carbon dioxide is a gas that makes the bubbles you see when you drop the tablets in water.
How can you measure the reaction rate?
You can measure the reaction rate by using a stopwatch to measure how long it takes for one tablet to stop fizzing. The faster it reacts, the sooner it stops fizzing.
What factors might affect the reaction rate?
- Size of the particles. (The reaction happens on the surfaces of the tablet. By splitting the tablets into pieces, maybe they will react more quickly.)
- Temperature (Heat is a result fast-moving molecules. Faster moving molecules may react more quickly.)
- Concentration of acid in the water. (Acid is necessary for the reaction. With more acid the reaction may speed up.)
You can do experiments on any of these variables, lets look at particle size as an example:
- 3 Clear glasses
- 15 Alka-Seltzer tablets
- Mortar and pestle
1) Fill a clear glass with 1 cup (or 250 ml) of water at room temperature.2) Drop a whole tablet in the water. 3) Measure and record the time until the tablet is completely dissolved. 4) Repeat two more times and average your results.
5) Repeat steps 1-4, using tablets that have been cut in halves, quarters, eighths, and ground into powder.
There are more great science project ideas at the