Final answer:
The addition of water to Alka-Seltzer tablets allows carbon dioxide to react.
Step-by-step explanation:
The addition of water to a test tube containing Alka-Seltzer tablets allows the carbon dioxide to react. When water is added to the tablets, the citric acid and sodium bicarbonate in the tablets react to produce carbon dioxide gas. This gas is responsible for the fizzy bubbles that are observed when Alka-Seltzer dissolves in water.