Carbon dioxide
Correct! An Alka-Seltzer tablet is composed of acetylsalicylic acid, citric acid, and sodium bicarbonate. When an Alka-Seltzer tablet is dropped into water it dissolves and the citric acid and bicarbonate react and release carbon dioxide gas. Carbon dioxide is also a combustion product when burning organic matter and fossil fuels; it is a greenhouse gas. It is also the product of aerobic respiration (humans exhale carbon dioxide) and is used by plants in photosynthesis.