Oxygen
Sorry, this answer is incorrect. Not oxygen but carbon dioxide is produced when the Alka-Seltzer tablets dissolve in water. Oxygen however is dissolved to some extent the water. Oxygen occurs in the free state as a diatomic gas, and is essential for oxygen breathing organisms. Oxygen is produced by plants through a process called photosynthesis, where plants, algae, and some bacteria convert carbon dioxide and water using sunlight to oxygen.