The Alka-Seltzer tablets contain sodium bicarbonate and citric acid. As the Alka-Seltzer tablet dissolves in water, the citric acid
reacts with the sodium bicarbonate to produce carbonic acid (Equation 1) and carbon dioxide (Equation 2). The carbonic acid
then reacts with the basic sodium hydroxide to change the pH of the solution (Equations 3 and 4). As the base is consumed, the
solution will slowly become more acidic, resulting in the colour changes.