Water switches on the reaction
In a dry tablet, citric acid and sodium bicarbonate can sit together without producing a burst of gas because the molecules have little freedom to move. Water dissolves them into charged particles, allowing the acid and bicarbonate to react. The result includes carbon dioxide gas, water, and sodium citrate, a dissolved salt.
The carbon dioxide first forms in the liquid, then gathers into bubbles. As those bubbles grow and escape, they disturb the tablet’s structure, helping it crumble and release its ingredients into the water. The fizz is therefore part of how the tablet disperses—not, by itself, evidence that a medicine is working.
Why the tablet is kept dry
The same reaction that makes an effervescent tablet convenient would be a problem during storage. Moisture can start the ingredients reacting before the tablet reaches a glass, weakening it and using up some of its fizzing capacity. That is why these products need protection from damp air and should be handled according to their packaging.
Once water is added, the tablet turns a solid dose into a drinkable solution or suspension, depending on what else is in the formulation. The bubbles do the visible work of breaking the tablet apart; the dissolved ingredients are what deliver the intended dose.
