Medicine

When the old vaudeville-style medicine shows promoting various patent medicines were in full swing, limewater was often used as a part of the act. The salesperson would have an audience member blow through a straw into a glass of limewater. Since the exhaled gas is carbon dioxide, the water would turn cloudy; the huckster then announced that this reaction proved that the audience member suffered from some ailment. If too much carbon dioxide comes into contact with the cloudy limewater, it will cause the calcium carbonate precipitate to redissolve to form soluble calcium bicarbonate.

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

The huckster had a patent medicine bottle filled with vinegar or some similar acid. He then would pour some of the acid into the glass of cloudy limewater. The acid reacted with the calcium carbonate, and the water would instantly clear. This demonstrated the potent effect of the nostrum he was selling to eliminate the "disease" demonstrated by the audience member.