Final answer:
Carbon dioxide from breath reacts with calcium hydroxide solution (limewater) to form an insoluble precipitate of calcium carbonate, making the solution turn milky.
Step-by-step explanation:
When your breath, which contains carbon dioxide (CO₂), is bubbled through a calcium hydroxide solution, also known as limewater, a chemical reaction takes place. The CO₂ reacts with the calcium hydroxide to form calcium carbonate, which is an insoluble precipitate. This precipitate causes the clear limewater to turn milky. The reaction can be summarized as Ca²⁺ (aq) + 2OH⁻(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). The change in the solution's appearance from clear to milky is a classic test for the presence of CO₂.