The solution turns milky when exhaled air is bubbled through it.
name: Lime water reacts with carbon dioxide producing insoluble calcium carbonate.
In the first part of the experiment the reaction between oxygen and carbon is demonstrated. Carbon is quite an inert material. It does not react with oxygen at normal conditions. But at high temperature it becomes possible to start the reaction. Heated carbon rod reacts with oxygen producing carbon dioxide. Glowing is the sign of the proceeding reaction. The presence of carbon dioxide could be detected by its action on lime water. Lime water is the name of a water solution of calcium hydroxide. Calcium hydroxide reacts with carbon dioxide forming insoluble calcium carbonate.
Similarly carbon dioxide produced during respiration could be determined using lime water and a special device. The air we exhale contains about 4-5% of carbon dioxide. So when it is bubbled through the lime water it turns the solution milky. At the same time the amount of carbon dioxide in the surrounding atmosphere is not enough for visible changes. The solution in the “inhaling” bottle stays clear.