Limewater can be used to detect carbon dioxide. If carbon dioxide is bubbled through limewater then it turns from clear to cloudy/milky in colour. This is why limewater used in a simple respirometer can show that more carbon dioxide is present in exhaled air compared to inhaled air.
Carbon dioxide (CO 2) is colorless and odorless, so you can't detect it through direct observation. You'll need to collect an air sample (or a CO 2 sample), then run one of several simple tests to identify the presence of the gas
