Chemical indicators for carbon dioxide are typically weak acids or bases that undergo a visible colour change when exposed to carbon dioxide gas. The most commonly used chemical indicator for carbon dioxide is phenolphthalein, which changes from colourless to pink in the presence of carbon dioxide. Other indicators include thymol blue, which changes from yellow to blue, and bromothymol blue, which changes from yellow to green. These indicators can be used to detect the presence of carbon dioxide in a variety of settings, such as in the air, in soil, and in water.