Carbon dioxide is a colourless gas with one carbon and two oxygen atoms. Its chemical formula is CO2. At low concentrations, the gas is odourless; however, it has a sharp, acidic odour at higher concentrations. Carbon dioxide is an essential trace of gas found in the Earth's atmosphere. The current concentration of about 0.04% by volume of Carbon dioxide is present in the atmosphere and acts as a greenhouse gas. Primary sources for atmospheric carbon dioxide are volcanic outgassing, combustion of organic matter, respiration process of living aerobic organisms, burning of various fossil fuels for power generation and transport. However, if the concentration of CO2 increases in the atmosphere, it can lead to global warming. Besides, a larger concentration of CO2 increases, it will lead to ocean acidification. This reduction in pH will affect the biological systems in the oceans.

Lime water is a colourless solution of Calcium hydroxide (Ca(OH)2) with a slight earthy smell. It is prepared by mixing calcium hydroxide with water and removing the excess undissolved solute, usually by filtration.
When CO2 is bubbled through lime water, the lime water changes to a milky white liquid. It is because the carbon dioxide reacts with lime water (Ca(OH)2) to form Calcium carbonate (CaCO3), which is an insoluble white precipitate.
Ca(OH)2 + CO2(g) 🡪 CaCO3(white precipitate) + H2O

One can ensure the presence of CO2 using this test.

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