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Chlorine dioxide is a chemical compound with the formula ClO2.  It is a synthetic, green-yellowish gas with a chlorine-like, irritating odour. Chlorine dioxide is a neutral chlorine compound, which is very different from elementary chlorine, both in its chemical structure as in its behaviour. Chlorine dioxide is a small, volatile and very strong molecule. In diluted, watery solutions it is a free radical. At high concentrations it reacts strongly with reducing agents. Chlorine dioxide is an unstable gas that dissociates into chlorine gas (Cl2), oxygen gas (O2) and heat. When it is photo-oxidised by sunlight, it falls apart. The end products of chlorine dioxide reactions are chloride (Cl-), chlorite (ClO-) and chlorate (ClO3-). At –59°C, solid chlorine dioxide becomes a reddish liquid. At 11°C chlorine dioxide turns into gas. It is 2 – 4 times denser than air. As a liquid, chlorine dioxide has a bigger density than water. One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. It does not hydrolyse when it enters water; it remains a dissolved gas in solution. Chlorine dioxide is approximately 10 times more soluble in water than chlorine. Chlorine dioxide can be removed by aeration or carbon dioxide.