Dichlorine monoxide

Cl2O is a reddish gas that, at subzero temperatures, is a dark brown liquid.

Dichlorine monoxide, Cl2O, is one of the “chlorineoxide” creators of the oxy-chlorine acids. It is a brownish yellow gas at room temperature that can explode at high concentrations when heated or subjected to sparks. It is best prepared by treating fresh yellow mercuric oxide with Cl2 gas:
2Cl2＋2HgO→HgCl2·HgO＋Cl2O
It can also be prepared by reaction of Cl2 gas with moist sodium carbonate, Na2CO3. Dichlorine monoxide is very water soluble. It is the anhydride of hypochlorous acid. Much of the Cl2O manufactured industrially is used to make hypochlorites. It is also the unstable decomposition product of calcium hypochlorite.