When chlorine is passed over iodine, iodine monochloride is formed as a brown liquid, followed quickly by yellow crystals of iodine trichloride. In a closed container an equilbrium mixture is formed involving chlorine, iodine monochloride and iodine trichloride:

Cl₂(g) + ICl(l) _∏ ICl₃(s)

This reaction is traditionally demonstrated using a U-tube or a combustion tube in a fume cupboard. A reduced-scale version of this experiment has recently been developed which allows the effect of temperature on the composition of the equilibrium mixture to be explored.

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