Chlorine displacing iodine. Chlorine gas bubbling through an aqueous solution of potassium iodide in a test tube. The reaction is turning brown due to the iodine's displacement. A clear solution of potassium iodide is seen at left in a glass bottle. This reaction is known as a displacement or substitution. The chlorine is said to be more reactive than iodine, so it replaces the iodine in the potassium salt, causing a brown aqueous iodine solution to form. The equation for this reactions is: Cl2 + 2KI ---> I2