chlorine gas bubbled into a potassium iodide solution in a test tube. explain why a dark brown colour is observed

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From Google:

As more chlorine is added, the iodine reacts with the excess chlorine to form iodine trichloride and the solution decolorizes. ... When chlorine gas is bubbled into an aqueous solution of potassium iodide, some of the iodide ions are oxidized to iodine.