What substance forms a brown color when chlorine is added to potassium iodide solution?

When chlorine is added to potassium iodide solution, a brown color develops due to the formation of iodine. In this reaction, chlorine, being a more reactive halogen than iodine, displaces iodine from potassium iodide. The displacement reaction leads to the generation of iodine, which is present in the solution as I₂.

Iodine has a characteristic brown color in solution, particularly when diluted in water. Hence, the presence of iodine in the solution after the reaction is what causes the observed color change to brown. This highlights iodine's role as a product of the redox reaction, with chlorine being reduced in the process while iodide ions are oxidized to iodine.