Chlorine gas is highly reactive, so to begin with we expect that a reaction is likely to occur even without the addition of heat. Although this situation is undoubtedly an aqueous solution containing dissolved KI, only the potassium iodide is mentioned in the problem, so we expect that the question is directed at a possible reaction between chlorine and potassium iodide.
In order to predict what will happen, let’s begin by writing the formulas of the substances we know. Chlorine, being a reactive element, exists as a diatomic gas, `Cl_2.` We get
`KI + Cl_2 -> ?`
We have a potential reaction involving two chemical species. Let’s consider the possible reaction types. This is clearly not decomposition, which starts with a single chemical species. It is not combustion, as it has no oxygen, and it is not a neutralization reaction, as the reactants are not an acid and base. Replacement is a possibility. This would be a single replacement, as we have an element and a compound. For single replacement reactions, we check the activity series to see if reaction will occur. There is an activity series for metal replacements and another for halogen replacements. We look at which we have both in the compound and in the element. Chlorine is a halogen and potassium iodide contains iodine, also a halogen, so this must be a halogen replacement.
The activity series tells us that a halogen higher on the Periodic Table will replace one that is lower on the Periodic Table. Thus we know chlorine will replace iodine. This means the chlorine can take the place of iodine in the compound, leaving iodine as an element.
For chlorine to take the place of iodine, it will form a negative ion to go with the positive potassium. Iodine, like chlorine, is diatomic (although it is a solid where chlorine is a gas). Thus the chlorine gas will react with the potassium iodide to yield potassium chloride, which is dissolved in the solution, and iodine. The equation can easily be balanced to give
`2 KI + Cl_2 -> 2 KCl + I_2`
Iodine, though only slightly soluble in water, is soluble in iodide solutions. At first, then, it will darken the solution, as it has an intense dark purple color, but if the reaction continues until the potassium iodide is all used up, it will precipitate as a solid.