The solubility of iodine in water increases as the concentration of potassium iodide increases due to the formation of triiodide ions (I₃^-).

Here's a breakdown:

1. Iodine in water: Iodine (I₂) is only slightly soluble in water. This is because iodine is a nonpolar molecule and water is a polar solvent. The interaction between them is weak.

2. Potassium iodide: Potassium iodide (KI) is soluble in water, dissociating into potassium ions (K⁺) and iodide ions (I^-).

3. Triiodide ion formation: When iodine is added to a solution containing iodide ions, a reaction occurs:

```

I<sub>2</sub> (aq) + I<sup>-</sup> (aq) ⇌ I<sub>3</sub><sup>-</sup> (aq)

```

The triiodide ion (I₃^-) is formed, which is soluble in water.

4. Le Chatelier's Principle: The formation of triiodide ions is an equilibrium reaction. Increasing the concentration of iodide ions (by adding more KI) shifts the equilibrium to the right, favoring the formation of more triiodide ions.

5. Increased solubility: As more triiodide ions form, more iodine is effectively "dissolved" in the solution, leading to an increase in the overall solubility of iodine.

In summary: The presence of iodide ions from potassium iodide promotes the formation of soluble triiodide ions, effectively increasing the solubility of iodine in water.