KI (potassium iodide) is used to dissolve iodine because it forms a soluble complex with iodine, known as the triiodide ion (I₃^-). This complex is soluble in water, allowing for the dissolution of iodine.

Here's how it works:

1. Reaction: When iodine (I₂) is added to a solution of potassium iodide (KI), it reacts with the iodide ions (I^-) present in the solution to form the triiodide ion (I₃^-):

```

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

```

2. Equilibrium: The reaction is an equilibrium, meaning it can proceed in both directions. However, the formation of the triiodide ion is favored, leading to the dissolution of iodine.

3. Solubility: The triiodide ion is soluble in water, unlike elemental iodine (I₂) which is only slightly soluble. This allows for the dissolution of a significant amount of iodine in the presence of KI.

Important Note: The amount of KI used will determine how much iodine can be dissolved. A higher concentration of KI will result in a higher concentration of triiodide ions and therefore a greater capacity to dissolve iodine.

Uses:

This reaction is used in various applications, including:

* Analytical Chemistry: To prepare standard solutions of iodine for titration.

* Medicine: As an expectorant and to treat hypothyroidism.

* Photography: In developing solutions to remove excess silver halide crystals from film.

Overall, the formation of the soluble triiodide complex is the key reason why KI is used to dissolve iodine.