In this activity, students see colours change in a solution, and learn how chemistry can explain this magical result.
This trick works because iodine is brown when dissolved in water and purple when dissolved in oil.
Pure iodine is violet, but when it’s dissolved in water, it accepts an electron from the oxygen atom, affecting how it absorbs light. When you shake the fluids, the iodine leaves the water and dissolves in the oil, and returns to its purple colour!
The reason the iodine molecules leave water to dissolve in oil is due to how polarity affects solubility. Water is polar; it has an uneven distribution of electrons. Oil is non-polar; electrons in molecules of this substance are distributed evenly. Iodine is also a non-polar molecule therefore it is more soluble in oil; "like dissolves like".