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Deep Purple Magic

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".

Objectives

  • Explain the importance of observation when doing science.

Materials

  • Per Demo or Student:
    small, clear jar
    water
    oil (mineral or baby oil)
    tincture of iodine
    *All chemicals and solutions are readily available from pharmacies or science supply stores.

Key Questions

  • What colour changes did you notice?
  • What part of the mixture is causing the colour change?
  • Why does the iodine change colour?
  • Does the iodine dissolve more readily in the oil or the water?
  • Why don’t the oil and water mix?

What To Do

Hint: perform the “magic trick” before you offer an explanation.

  1. Fill the jar about ½ full of water and add iodine until it is about the colour of tea.
  2. Hold it up and slowly add an equal amount of oil so that the students can see the clear oil on top of the amber-coloured water.
  3. Tighten the cap and shake for about 30 seconds. The two layers will need a few moments to form. The bottom layer will be much lighter and the top will be a beautiful purple colour.

Vocabulary:

  • Solution: A mixture of two or more substances, usually in liquid form.
  • Polar molecule: a molecule that has an uneven distribution of molecules from one end to the other.

Extensions

  • Can you predict what would happen if you didn’t have equal amounts of oil and water in the bottle? Test it and see if your results match your predictions.
  • Take a slice of potato and add a drop or two of iodine onto it. What colour change do you observe?
  • Investigate what is in the potato that causes the colour change when iodine is applied.

Other Resources

Science World | YouTube | Using Purple Cabbage to do some Chemistry!