Elephant’s Toothpaste

Materials

• 50 mL to 100 mL 30% hydrogen peroxide (this is expensive, though, but a lower conc. will do just as well. 30% hydrogen peroxide is a strong oxidizer).

• 10 mL saturated potassium iodide solution (To prepare this, dissolve 100 g of potassium iodide in 70 mL of water. You can prepare this solution ahead of time and store it for future use.)

• 10 mL liquid soap or dishwashing liquid

• food colouring

• 2 plastic bin liners

• a measuring cylinder (at least 500ml)

• a pair of scissors, a pair of safety goggles and a pair of rubber gloves

H₂O₂ is an oxidising agent!

Procedure

1. Put on the safety goggles and gloves.

2. Use the scissors to cut one of the bin bags down one side and across the bottom. Open the bag and spread it over the demonstration area. Save the remaining bag for cleanup.

3. Place the graduated cylinder on the open bag.

4. Fill the cylinder to about ¼ full with 30% hydrogen peroxide.

5. Add from 5 mL to 10 mL liquid soap or dishwashing liquid.

6. Sprinkle some food coloring on the inside wall of the cylinder.

7. Add 10 mL saturated potassium iodide solution.

8. STAND BACK! In a few seconds a column of foam will rise out of the cylinder and overflow onto the open bag.

The Chemistry

This activity demonstrates the decomposition of hydrogen peroxide catalyzed by potassium iodide. The rapid production of oxygen causes the mixture to foam, rise, and overflow the cylinder. The 2-step decomposition reaction is written as follows:

            a.  H₂O_2(aq) + I ^-_(aq)  à  H₂O_l + OI ^-_(aq)    (rate determining step)

            b.  H₂O_2(aq) + OI ^-_(aq)  à  H₂O_l + O_2(g) + I ^-_(aq)

You can reveal the presence of oxygen in the foam by performing a glowing splint test. Place a glowing splint in the foam and it will relight, indicating that oxygen is present. Do not drop the splint into the cylinder. The brown color of the foam indicates that iodine is present. Iodine can stain clothing and skin, so avoid contact with the foam. This demonstration is a fun, attention-getting way to introduce topics such as kinetics, rate laws, decomposition, oxidation/reduction, and gas production or limiting reagents.

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