The catalytic decomposition of hydrogen peroxide
On its own, hydrogen peroxide slowly decomposes into oxygen and water.
2 H2O2(aq) → O2(g) + 2 H2O(l)
For this reason, fresh bottles of drugstore hydrogen peroxide, labeled 3%, are actually 6%. This is so the hydrogen peroxide remains above 3% at the expiration date, if stored properly. When catalyzed, the rate of decomposition of the hydrogen peroxide increases. Suitable catalysts include yeast, manganese dioxide, “rusty” manganese metal and potassium iodide.
Catalase, an enzyme present in our blood, also catalyzes the decomposition of peroxides, including hydrogen peroxide, which would otherwise be harmful. If you have ever used hydrogen peroxide to clean an open wound, you probably have observed the rapid formation of gas bubbles. These gas bubbles are filled with oxygen from the decomposition of the hydrogen peroxide.
In the demonstration described below potassium iodide is used to catalyze the hydrogen peroxide. More specifically, it is the iodide ion that catalyzes the reaction. The proposed mechanism is as follows:
H2O2(aq) + I-(aq) → IO-(aq) + H2O(l)
H2O2(aq) + IO-(aq) → I-(aq) + O2(g) + H2O(l)
The first step is believed to be the rate determining (slow) step. Recall that catalysts work by proceeding along a different reaction pathway of lower activation energy. Note that the catalyst is involved in the reaction mechanism but is returned to its original form in the final step, enabling it to catalyze the decomposition of additional peroxide molecules.
The iodide is not a perfect catalyst because some reacts with the hydrogen peroxide to form iodine and water. This reduces the amount of iodide available to catalyze the decomposition of the hydrogen peroxide.
2 H+(aq) + 2 I-(aq) + H2O2(aq) → I2(aq) + 2 H2O(l)
I was enticed the first time I saw this demonstration at the ChemEd 89 conference; unfortunately I do not recall the presenter. Since them it has become part of our repertoire of outreach demonstrations. The large scale ‘Elephant’s toothpaste’ demonstration uses 100 mL of 30-35% hydrogen peroxide, a squirt of dish washing liquid, and a few drops of food coloring (just for aesthetics) mixed together in a 1-L cylinder (we use non-graduated cylinders). The cylinder is placed in a plastic tub to catch the overflow and a tarp covers the table to prevent iodine stains in case the mixture spills out of the tub onto the table.
A concentrated solution of potassium iodide is then added to the mixture. Soap bubbles filled with oxygen rise up and out of the cylinder. Condensed steam can be seen coming from the cylinder because the reaction is exothermic. A brown coloration due to the iodine is visible in the soap bubbles. When a glowing wooden splint is inserted into the bubbles it reignites showing that the bubbles are filled with oxygen. A small scale hands-on activity may be performed similarly but with the following changes: 30 mL of 15% hydrogen peroxide, 5 mL of potassium iodide solution, 100-mL graduated cylinders, plastic cafeteria trays and without testing for oxygen (although you can have the participants do this test with close supervision).
- Decomposition reactions
- Exothermic processes
- Reaction mechanisms
- Chemical test for oxygen
- 1-L cylinder
- Plastic tub (to catch overflow)
- Tarp (to cover table)
- 100 mL of 30 – 35% hydrogen peroxide
- Dish washing soap (1 – 2 squirts)
- Food coloring (5-10 drops)
- ~6 – 8 g of KI dissolved in ~20 mL of deionized water (do not use tap water)
- Wooden splints and candle or other means of lighting splints
- Wear protective goggles, gloves and lab coat or apron; tie back long hair if testing for oxygen.
- Avoid coming in contact with the hydrogen peroxide; it will blister the skin immediately. If you do come in contact, flush area with copious amounts of water and remove any clothes that have come in contact.
- Do not touch the product of this reaction or allow others to do so; it contains iodine and unreacted hydrogen peroxide. It is also quite hot.
- Remove gloves before conducting the test for oxygen; open flames can melt the gloves. Be sure the burning wooden splint is extinguished before placing in the trash (dip in water).
- Prepare the potassium iodide solution in advance unless you want to show an example of an endothermic process. Dissolve the potassium iodide in deionized water and place the solution in a sealed container.
- We measure the 100 mL of hydrogen peroxide in advance and place it in a clean, empty bottle labelled 30 – 35% hydrogen peroxide. Doing so eliminates the need to do this in front of the audience and having excess in the demonstration area.
- Cover the demonstration table with the protective tarp.
- Stand the 1-L cylinder in the center of the plastic tub.
- Add 100 mL of 30 – 35% hydrogen peroxide to the empty cylinder.
- Add one or two good squirts of dish washing liquid to the hydrogen peroxide; and mix.
- Add about 5-10 drops of food coloring to the mixture and mix (optional).
- Ensuring that the cylinder is in the center of the tub; quickly but carefully, add the potassium iodide solution and stand back.
- Upon completion of the reaction, light the wooden splint using a candle flame or other source of fire.
- Holding the lighted wooden splint downward at about a 45o angle, allow the wooden splint to burn until intense red embers are visible. Blow out the flame and immediately insert the glowing splint into the bubbles. As soon as it relights, pull the splint back out — water present will extinguish the flame if it is not pulled back out. Repeat several times placing the glowing splint in different areas of bubbles. Those remaining in the cylinder seem to have the greatest amount of oxygen.
- Wearing gloves, set down the cylinder on its side in the tub to avoid breakage. The cylinder and tub along with soapy mixture can be washed out in the sink.
- Be sure wooden splints are completely extinguished. They can then be put in the trash.
Zeena Bhakta performed this demonstration for a large audience of tutors and tutees on campus. The following are her thoughts about learning and performing ‘Elephant’s toothpaste’.
I first presented ‘Elephant’s toothpaste’ at the Tutors and Tutees outreach event. This event was set up as a large-scale demonstration with an audience that was mainly comprised of young elementary and middle school students. We introduced ‘Elephant’s toothpaste’ in the format of a story; we asked “Have you ever been to the zoo?” and explained how important it was for elephants to brush their teeth, just like we do. We used a single 1000-mL graduated cylinder to ensure improved visibility and minimal safety hazards. Given the demonstration circumstances, this was a very effective way of presenting ‘Elephant’s toothpaste’ to the community; the entire audience was able to witness the phenomena and we were able to practice chemical safety simultaneously.
My second time demonstrating ‘Elephant’s toothpaste’ was on a much smaller scale, presenting the demo to a few of my classmates. I was able to adjust my demonstration technique and scientific explanation accordingly. For example, I skipped the storytelling aspect of the demonstration completely. I mentioned it as a technique used for a young audience, but then went straight into the scientific detail. It was necessary for them to be knowledgeable about the science behind this experiment.
Throughout the demonstration, I focused on chemical details such as the catalytic activity of potassium iodide and the decomposition of hydrogen peroxide into oxygen gas and water. I performed the splint test to prove that the gas was indeed oxygen. Providing a detailed scientific explanation and an intimate, hands-on experience was an effective way of teaching the foundations of this experiment to future demonstrators.
The ‘Elephant’s toothpaste’ demonstration is no doubt one of the most simple, yet more engaging experiments Duke Chemistry Outreach has to offer, and the student feedback proved just that. It was a topic of discussion long after the demonstration was performed. Several students asked for it to be repeated and others asked about the experiment itself, delving into more scientific detail.
Based on how this demonstration can be modified to accom-modate audiences of varying ages and scientific background, it is clear that ‘Elephant’s toothpaste’ is an outreach classic.
*Zeena Bhakta is a senior at Duke University, majoring in chemistry, with minors in psychology and Spanish. She plans to take a gap year upon graduation to explore her options.
**Dr. Kenneth Lyle is a lecturing fellow at Duke University. The Powell Family Trust, the Duke-Durham Neighborhood Partnership, and Biogen Idec – Research Triangle Park, fund the Duke Chemistry Outreach Program.