Decomposing Hydrogen Peroxide Demonstration (Elephant Toothpaste)
Lesson 2 of 11
Objective: Students will be able to compare reaction rates between different concentrations of hydrogen peroxide as it decomposes into water and oxygen.
PE: MS-PS1-2 - Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
DCI: PS1.B – Chemical Reactions: Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substance are regrouped into different molecules, and these new substances have different properties from those of the reactants. The total number of each type of atom is conserved, and thus the mass does not change. Some chemical reactions release energy, others store energy.
S&E Prac: (8) Obtaining, evaluating, and communicating information.
CCC: Energy and Matter
This demonstration will provide your students with evidence of a chemical reaction (MS-PS1.2). The chemical reactions they observe will show that the total number of atoms stays the same, but changes form, in addition this chemical reaction will release energy (PS1.B). They will obtain and evaluate the information presented (SP8) while studying the broad topic of energy and matter (CCC: Energy and Matter).
Elephants' Toothpaste Demonstration decomposes hydrogen peroxide using a catalyst and changes into water and oxygen. The rapidly released oxygen will be captured in liquid soap to form a column of oxygenated soap that looks like toothpaste for an elephant.
The goal of this lesson is to introduce a potentially dangerous reaction that the students can use as a point of comparison to when they actually get to perform a similar reactions: Catalyst Lab (Elephant's Toothpaste). New vocabulary is introduced (exothermic, decomposition, catalyst, concentration). They are expected to use this new vocabulary when they discuss chemical reactions.
Materials: 30% hydrogen peroxide (purchase through a chemical supply store), 3% hydrogen peroxide (purchase at any drug store), food coloring, 2 large graduated cylinder (150mL) or 2 2-litre soda bottles, potassium iodine (KI) or Fleishman’s Active Yeast, large aluminum baking tray, two medium sized beakers, sir rod, liquid soap, distilled water.
I perform this demonstration as a comparison between two different concentrations of hydrogen peroxide. TIP: If you are unable to get 30% hydrogen peroxide you can substitute Clairoxide hair bleach by Clairol, which is roughly 12% concentration, it works but it is not as spectacular.
- Set two identical, large graduated cylinders side-by-side in the large aluminum baking tray.
- Pour 50 mL of 30% hydrogen peroxide into one graduated cylinder and 50 mL of 3% hydrogen peroxide in another.
- Add a few drops of food coloring to each graduated cylinder for effect. TIP: I let the kids pick the colors, too many colors produces black toothpaste. I like to dribble the color down the side of the cylinder to give the resulting column of O2 a striped look.
- Pour 20 mL of distilled water into each beaker and add one spoonful of potassium iodine (KI) and stir until dissolved. TIP: I teach my students to use the handle of a spoon to get powdered solids out of a jar. It is not as clumsy as using the intended end of the spoon.
- Add one squirt of liquid soap to each graduated cylinder. The soap is not part of the reaction, it helps trap the oxygen gas to the products are visible.
- Place the following chemical reaction on the board:
Hydrogen peroxide produces water + oxygen with the aid of a catalyst.
Referring to the chemical equation, I explain that the students are going to watch hydrogen peroxide being torn into water and oxygen gas. This will occur very slowly over several years, but with a catalyst (KI) the reaction can occur in a few seconds. A catalyst is not consumed or part of the reaction, its only purpose is to accelerate the reaction. Catalysts are written above the arrow in chemical equations.
The reaction will produce heat (exothermic), decompose hydrogen peroxide into simpler water and oxygen. It also will be accelerated with the aid of a catalyst. They will be able to compare two concentrations, recalling that chemical concentration also will accelerate a reaction.
I like to set up the entire demonstration as the kids watch. I also encourage them to film the demonstration and post it to their social media sites. I make a big dramatic countdown and then stand in front of the class doing nothing. I explain that this reaction is now occurring, but will take several years to complete. This usually emits many groans and moans from the kids as they were duped into filming nothing exciting.
I explain that a catalyst (KI) will be able to speed this reaction up. I explain that it is dissolved in distilled water, because the catalyst will be able to interact with the hydrogen peroxide faster if it a liquid that it were left in its solid form.
For the second countdown I simultaneously pour the liquid catalyst into each graduated cylinder and step back for the reaction to occur. You will witness a rapid column of oxygenated soap rise from the graduated cylinder containing 30% hydrogen peroxide and a much slower rise of oxygenated soap from the 3% hydrogen peroxide.
After the reactions have slowed I carefully call up sets of students and have them scoop a handful of warm soap into their hands to prove the reaction was exothermic. It is only soap, but the food coloring can stain hands and clothes.
To prove the existence of oxygen in the soap I hold a lighter next to the soap column and a crackling sound can be heard.
I review with the students that they observed a(n):
- exothermic reaction (releases heat)
- decomposition reaction (complex molecule torn into simpler molecules)
- catalyst (chemical speeds up reaction, but not consumed)
- concentration (higher percentage reacts faster)
As part of their review assignment students have to draw four pictures on one sheet of paper in their interactive notebook reviewing these four points (exothermic, decomposition, catalyst, concentration). These pictures must use color, scientific terms, and a clear explanation of the terms.