Wednesday, April 20, 2011

Elephant's Toothpaste

One of the most exciting classes I conduct in my chemistry classroom is the Elephant's Toothpaste demonstration.  While, this is an old standard and some students have seen it on You Tube or in other science classes, it never fails to capture the student's attention and create opportunities for discussion.

I conduct the demo in a 5 L graduate cylinder. I place the cylinder in an equipment tray placed on a tarp to protect the floor from overflow.

Pour 250 mL of 30% Hydrogen Peroxide (H2O2) to the cylinder and add 1-2 tablespoons of dish soap to the peroxide.  Dawn is preferable, but any dish soap will work.  I drip food coloring down the sides of the cylinder to provide the toothpaste striping. I then add 4-6 grams of Potassium Iodide (KI)  to the solution. Step back and enjoy the reaction, both from the cylinder and the students. 

I do not ask students to complete a lab write-up for this demonstration.  I instead use the opportunity to  discuss rates of reaction. I will have students touch the surface of the cylinder with the back of their hand to feel the heat produced by the reaction and discuss endothermic and exothermic reactions.   There are a multitude of topics that can be discussed from this reaction.

However most important is the discussions that start with the students questioning, why does that happen? Will other types of salt do that? What if we used ....?  Why did we add the soap?  That is when the thinking begins and the learning takes over.

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