We have taken a brief look at an endothermic process, seeing that dissolving epsom salts in water lowered the temperature of the water. This week, we will examine a process that is exothermic, which means that it gives off heat. To try this, you will need:
- a bowl
- a thermometer
- 3% hydrogen peroxide (from the grocery or pharmacy)
- pencil and paper
Pour about 1/4 cup of hydrogen peroxide into the bowl. Find its temperature with the thermometer, and write it down, labeling it "Starting temperature." Stick your finger into the liquid, and it should feel cool. Normally, you don't go around sticking your fingers into chemistry experiments, but in this case, we know that hydrogen peroxide is not harmful.
Sprinkle about 1/4 of a teaspoon of yeast into the bowl, and give it a stir. The mixture should start to foam, and its temperature should increase quite a bit. If you stick your finger into the liquid, it should be getting warm.
OK, so why did it get so warm? Think back to last week's experiment. We said that whenever anything changed, energy is involved. Last week, the process of dissolving the epsom salts in water needed energy, so it absorbed heat energy from the surrounding water, making it cooler.
This time, we are looking at a different process. Hydrogen peroxide has the chemical formula H2O2, telling us that each molecule is made up of two atoms of hydrogen and two atoms of oxygen. Looking at the formula, it looks very much like the formula for water, H2O, and it is. The extra oxygen atom is not very stable, so it is easy for it to be released. That is what formed the bubbles you saw. The gas inside was oxygen. (See the Extra Oxygen video to learn more.
The yeast contains an enzyme called catalase, cause the hydrogen peroxide to break apart into water and oxygen gas. In the process, energy that was stored in the chemical bond is released as heat, causing the liquid to get quite warm.
Any time something changes; you need energy. Sometimes energy is absorbed, and sometimes it is given off, but energy is always involved in change. That energy can be heat, as with these experiments, but it can also be other forms of energy, such as light, electricity, motion, etc.
Now it is time for me to gain some energy and lose some at the same time, by having a nice bowl of ice cream. The calories (stored chemical energy) will warm me up, after the cold ice cream absorbs heat as I eat it. What a wonder-filled way to study energy!
Have a wonder-filled week.