Usually on Wednesdays we feature Bug of the Week, so today’s chemistry lesson is inspired by insects.

Dr. Thomas Eisner was a very curious man. He was particularly curious about insects, like the beetle shown in this video (has pop-up ad).

When he discovered these particular beetles, Dr. Eisner began to experiment with them because he wanted to know what and how they were squirting. One of the unexpected things he found out was that the temperature of the spray the beetle released was very hot, nearly 100° C. How could that be?

Endothermic and Exothermic Reactions

Sometimes chemical reactions between two or more substances give off or take in energy, often in the form of heat. In exothermic reactions, heat energy is given off during the reaction and the temperature increases. In endothermic reactions heat energy is removed by the reaction (“taken in”) and the temperature of the reactants decreases.

Below are two chemical reactions that use household products. Find out whether they are exothermic or endothermic.

Notes:  These activities are messy fun, so perform them in a sink, tub, or outdoors in an area where wet spills are not a problem. Also, scientists never eat or drink their experiments!

Reaction 1.

Materials:

• About 26 g lemonade drink mix* (make sure the primary ingredient is citric acid)
• Baking soda
• Liquid measuring cup
• 1/4 cup dry measuring cup
• Water (room temperature)
• Large Styrofoam cup (to help insulate the reaction)
• Container (glass or cup) to mix the drink mix and water in
• Thermometer or temperature probe
• Spoon

* Math alert:  Originally I used Crystal Light pink lemonade mix, which came in 3.68 g packets (see the serving size information for the number of grams per packet). I used 7 packets for about 28 g. Then I switched to an off brand, and the packets were 2.6 g each (10 packets for 26 g). It worked just as well. Also, check the amount of vitamin C information on the labels. The raspberry-flavored lemonade mix contained significantly more citric acid for some reason.

Procedure 1:

1. Measure 100 mL (approx. 1/3 cup) of room temperature water and pour into in a container.
2. Add 26 g of drink mix into the water in the container. Stir until completely dissolved.
3. Use the thermometer or probe to measure the temperature of the solution, taking care not to rest the thermometer or probe on the bottom or side of the cup.
4. Measure 1/4 cup baking soda into the Styrofoam cup.
5. In a sink or similar area, quickly add the drink mix solution to the baking soda in the Styrofoam cup.
6. As the reaction starts to slow, take the temperature again. (You may want to let the children touch the solution and compare to the room temperature water. They will need to wash their hands afterwards.)

Did the temperature of the contents of the Styrofoam cup  go up or down?

Reaction 2:

Materials:

• 2 teaspoons active yeast
• Liquid measuring cup
• Measuring teaspoon
• Water at room temperature
• Cup or similar container for mixing yeast
• Large Styrofoam cup
• Thermometer or temperature probe
• Hydrogen peroxide (be sure to use 3 %, the kind sold for home use) – held at room temperature
• Spoon

Procedure 2.

1. Measure 100 mL of room temperature water (approx. 1/3 cup) and pour into the yeast-mix container.
2. Mix the 2 teaspoons dry yeast into the water. Stir until thoroughly mixed.
3. Take the temperature of the solution, taking care not to rest the thermometer on the bottom or side of the cup.
4. Add 100 mL (about 1/3 cup) 3% hydrogen peroxide to the Styrofoam cup.
5. Take the temperature of the hydrogen peroxide, taking care not to rest the thermometer on the bottom or side of the cup. The two solutions should be roughly the same temperature.
6. In a sink or similar area, add the yeast solution to the hydrogen peroxide in the Styrofoam cup.
7. As the reaction starts to slow, take the temperature again. (You may want to let the children touch the solution and compare to the room temperature water. They will need to wash their hands afterwards.)

Did the temperature go down or up?

Which reaction was endothermic? Which reaction was exothermic? Let us know what you find out.

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If you are interested in learning more about the beetles, read the first chapter in:

For Love of Insects by Thomas Eisner

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For more chemistry activities, check the Table of Contents for Chemistry Week page.