Hot and Cold Volcano: Easy Endothermic and Exothermic Reactions

Learn about endothermic and exothermic reactions by making cold and hot chemical volcanoes. This project is more entertaining than mixing chemicals in beakers and measuring their temperatures. The volcanoes use common, safe household ingredients.

Endothermic and Exothermic Reactions

An endothermic chemical reaction absorbs energy from its environment, so the reaction feels cold. An exothermic reaction releases energy, so the reaction feels warm. Common examples of endothermic processes include photosynthesis, dissolving ammonium chloride in water (cold pack reaction), sublimating dry ice into carbon dioxide, and melting ice. Examples of exothermic processes include burning wood, polymerization of resin, the thermite reaction, mixing acids and bases, dissolving detergent, and condensing rain from water vapor. Endothermic and exothermic chemical reactions both require energy input (the activation energy) to occur. Spontaneous reactions happen when there is already enough energy in the system to supply the activation energy. But, exothermic reactions release more energy than they absorb. Endothermic reactions continue to absorb heat from their environment as they proceed.

Hot and Cold Chemical Volcanoes

The materials for this project are:

• 2 Erlenmeyer flasks or model volcanoes
• Baking soda (sodium bicarbonate)
• Vinegar (weak acetic acid)
• Dry yeast
• Peroxide (3% hydrogen peroxide)
• Liquid dishwashing detergent (e.g., Dawn)
• Water
• Food coloring
• Thermometer (optional)

Erlenmeyer flasks are conical like a volcano and transparent, so you can watch the chemical reaction. However, you can build and decorate model volcanoes by covering plastic bottles with flour and water, clay, or paper mache. If you like, you can just peel the labels off bottles and use them like that.

The baking soda and vinegar volcano is the cold volcano.

1. Fill the “volcano” about halfway with water. Stir in a couple of heaping spoonfuls of baking soda, a squirt of detergent, and a few drops of blue food coloring.
2. Pour vinegar into the volcano to make it erupt. You can recharge the volcano with more baking soda and vinegar.
3. Touch the liquid to feel that it is cool. A more scientific approach is to measure the temperature of the liquids before and after the eruption with a thermometer.

The yeast and peroxide volcano is the hot volcano (warm, really).

1. Fill the volcano most of the way full with household peroxide. Add a squirt of liquid detergent and some food coloring. Red or orange are nice warm color choices. Note: This project works with a higher percentage of hydrogen peroxide (e.g., 6%) to give a hotter reaction, but then it is not safe to touch because the peroxide is a strong oxidizer.
2. Start the eruption by pouring in a packet of dry active yeast.
3. Feel the warmth of the reaction or measure the temperature change with a thermometer.

How It Works

The classic baking soda and vinegar chemical volcano is an example of an endothermic reaction. Baking soda (sodium bicarbonate) reacts with vinegar (acetic acid) to produce carbon dioxide gas, water, and sodium acetate:

NaHCO₃ + HC₂H₃O₂ → NaC₂H₃O₂ + H₂O + CO₂

The detergent captures the carbon dioxide gas, making the bubbles or “lava.” The reaction actually proceeds in two steps: a double displacement reaction and a decomposition reaction. Sodium bicarbonate and acetic acid form sodium acetate and carbonic acid:

NaHCO₃ + HC₂H₃O₂ → NaC₂H₃O₂ + H₂CO₃

Carbonic acid then decomposes into water and carbon dioxide gas:

H₂CO₃ → H₂O + CO₂

The decomposition reaction absorbs energy to break the chemical bonds in complex molecule. In this reaction, more energy is needed to break chemical bonds than to form new ones, so the overall reaction is endothermic.

The yeast and peroxide chemical volcano is an example of an exothermic reaction. Another use of the reaction is for the kid-friendly elephant toothpaste demonstration. Hydrogen peroxide decomposes into water and oxygen gas:

2H₂O₂ → 2H₂O + O₂

This reaction occurs slowly in a bottle of hydrogen peroxide, so it eventually loses its effectiveness. The yeast contains the enzyme catalase, which catalyzes the reaction so it proceeds much more quickly than normal. The “lava” forms as the detergent forms bubbles around the escaping oxygen gas.

This is another example of a decomposition reaction, but this time it is exothermic because more energy is released breaking bonds than forming them.

References

• American Chemical Society. “Heat Up to Some Cool Reactions” (PDF).
• PS21. “Dissolving Energy” (PDF).