An endothermic reaction is a chemical reaction that absorbs heat from its environment. Like any chemical reaction, an endothermic reaction requires activation energy to proceed. Then, it continues to absorb energy. Such reactions feel cold. In contrast, exothermic reactions release more heat than they absorb and feel hot.
Not all endothermic reactions are safe to touch, so their temperature change must be measured with a thermometer. This simple chemistry demonstration is an endothermic reaction that uses simple ingredients and produces a cooling effect that’s safe to touch.
All you need are three ingredients:
- Citric acid
- Baking soda (sodium bicarbonate)
You can find citric acid and baking soda at the grocery store. If you have trouble finding either material, a fizzy bath bomb contains these ingredients (plus oil, scents, and colors). These ingredients are safe enough that even young children can experience the reaction.
The Endothermic Reaction
Mix together some citric acid and baking soda in a bowl. Add a bit of water and feel the temperature change! The chemical reaction yields sodium citrate, carbon dioxide, and water:
H3C6H5O7(aq) + 3 NaHCO3(s) → Na3C6H5O7(aq) + 3 CO2(g) + 3 H2O(l)
If you like, you can add a squirt of dishwashing liquid to trap the carbon dioxide and make bubbles. When the reaction concludes, the temperature of the solution eventually returns to room temperature. Holding a bath bomb in water is another way to experience the temperature change. Once you are finished with the reaction, it’s safe to wash down the drain.
Increase the “science” by making predictions about what happens in the reaction. It’s a great opportunity to discuss the difference between chemical and physical changes, since the temperature change and bubbles produced by the mixture are clear indications that a chemical reaction occurs.
More Endothermic Reaction Examples
Endothermic reactions are common. Some examples include:
- Melting ice into water
- Evaporating liquid water
- Depressurizing a can
- Making salt (NaCl) from its elements
- The baking soda and vinegar reaction
A really cold endothermic reaction is the chemical reaction between barium hydroxide and ammonium thiocyanate. This reaction reaches temperatures of -20°C or -30°C and is cold enough to produce frostbite!
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