The bonds between the atoms of substances are broken during a chemical reaction. Atoms will, however, form new bonds as they rearrange themselves during the reaction.

Some chemical reactions give off energy in the form of sound, light, or heat. These are called exothermic reactions. Other reactions absorb energy. These are called endothermic reactions.

We used sodium carbonate (NaCO) and sodium bicarbonate (NaHCO) as examples of exothermic and endothermic processes. It’s an easy, no mess experiment. (We used Arm & Hammer because that was what we had in our cupboard—no product placement here! Washing soda is easy to find at the grocery store: just look in the detergent section.)

For a printable version of this project, click here.

• Washing Soda (NA₂CO₃)

• Baking Soda (NAHCO₃)

• Two plastic cups

• 100 millilitres of room temperature water

• Spoon

1. Pour 50 millilitres of the water into one cup, the other 50 millitres in the other cup.

2. Place a spoonful of washing soda in one cup and stir.

3. Place a spoonful of baking soda in the other cup and stir.

4. Grasp a cup in each hand.

The cup containing the water and washing soda should feel warmer than the cup with water and baking soda. The washing soda/water mixture goes through an exothermic process because the washing soda (sodium carbonate) is ionized—it shares electrons with the water. (See Bond. Chemical Bond, page 23.) The baking soda/water mixture is probably weakly endothermic but may just feel cooler relative to the other mixture.

To create an endothermic process you could use ammonium nitrate with water. That solution will feel cooler to the touch. (Ammonium nitrate is available from some garden stores.) Combining salt and ice-water also works.
One common endothermic reaction you don’t have to create yourself is a lightning strike. In lightning strikes, air is heated and the nitrogen and oxygen combine to form nitric oxide.