Materials:
1. 1.5 M (30%) citric acid solution,
2. (dissolve in warm water, let cool to room temperature before use; each pair needs 30 ml)
3. Sodium bicarbonate (NaHCO3) powder (10 g for each pair) + spatula
4. Thermometer
5. 1 plastic tray for each pair for containing spills
Students do:
1. collect goggles, 100 ml beaker, thermometer
2. dispense 30 ml citric acid solution into 100 ml beaker
3. measure starting temperature (make sure it is stable)
4. ad 1 spatula of sodium bicarbonate and stir gently using the thermometer
5. when the fizzing subsides, record the temperature
6. repeat adding 1 spatula of sodium bicarbonate, stirring and recording
Discussion: the production of carbon dioxide gas bubbles and the temperature change indicate that a chemical reaction takes place. Adding universal indicator would have demonstrated neutralisation by colour change (pH 4 > pH 7).
The reaction can be used for making cold packs to reduce the pain and damage caused by burns, sprained ankles or bruises.