Endothermic reaction


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.