Grade 12 Physical Sciences students investigating the effect of concentration on reaction rate using the reaction between sodium thiosulphate and hydrochloric acid.
When hydrochloric acid is added to a clear solution of sodium thiosulphate, one of the products formed is yellow sulphur. Sulphur turns the clear solution cloudy. If a piece of paper with a cross marked on it is placed under the reaction flask, the time taken for the cross to disappear can be used to calculate the rate of the reaction. The experiment is repeated with different concentrations of sodium thiosulphate solution. This experiment complements theoretical teaching done in class and forms part of each student's school-based assessment mark.