Sodium thiosulphate reacts with diluted acid to give sulphur dioxide, sulphur and water. Both sodium thiosulphate and diluted hydrochloric acid are colorless solution. Sulphur dioxide is a very soluble gas and dissolves completely in the aqueous solution. The sulphur formed, however, is not soluble and will exist in the mixture as white or yellow precipitate (or colloidal). It makes the reaction mixture becomes opaque as the reaction occurs. Therefore, we can study the reaction rate by monitoring the opaqueness of the reaction. This can be easily done by measuring the time taken for forming a certain amount of precipitate.