Rates of reaction experiment. This experiment allows variables such as the concentration of a reactant to be investigated. Sodium thiosulphate (Na2S2O3) is a clear liquid, which reacts with hydrochloric acid (HCl) to form an opaque solution. A cross drawn onto paper is placed under the reaction to allow the end point to be confirmed. The experiment is timed from when the reactants are mixed until a precipitate of sulphur is formed which obscures the cross.