Rates of reaction experiment. This experiment allows variables such as the concentration of a reactant to be investigated. Sodium thiosulphate (Na2S2O3, left) is a clear liquid, which reacts with hydrochloric acid (HCl, upper centre) to form an opaque solution (right). A cross (left) 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 the cross disappears from sight.