Start your stopwatches, and get ready to learn about reaction rates. Mixing different solutions of sodium thiosulfate with an acid will result in sulfur precipitate forming at different speeds. Will students keep track?

Create a lesson of discovery where students put their knowledge of concentrations and reactions into practical use. This experiment uses simple chemicals and equipment found in your learning space. 

This experiment should take 60 minutes.



  • Eye protection
  • Conical flask, 250 cm3
  • Measuring cylinder, 100 cm3


  • Sodium thiosulfate solution 50 g dm–3
  • Hydrochloric acid 2 mol dm–3

Health, safety and technical notes

  • Read our standard health and safety guidance
  • Always wear eye protection.
  • Sodium thiosulfate solution 50 g dm–3 is low hazard (see CLEAPPS Hazcard HC095a).
  • Dilute hydrochloric acid is low hazard at the concentration used (see CLEAPSS Hazcard HC057a).


  1. Put 50 cm3 of sodium thiosulfate solution in a flask.
  2. Measure 5 cm3 of dilute hydrochloric acid in a small measuring cylinder.
  3. Add the acid to the flask and immediately start the clock. Swirl the flask to mix the solutions and place it on a piece of paper marked with a cross.
  4. Look down at the cross from above. When the cross disappears, stop the clock and note the time. Record this in the table provided on the student sheet.
  5. Repeat this using different concentrations of sodium thiosulfate solution. Make up 50 cm3 of each solution. Mix different volumes of the sodium thiosulfate solution with water as shown in the table.
  6. As soon as possible, pour the solution down the sink (in the fume cupboard if possible) and wash away.


  1. Calculate the concentration of sodium thiosulfate in the flask at the start of each experiment. Record the results in the table provided on the student sheet.
  2. For each set of results, calculate the value of 1/time. (This value can be taken as a measure of the rate of reaction).
  3. Plot a graph of 1/time taken on the vertical (y) axis and concentration on the horizontal (x) axis.


  • Sulfur dioxide (a toxic gas) forms as a by-product. Ensure good ventilation. If there are asthmatics in the class, the risk assessment should take this into account and necessary adjustments made, such as working in a fume cupboard, reducing the scale etc.
  • As soon as the reaction is complete, pour the solutions away, preferably into the fume cupboard sink. Wash away with plenty of water.
  • A light sensor can be used to monitor the precipitation on a computer. The result, in the form of graphs on the computer, can be analysed using data logging software.
  • A light sensor clamped against a plastic cuvette filled with the reactants substitutes for a colorimeter. The data logging software shows the turbidity on a graph, and this tends to yield more detail than the standard end-point approach. The rate of change can be measured using the slope of the graph or the time taken for a change to occur.