Core practical

Investigate the rate of a reaction by observing a colour change

There are a number of ways to investigate the rate of a reaction in Chemistry. This is an outline of the required steps to undertake one of these methods. It is important in this core practical to use appropriate apparatus to make and record a range of measurements accurately, including mass, time, temperature and volume.


To investigate the effect of changing the temperature on the rate of a reaction.

Sodium thiosulfate solution reacts with dilute hydrochloric acid:

Sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur

Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

The sulfur forms a cloudy yellow-white precipitate during the reaction. The time taken for this to achieve a given cloudiness provides a way to measure the reaction time.


The result of adding dilute acid to a transparent sodium thiosulfate solution is a more opaque solution.
  1. Using a measuring cylinder, add 50 cm3 of dilute sodium thiosulfate solution to a conical flask.
  2. Place the conical flask on a piece of paper with a black cross drawn on it.
  3. Using a different measuring cylinder, add 10 cm3 of dilute hydrochloric acid to the conical flask. Immediately swirl the flask to mix its contents and start a stop clock.
  4. Measure and record the temperature of the reaction mixture.
  5. Look down through the reaction mixture. When the cross is no longer visible, record the time on the stop clock.
  6. Measure and record the temperature of the reaction mixture, and clean the apparatus as directed by the teacher.
  7. Repeat steps 1 to 6 with different starting temperatures of sodium thiosulfate solution.


Record the results in a table. This table gives some example results.

Temperature (°C)Reaction time (s)Rate \frac{1000}{time}(/s)


  • Calculate 1000/time for each temperature. This value is proportional to the rate of reaction.
  • Plot a graph to show:
    • reaction rate (/s) on the vertical axis
    • temperature (°C) on the horizontal axis
    • a curve of best fit
Graph showing the reaction rate rising sharply with the temperature.

Describe the effect of increasing the temperature of the reaction mixture on the rate of reaction. Use the graph to help.

The rate of reaction increases as the temperature increases. The rate increases by a greater amount at higher temperatures.



Suggest a reason why the same person should look at the black cross each time.

Different people may decide that they cannot see the cross at different amounts of cloudiness, leading to errors in deciding when to take the reaction time.

Hazards, risks and precautions

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

Hot sodium thiosulfate solutionBurns to the skinDo not heat above 60°C
Sulfur dioxideCan cause irritation to the eyes and lungs, particularly to people with asthmaMake sure the laboratory is well ventilated and avoid breathing directly over the top of the flask

Fran Scott demonstrates how to measure the rate of reaction and how to increase it

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