Investigate the effects of changing the conditions of a reaction on the rate of a reaction by observing a colour change.

It is important in this practical to use appropriate apparatus to make and record a range of measurements accurately, including time, temperature and volume. This includes the safe use of apparatus, and monitoring chemical changes.

This outlines one way to carry out the practical. Eye protection must be worn.


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 you can no longer see the cross, record the time on the stop clock.
  6. Clean the apparatus as directed by your teacher.
  7. Repeat steps 1 to 6 with different starting temperatures of sodium thiosulfate solution. The sodium thiosulfate solution is heated by placing it in a hot water bath for different lengths of time.


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

Temperature of reaction mixtureReaction timeRate \frac{1000}{time}(/s)
18°C80 (s)12.5
29°C57 (s)17.5
42°C32 (s)31.3
49°C20 (s)50.0


  1. Calculate 1000/time for each temperature. This value is proportional to the rate of reaction.
  2. 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 your graph to help you.

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 avoid breathing directly over the top of the flask

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