Disappearing X Experiment. Coursework about sodium thiosulphate and hydrochloric acid. Method, Results, Fair Testing, Evaluation included. Was marked with a passing grade.

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Disappearing X Experiment


I am going to investigate the rate of reaction between sodium thiosulphate (thio) and hydrochloric acid.

Hydrochloric Acid + Sodium Thiosulphate "" sodium chloride + water + sulphur

The reactants are both colourless, but one of the products, sulphur, is yellow, so we can use this fact to follow the rates of reaction. I am going to investigate how the concentration of the acid effects the speed of the reaction.


1 small measuring cylinder

1 large measuring cylinder

1 small beaker

1 large beaker

Hydrochloric Acid

Sodium Thiosulphate

Cross drawn on paper

Conical flask




First of all we measured out 25cm3 acid and 25cm3 sodium thiosulphate. Then we drew a cross onto a piece of paper and put an empty conical flask on top of the paper. Next, we poured the 25cm3 acid and 25cm3 thio into the conical flask and started the stop clock. We timed how long it took for the solution to turn cloudy so that the cross could no longer be seen. We then decided to change the experiment to see if the concentration of the acid affected the rate of reaction. To do this we added water to the acid and did the experiment again 4 times, decreasing the concentration of the acid by 5cm3 each time. We did this by measuring out the acid in a beaker and pouring it into a measuring cylinder along with the water as well. We then added the contents of the cylinder to the conical flask along with the 25cm3 sodium thiosulphate.


I changed the concentration of the hydrochloric acid by adding water to it.

25 Acid / 0 Water / 25 thio

20 Acid / 5 Water / 25 thio

15 Acid / 10 Water / 25 thio

10 Acid / 15 Water / 25 thio

5 Acid / 20 Water / 25 thio

Instead of changing the concentration of the acid, we could have added water to the thio, like this:

25 Acid / 25 thio / 0 Water

25 Acid / 20 thio / 5 Water

25 Acid / 15 thio / 10 Water

25 Acid / 10...
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