The effect of concentration on the reaction rate:

III) Method:

• We put 50cm3 of 40g/dm3 sodium of thiosulphate solution in a flask and then we put 5cm3 of hydrochloric acid and we start the timer. When we could not see the cross anymore, we stop the timer and we note the time.
• Then we redo the processes with different concentration of sodium thiosulphate solution by adding water.

IV) The table of results:

V) Graph:

In this experiment there is three types of variable:  the independent variable which is the concentration of sodium thiosulphate solution; the dependent variable which is the rate of reaction and the constant variable which is the temperature, the volume of hydrochloric acid and the volume of water.

The graph shows that if you double the concentration it will double the rate of reaction. It may happen like that, but the relationship may well be more complicated.

Conclusion:  

This reaction involves collision between two different types of molecules: molecules of hydrochloric acid and molecules of sodium thiosulphate solution.  

In order for any reaction to happen, those particles must first collide. If the concentration is higher, the chances of collision are greater and the reaction happens quickly.