Suggested practical

Investigating the effect of concentration on rate of diffusion


To determine if higher concentrations diffuse faster.


  1. Cut a 1 cm3 cube of agar. The agar should be made using 0.1 M sodium hydroxide and coloured with phenolphthalein indicator to produce pink-coloured alkaline agar.
  2. Place a 1 cm3 cube of agar into a 0.5 M solution of hydrochloric acid for two minutes.
  3. Remove the cube and wash it in water to stop the reaction.
  4. Cut the cube in half and measure the distance in millimeters that the acid has caused the agar to become colourless from the outside, inwards towards the centre.
  5. Repeat the experiment a further two times and calculate a mean value.
  6. Repeat using 1, 1.5 and 2 M solutions of hydrochloric acid.


  • Hydrochloric acid is corrosive. If it touches skin it should be washed off.
  • Goggles should be worn at all times.


Concentration of buffer solution (M)Distance of colour change into cube (mm)


As the concentration of hydrochloric acid increased the distance the colour change of the agar, from pink to colourless, also increased. This shows that greater diffusion of the hydrochloric acid into the agar occurred at higher concentrations.