ZIMSEC O Level Combined Science Notes: Experiment: Investigating the neutralisation process

Aim: To find out what happens to the pH of an acid when a base is added

Materials: vinegar, plastic beaker, measuring cylinder, stirring rod, spatula/plastic teaspoon, slaked lime (calcium hydroxide), universal indicator paper


  1. Place about 10 cm3 of vinegar in a beaker
  2. Dilute it with an equal volume of water
  3. Remove one drop/small amount of the solution and test its pH using the universal indicator paper
  4. Add a spatula measure of slaked lime to the vinegar and stir
  5. Test the pH of the resulting solution as in step 3 above making sure to avoid collecting particles of the slacked lime that in the mixture
  6. Repeat steps until the pH of the solution no longer changes after each step


  • At first the vinegar has a low pH i.e. it is acidic
  • The pH of the mixture/solution increases with each spatula of slaked lime added
  • At some point the pH reaches 7 i.e. the solution is neutral and is neither acidic nor alkaline
  • The pH of the mixture continues to increase until it reaches 12
  • After the pH reaches 7 it suddenly reaches 11-12
  • Then it stops increasing


  • Acids can be neutralised by adding bases
  • The acid reacts with the base to form salt and water
  • The equation can be represented as:
  • \text{base+acid}\rightarrow\text{salt+water}
  • As soon as the acid is neutralised and there is no acid remaining to react with the base the pH increases suddenly to 12
  • Slaked lime (lye) has a pH of around 12.4 so once the acid is neutralised the Universal paper will in fact be showing the pH of the residual base
  • It is important not to pick up the slaked lime during the measuring process outlined in step 3 as this would give pH readings as the test would show the pH of the lime rather than the actual mixture.

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