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

Method

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

Observations/Results

• 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

Conclusion

• 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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