Aim: To investigate how the pH of a solution changes as calcium oxide (a base) reacts with dilute hydrochloric acid.

Method:

1. Measure the Volume of Hydrochloric Acid:

• Use a pipette or measuring cylinder to measure a set volume of dilute HCl (e.g., 25 cm³).
• Pour the measured HCl into a conical flask.

2. Weigh the Calcium Oxide:

• Using a mass balance, measure a fixed mass of calcium oxide (CaO) (e.g., 1g).
• Make sure the mass is accurate for each repeat of the experiment.

3. Add Calcium Oxide to Hydrochloric Acid:

• Carefully add the calcium oxide to the flask containing the hydrochloric acid.
• The reaction between the calcium oxide and hydrochloric acid will start immediately, producing calcium chloride and water.

4. Monitor the Reaction and Measure the pH:

• Wait for the reaction to complete (the bubbling and fizzing should stop as the acid reacts with the base).
• Use a pH probe or universal indicator to record the pH of the solution.
• Note down the pH reading after each addition of calcium oxide.

5. Repeat the Process:

• Add more calcium oxide and measure the pH again, repeating until no more reaction occurs (e.g. when calcium oxide remains sitting at the bottom of the flask).
• This indicates that all the acid has reacted, and any additional calcium oxide cannot dissolve or react.

Expected Results:

• As more calcium oxide is added, the solution's pH will gradually increase.
• Initially, the pH will be acidic (less than 7), but as more calcium oxide is added, the pH will rise towards neutral (pH 7).
• If enough calcium oxide is added, the pH may become alkaline (greater than 7), as the calcium oxide neutralises all the hydrochloric acid and the excess base remains unreacted.