Core practical: pH changes in neutralisation

Core practical 3.6 follows the pH as calcium hydroxide (or calcium oxide) is added to dilute hydrochloric acid.

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The method

1. Measure a fixed volume of dilute hydrochloric acid (for example 50 cm3) into a beaker.
2. Measure the pH with a pH meter or universal indicator paper.
3. Add a small measured mass of calcium hydroxide powder (for example 0.3 g) and stir.
4. Measure the pH again. Repeat, adding the same mass each time, until the pH stays the same.
Wear eye protection: the acid and calcium hydroxide are both irritants.

The results

Plot pH (y-axis) against total mass of calcium hydroxide added (x-axis).
At first the pH is low (acidic) and rises slowly. Near neutralisation it rises quickly through 7. Once the acid is used up, excess calcium hydroxide makes the solution alkaline, so the pH rises above 7 and then levels off.
calcium hydroxide + hydrochloric acid → calcium chloride + water
Ca(OH)2 + 2HCl → CaCl2 + 2H2O

What neutralisation is

Neutralisation is the reaction between an acid and a base to make a salt and water.
In every acid-alkali neutralisation: H+(aq) + OH−(aq) → H2O(l)
A pH meter gives more precise readings than universal indicator, which is judged by eye from colours.
Worked example

A student adds 0.3 g of calcium hydroxide at a time. The pH first goes above 7 after the eighth addition. What total mass had been added?

  1. Total mass = number of additions × mass each time
  2. = 8 × 0.3

Answer: 2.4 g

Key idea

Add measured masses of calcium hydroxide to a fixed volume of acid, stir, and record the pH each time. The pH rises slowly, then quickly through 7, then levels off above 7. Neutralisation: H+ + OH− → H2O.

Key words
Neutralisation
The reaction between an acid and a base to make a salt and water. H^{+} + OH^{-} → H_{2}O.
Acid
A substance that releases hydrogen ions, H^{+}, in water (a proton donor). Its solution has a pH below 7.
Indicator
A dye that changes colour depending on pH, such as litmus, methyl orange or phenolphthalein.
Solution
A mixture formed when a solute dissolves in a solvent.
Base
A substance that neutralises an acid to form a salt and water (a proton acceptor), such as a metal oxide or hydroxide.
Salt
The compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.

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Quick check

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Extra practice

2 more questions

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  1. A student adds 0.20 g of calcium oxide at a time to acid. How much has been added in total after 8 additions?

    Show the answer

    Answer: 1.6 g

    0.20 × 8 = 1.6 g

  2. Universal indicator paper reads "about pH 4" while a pH meter reads 4.27. What does this show?

    1. The pH meter has a higher resolution
    2. The meter neutralises the acid
    3. The paper is more accurate
    4. Both are equally precise
    Show the answer

    Answer: The pH meter has a higher resolution

    A pH meter gives more precise readings.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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