Neutralisation Does Not Always Give pH 7

What students typically write

“Neutralisation always produces a solution with pH 7.” Or: “If you mix an acid and an alkali, the pH is always 7.”

Students learn that neutralisation is “acid + base gives salt + water” and that water has pH 7, so they conclude that neutralisation always reaches pH 7.

Why it is wrong

A solution of pH 7 requires that the concentrations of H+ and OH- ions are exactly equal. This happens when a strong acid reacts with a strong alkali in the exact mole ratio demanded by the equation, and no excess of either remains.

In practice, several scenarios give a pH that is not 7:

1. Excess acid remains. If you add too much acid or too little alkali, the solution will be acidic (pH below 7).
2. Excess alkali remains. If you add too much alkali or too little acid, the solution will be alkaline (pH above 7).
3. Weak acid + strong alkali. The salt formed (for example, sodium ethanoate) produces a slightly alkaline solution.
4. Strong acid + weak base. The salt formed (for example, ammonium chloride) produces a slightly acidic solution.

At IGCSE level, the syllabus focuses on the first two scenarios. The most common exam context is titration, where reaching pH 7 requires adding exactly the right volume of acid to exactly neutralise the alkali, which is why indicators are used to detect the end point.

What the mark scheme actually wants

A typical question: “A student adds sodium hydroxide solution to hydrochloric acid. Explain why the pH of the mixture may not be exactly 7.” [2]

Marking points:

1. If excess acid/alkali is present (1 mark)
2. The solution will be acidic/alkaline rather than neutral (1 mark)

The examiner wants you to recognise that stoichiometry matters. Simply saying “neutralisation gives pH 7” without qualification is incomplete.

Worked example: wrong vs right

Question: 25.0 cm3 of 0.10 mol/dm3 NaOH is mixed with 20.0 cm3 of 0.10 mol/dm3 HCl. Predict whether the final pH will be 7, above 7, or below 7. Explain your answer. [2]

Wrong answer: “pH 7, because it is a neutralisation reaction.” This scores 0/2. It ignores the volumes and assumes neutralisation always means pH 7.

Right answer: “Moles of NaOH = 0.025 x 0.10 = 0.0025 mol. Moles of HCl = 0.020 x 0.10 = 0.0020 mol. There is excess NaOH (0.0005 mol unreacted), so the solution is alkaline with a pH above 7 (1)(1).” This scores 2/2.

How to avoid this mistake

Before writing “pH 7”, check two things:

1. Are the acid and alkali both strong? (If one is weak, pH will not be exactly 7 even at the equivalence point.)
2. Are they mixed in exactly the right stoichiometric ratio? (Calculate moles of each to check.)

Only if both conditions are met can you confidently say pH 7. In all other cases, state whether the solution is acidic or alkaline and explain why.

For titration technique and end-point detection, see acids, bases and indicators. Salt preparation by neutralisation is covered at preparation of salts.