Identifying unknown solutions from pH data

A recurring task: given several solutions of the SAME concentration and their measured pH values, identify which named substance each one is.

The identification method

Classify each candidate substance FIRST: is it a strong base, a weak base, or neutral (neither acidic nor basic)?
A STRONG base (e.g. NaOH) fully ionises, giving the HIGHEST pH of the group at a given concentration.
A WEAK base or a salt that hydrolyses to be mildly basic (e.g. Na₂CO₃, whose carbonate ion reacts with water) gives a MODERATELY high pH — lower than a strong base at the same concentration, but still above 7.
A NEUTRAL salt (e.g. NaCl, from a strong acid and strong base) gives pH ≈ 7, since neither ion reacts significantly with water.
Match the HIGHEST measured pH to the strong base, the MIDDLE pH to the weakly-basic salt, and pH ≈ 7 to the neutral salt.

Worked example Identifying NaOH, NaCl and Na₂CO₃ from pH data Three 0.1 mol L⁻¹ solutions of NaOH, NaCl and Na₂CO₃ have measured pH values of 13.0, 7.0 and 11.6 (in some order). Identify each and justify with equations. HIGHEST pH (13.0) = NaOH: a strong base, fully ionising as NaOH(aq) → Na⁺(aq) + OH⁻(aq), giving the highest [OH⁻] and therefore the highest pH. LOWEST pH (7.0) = NaCl: the salt of a strong acid and strong base — neither Na⁺ nor Cl⁻ reacts significantly with water, so the solution is neutral. MIDDLE pH (11.6) = Na₂CO₃: the carbonate ion is a weak base, reacting partially with water: CO₃²⁻(aq) + H₂O(ℓ) ⇌ HCO₃⁻(aq) + OH⁻(aq) — producing some OH⁻, but far less completely than NaOH's full ionisation, giving a pH between the other two. ✓ The justification must include the RELEVANT EQUATION for each substance's behaviour in water, not just a stated classification — "NaOH is a strong base" alone, with no equation, is an incomplete "fully explain". ✕

Assuming every sodium salt is neutral

Not every salt of sodium is neutral — a salt formed from a strong base and a WEAK acid (like Na₂CO₃, from carbonic acid) hydrolyses to produce a mildly basic solution. Only a salt from a strong acid AND a strong base (like NaCl) is truly neutral.

Solution	Classification	Relative pH
NaOH	Strong base — fully ionises	Highest
Na₂CO₃	Salt of a strong base + weak acid — weakly basic	Middle
NaCl	Salt of a strong base + strong acid — neutral	Lowest (≈7)

Practice for this standard covers the whole topic — flashcards and quiz.