SPEC 4.4.2.4 • PAPER 1
The pH Scale & Neutralisation
What pH measures, what acids and alkalis release in solution, and the universal ionic equation that defines every neutralisation. Plus the logarithmic scale (HT) — and why "linear thinking" loses the mark every year.
The pH Scale: 0 to 14
The pH scale is a measure of the acidity or alkalinity of an aqueous solution. It runs from 0 to 14, with 7 being neutral.
Spec 4.4.2.4
Aqueous solutions of acids have pH less than 7; aqueous solutions of alkalis have pH greater than 7. A solution with pH 7 is neutral — the pH of pure water at room temperature.
What Acids and Alkalis Release
The pH scale measures the concentration of a specific ion in solution — the hydrogen ion, H⁺.
Acid
Acids produce hydrogen ions, H⁺(aq), in aqueous solution. (Spec 4.4.2.4 verbatim.)
Example: HCl(aq) → H⁺(aq) + Cl⁻(aq)
Alkali
Aqueous solutions of alkalis contain hydroxide ions, OH⁻(aq). An alkali is a soluble base.
Example: NaOH(aq) → Na⁺(aq) + OH⁻(aq)
Core Fact
Higher [H⁺] in solution → lower pH (more acidic). Higher [OH⁻] in solution → higher pH (more alkaline). The two ions react to form water — that's why neutralisation works.
Two Ways to Measure pH
Universal / Wide Range
A liquid or paper indicator that changes through a colour range: red → orange → yellow → green → blue → purple as pH rises 0 to 14.
Use: measures the approximate pH (the spec's word) by comparing to a colour chart.
pH Probe / Meter
Electronic device giving a precise digital pH reading.
Use: quantitative work — when an exact pH value is needed (e.g. plotting a pH curve).
Spec Language
Spec 4.4.2.4 says "universal indicator or a wide range indicator" — both are accepted answers. Both give an approximate pH. For a precise value, name the pH probe.
The Universal Neutralisation Equation
When an acid reacts with an alkali, the hydrogen ions from the acid combine with the hydroxide ions from the alkali to form water:
H⁺(aq) + OH⁻(aq) → H₂O(l)
Why It's Universal
This ionic equation describes every acid–alkali neutralisation. It doesn't matter whether the acid is HCl, H₂SO₄, or HNO₃, or whether the alkali is NaOH, KOH, or LiOH. The metal cations (Na⁺, K⁺) and acid anions (Cl⁻, SO₄²⁻) are spectator ions — they take no part in the reaction.
At the End Point
When neutralisation is complete, moles of H⁺ = moles of OH⁻. The solution contains only the salt and water. This is the principle behind titration calculations (covered in 4.06).
Worked Example — Writing the Ionic Equation
Q: Write an ionic equation for the neutralisation of hydrochloric acid with potassium hydroxide.1 MARK
Citation: P1 Jun 2021 (Nov21) Q05.2 — verbatim AQA stem.
1
Write the full equation in ions. HCl(aq) → H⁺ + Cl⁻; KOH(aq) → K⁺ + OH⁻; product KCl(aq) → K⁺ + Cl⁻.
2
Cancel spectator ions. K⁺ and Cl⁻ appear unchanged on both sides — they take no part in the reaction.
3
Write what's left: H⁺ from the acid + OH⁻ from the alkali → H₂O.
✓
Mark-scheme answer: H⁺ + OH⁻ → H₂O. AQA mark scheme: "ignore state symbols" — accepted but not required.
Mark Economy
Don't write the full molecular equation (KCl + H₂O) when the question asks for an "ionic equation" — that's a different answer. The same H⁺ + OH⁻ → H₂O works for any strong acid + strong alkali pairing.
What Happens When Acid Is Diluted
When water is added to an acidic solution, the pH moves towards 7. But "moves towards 7" alone is not enough in a mark-scheme answer.
Two Marking Points
The direction: the pH increases (for an acid being diluted).
The reason: the concentration of hydrogen ions decreases (per unit volume) — the same number of H⁺ ions is now spread through more water, so [H⁺] falls.
Why pH Rises
pH measures the concentration of H⁺ ions. Adding water doesn't remove H⁺ — it just dilutes them. The total amount of H⁺ stays roughly the same; the concentration drops because the volume rises. Lower [H⁺] = higher pH.
Examiner Alert — Direction Matters
"Two thirds of students realised that the pH would increase. Some did not give the direction of the change, saying the pH would become 'closer to 7' but not saying from which direction. Far fewer were able to articulate the reason in terms of the decrease in concentration of hydrogen ions." — AQA Examiner Report, P1 Jun 2024 Q09.2
Student wroteWhy it failed
"pH becomes closer to 7"No direction stated. Could be acid (going up) or alkali (going down). Mark not awarded.
"the acid is diluted"Restates the question. Doesn't explain the pH change in terms of ions. Mark not awarded.
"there are fewer particles"Vague. Mark scheme requires reference specifically to hydrogen ions .
Mark-Scheme Phrases
MP1: "pH increases" | MP2: "concentration of hydrogen ions decreases" — use these exact phrases.
Worked Example — pH on Dilution
Q: Explain what happens to the pH of an acid as the acid is diluted with water.2 MARKS
Citation: P1 Jun 2024 Q09.2 — verbatim AQA stem.
MP1
The pH increases. Direction stated — acid being diluted means pH moves up towards 7.
MP2
Because the concentration of hydrogen ions (H⁺) decreases per unit volume. Reason given in terms of the marked ion.
★
Full-mark answer:"The pH increases because the concentration of hydrogen ions decreases." Two clauses joined by "because" = both marks.
The pH Scale Is Logarithmic HT Only
The pH scale is not linear. Each one-unit change in pH corresponds to a factor-of-ten change in the concentration of H⁺ ions.
pH change of n → [H⁺] changes by 10ⁿ
pH change[H⁺] changeExample
pH falls by 1[H⁺] × 10pH 4 → pH 3 means 10× more H⁺
pH falls by 2[H⁺] × 100 (10²)pH 5 → pH 3 means 100× more H⁺
pH falls by 3[H⁺] × 1000 (10³)pH 6 → pH 3 means 1000× more H⁺
pH rises by 1[H⁺] ÷ 10pH 3 → pH 4 means 10× less H⁺
Remember
If pH decreases, the solution is more acidic, so [H⁺] multiplies. If pH increases, the solution is less acidic, so [H⁺] divides. Always count the unit difference, then raise 10 to that power.
Examiner Alert — Linear ≠ Logarithmic HT Only
"Over a third of students scored the mark in this high demand question. As the pH has decreased by a factor of 2, the concentration of hydrogen ions increases by a factor of 10². However, many students did not realise this and attempted to calculate a concentration by applying a linear, instead of logarithmic, scale factor. Some students divided by 100 instead of multiplying." — AQA Examiner Report, P1 Jun 2023 Q05.6
✗ Wrong
"pH drops by 2 → [H⁺] × 2"Treated pH as linear. The real factor is 10² = 100. Cited in Jun23 ER.
✓ Right
"pH drops by 2 → [H⁺] × 100"Logarithmic factor 10ⁿ. Direction up (pH falling = more H⁺).
✗ Wrong
"pH drops by 2 → [H⁺] ÷ 100"Right factor (100), wrong direction. pH falling means [H⁺] going UP. Cited in Jun23 ER.
✓ Right
Multiply, then check directionMore acidic = more H⁺. Sanity-check the direction every time.
Worked Example — Logarithmic Calculation HT Only
Q: A solution of HCl with pH 4.5 has [H⁺] = 3.16 × 10⁻⁵ mol/dm³. What is [H⁺] in a solution of HCl with pH 2.5?1 MARK
Citation: P1 Jun 2023 Q05.6 — verbatim AQA stem.
1
pH difference: 4.5 − 2.5 = 2. The pH has decreased by 2.
2
Direction: pH decreasing means more acidic, so [H⁺] is increasing. We multiply.
3
Factor: 10ⁿ where n = 2, so factor = 10² = 100.
4
Calculate: 3.16 × 10⁻⁵ × 100 = 3.16 × 10⁻³ mol/dm³.
✓
Sanity check: pH 2.5 is more acidic than pH 4.5. The new [H⁺] should be larger. 10⁻³ > 10⁻⁵ ✓
Common Errors → Mark-Scheme Phrases
Common errorCorrect mark-scheme phrase
"pH changes" (no direction)"pH increases " / "decreases" — state direction
"pH becomes closer to 7"State direction explicitly + concentration change
"the acid is diluted""the concentration of hydrogen ions decreases"
"pH drops by 2 → [H⁺] × 2" (HT)"pH drops by 2 → [H⁺] × 100 (10²)"
"H⁺ + OH⁻ → water" (no formula)"H⁺ + OH⁻ → H₂O " (chemical formula)
"pH measures acidity" (vague)"pH measures the concentration of H⁺ ions "
Pattern
Vague language loses marks. Replace "changes" with "increases / decreases", replace "diluted" with "[H⁺] decreases", and always name the specific ion. The mark scheme rewards chemical precision.
Practice 1 — Foundation
Write an ionic equation for the neutralisation of nitric acid (HNO₃) with sodium hydroxide (NaOH).1 MARK
★
Answer: H⁺ + OH⁻ → H₂O
MP
Marking point: correct ionic equation showing only H⁺, OH⁻ and H₂O — 1 mark. State symbols (aq, l) not required.
!
Why it works: Na⁺ and NO₃⁻ are spectator ions — unchanged on both sides of the full equation, so they cancel. Only H⁺ and OH⁻ react.
Practice 2 — Application
A potassium hydroxide solution has a pH of 13. Water is added to the solution. Explain what happens to the pH of the solution.2 MARKS
MP1
The pH decreases (towards 7) — direction stated.
MP2
The concentration of hydroxide ions (OH⁻) decreases — correct ion named for an alkali.
★
Full-mark answer:"The pH decreases because the concentration of hydroxide ions decreases (per unit volume)."
!
Note: mirrors the AQA-marked acid-dilution pattern (P1 Jun 2024 Q09.2) — same logic, OH⁻ instead of H⁺.
Practice 3 — HT Analysis HT Only
Solution A has pH 6. Solution B has pH 3. By what factor is the concentration of H⁺ ions in Solution B greater than in Solution A?1 MARK
1
pH difference: 6 − 3 = 3.
2
Direction: Solution B has lower pH → more acidic → [H⁺] is larger.
3
Factor: 10³ = 1000.
★
Answer: [H⁺] in B is 1000× greater than in A.
⚠️
Common wrong answers: "3×" (treating pH as linear); "÷ 1000" (right factor, wrong direction).
Lesson Checkpoint
You should now be able to:
Foundation Outcomes
State the pH scale: 0–14, with 7 as neutral, less than 7 acidic, greater than 7 alkaline
Identify what acids and alkalis release: H⁺(aq) and OH⁻(aq)
Write the universal neutralisation equation: H⁺ + OH⁻ → H₂O
Choose between universal indicator (approximate) and pH probe (precise)
Explain pH change on dilution: state direction + change in [H⁺] or [OH⁻]
HT Outcomes
Apply the logarithmic scale: pH change of n → [H⁺] changes by factor of 10ⁿ
Calculate new [H⁺] from a pH change, multiplying or dividing as appropriate
Next Lesson
4.06: Titrations and Strong vs Weak Acids (RP2). Now we use this neutralisation principle to determine concentrations precisely — and meet the strong/weak acid distinction that depends directly on the H⁺ ion ideas from this lesson.
Exam-Ready Summary
The Four Essentials
pH 0–14, 7 neutral. Acids release H⁺; alkalis release OH⁻.
Universal ionic equation: H⁺ + OH⁻ → H₂O.
Dilution of an acid: pH increases (state direction); [H⁺] decreases (name the ion).
HT logarithmic: pH change of n → [H⁺] changes by 10ⁿ. Always count units, never multiply linearly.
Top 3 Lost Marks
"pH becomes closer to 7" without stating direction (Jun24 Q09.2).
"the acid is diluted" without naming H⁺ (Jun24 Q09.2).
Treating pH as linear instead of logarithmic — "× 2" instead of "× 100" (Jun23 Q05.6).
Exam Technique
For any 2-mark "explain pH change" question: write two clauses joined by "because". First clause = direction. Second clause = ion concentration. That's the full mark scheme in one sentence.
Next: The pH Scale: 0 to 14