Acids give H⁺ ions; alkalis give OH⁻ ions
An acid in aqueous solution is a source of hydrogen ions, ; an alkali is a soluble base that is a source of hydroxide ions, . The words in aqueous solution matter: a covalent acid like hydrogen chloride only releases when dissolved in water — in an organic solvent it stays as covalent molecules and is not acidic.
The pH scale runs 0–14 in five bands
The pH scale runs 0 to 14: strongly acidic 0–3, weakly acidic 4–6, neutral 7, weakly alkaline 8–10, strongly alkaline 11–14. A lower pH means more ions; a higher pH means more ions. Universal indicator gives only an approximate pH — match its colour to a chart (red ≈ acidic, green ≈ neutral, purple ≈ alkaline); a pH meter is precise.
Neutralisation: H⁺ + OH⁻ → H₂O
Neutralisation is the reaction of an acid with a base or alkali to give a salt and water. Every acid–alkali neutralisation shares the ionic equation At the end point the moles of from the acid equal the moles of from the alkali, which is why the mole ratio must be used in every titration calculation.
Drawn from real examiner reports.
Neglecting the titration mole ratio
Sulfuric acid gives two moles of per mole, so it reacts with twice its own moles of a 1:1 alkali: . Candidates find the moles correctly but then assume 1:1, forgetting the ×2. Always read the mole ratio off the balanced equation before finding the unknown concentration.
June 2024 Paper 2CR examiner report (Introduction): "the stoichiometric ratio was neglected, and candidates were then unable to score full marks." The titration question was Q05.
Indicator colours the wrong way round
Litmus is red in acid, blue in alkali; phenolphthalein is colourless in acid, pink in alkali; methyl orange is red in acid, yellow in alkali (orange at neutral). Candidates reverse these — writing phenolphthalein "pink to colourless" the wrong way round, sometimes confusing an indicator with bromine water. Learn each as acid first, alkali second.
June 2024 Paper 2C examiner report Q05(b)(ii): indicator colours were "sometimes seen the wrong way round... perhaps confusing bromine water and indicators in titrations."
"Acidic" alone, not naming H⁺ ions
When explaining why a solution is acidic, writing only "it is acidic" earns no credit — the mark scheme wants the cause: the presence of (hydrogen) ions, or the acid named. Acids only release ions in aqueous solution; a covalent acid in an organic solvent stays as molecules and is not acidic.
June 2024 Paper 2C examiner report Q07(b): for HCl in water, "just stating that it is acidic was not enough... Either H+ ions or hydrochloric acid were needed for the mark."
Titration method slips
The most-missed mark is adding from the burette dropwise only near the end point, not from the start. Other slips: using a pipette for the variable volume instead of a burette; putting the wrong reagent in the burette; forgetting to swirl the flask; and not knowing concordant titres agree within before averaging.
June 2024 Paper 2CR examiner report Q05(a)(ii): few scored M4 (adding dropwise near the end point); common errors were using a pipette instead of a burette, adding dropwise from the start, forgetting to swirl, and not grasping concordant results.
Not converting cm³ to dm³
Volumes are given in , but needs in . Convert by dividing by 1000 (not 100): . Using the wrong factor scales every mole figure and the final concentration by a power of ten, so the accuracy marks are lost even when the method is right.
June 2023 Paper 2C examiner report Q4(f): a worked error was dividing $\text{cm}^3$ by 100 not 1000. Always convert $\text{cm}^3$ to $\text{dm}^3$ by dividing by 1000.
Strong ≠ concentrated acid
Strong describes ionisation: a strong acid is fully ionised in water. Concentrated describes amount: a large amount of solute per unit volume. They are independent — a dilute strong acid is still strong. A weak acid is only partially ionised, so it has fewer ions and a higher pH than a strong acid of equal concentration.
(common Edexcel chemistry error)
The four-step titration scaffold
A reliable scaffold: (1) balanced equation → mole ratio; (2) convert the known volume to , then ; (3) apply the ratio for the other solution's moles; (4) divide by its volume in for .
Show working in a "show that"
In a "show that" question the answer is already given, but you must still write every line of working or the method marks are lost. Showing each step also earns error-carried-forward marks: if you slip early, later steps done correctly on your own figure can still score.
Always state the unit (mol/dm³)
Every concentration answer needs its unit — (or for a mass concentration). A correct number with no unit is routinely marked wrong, so write the unit on the final line as you state the value.
| pH range | Classification |
|---|---|
| 0–3 | Strongly acidic |
| 4–6 | Weakly acidic |
| 7 | Neutral |
| 8–10 | Weakly alkaline |
| 11–14 | Strongly alkaline |
Key formula for titrations: where = amount (mol), = concentration () and = volume in . Remember . Mass concentration: (relative formula mass).
Define an acid in terms of ions.
Sodium hydroxide is the alkali used in many titrations.
Show that the relative formula mass () of sodium hydroxide, , is 40.
(: Na = 23, O = 16, H = 1.)
