Acids, Bases, Alkalis and pH
Acids release H⁺ ions; alkalis release OH⁻. The two definitions and the indicator colours are pure recall — and both were recorded going wrong repeatedly.
1. The definitions
The simple (Core) definitions:
An ACID produces H⁺ ions in aqueous solution. A BASE neutralises an acid. An ALKALI is a soluble base, producing OH⁻ ions in solution.
The Brønsted–Lowry (Extended) definitions:
An ACID is a PROTON (H⁺) DONOR. A BASE is a PROTON (H⁺) ACCEPTOR.
Use the proton donor/acceptor definitions when the question involves a reaction; use the H⁺/OH⁻ definitions for aqueous solutions. Confusion about which to use was recorded — if the question mentions protons or a Brønsted–Lowry context, use the donor/acceptor pair.
All alkalis are bases, but not all bases are alkalis — only the soluble ones.
2. The pH scale
The pH scale runs from 0 to 14.
| pH | Meaning |
|---|---|
| 0–6 | acidic (the lower the number, the stronger) |
| 7 | neutral |
| 8–14 | alkaline (the higher the number, the stronger) |
Be precise with pH ranges. Tutors flagged this — confusing the ranges for acids and alkalis was recorded, and so was uncertainty about the range for a strong acid (pH 0–2).
A lower pH means a MORE acidic solution. pH 1 is far more acidic than pH 6.
3. Indicators
| Indicator | In acid | In neutral | In alkali |
|---|---|---|---|
| Litmus | red | purple | blue |
| Methyl orange | RED | orange | YELLOW |
| Phenolphthalein | colourless | colourless | PINK |
| Universal indicator | red/orange | green | blue/purple |
Methyl orange is RED in acid and YELLOW in alkali. Its colour change was recorded wrong twice — and note it is the opposite way round from litmus in terms of which end is red.
Red means ACID, not alkali. A student asked “red is a sign for alkali, right?” — it is not. Red = acid for both litmus and methyl orange.
Universal indicator is GREEN in neutral solution, going red at low pH and purple at high pH. Not knowing its neutral colour, and its colour in strong acid, were both recorded.
Universal indicator gives a pH RANGE, not a sharp endpoint — which is why it is not used in titrations.
For a two-mark question: state the indicator used and the expected colour change — tutors flagged both halves are needed.
4. Strong and weak acids
A STRONG acid is COMPLETELY ionised (dissociated) in water — all its molecules release H⁺. A WEAK acid is only PARTIALLY ionised — only some molecules release H⁺, and the process is reversible.
| Strong | Weak | |
|---|---|---|
| Ionisation | complete | partial |
| Arrow | → | ⇌ |
| pH (same concentration) | lower | higher |
| Examples | HCl, HNO₃, H₂SO₄ | ethanoic acid, citric acid, carbonic acid |
Use a REVERSIBLE arrow (⇌) for weak acids. Tutors flagged this directly, and confusion about the arrows was recorded.
Complete vs partial ionisation is the distinction — recorded as a confusion twice. “Ionisation” and “dissociation” mean essentially the same thing here: splitting into ions in water.
STRONG is not the same as CONCENTRATED. Strong describes how fully it ionises; concentrated describes how much is dissolved. You can have a dilute strong acid or a concentrated weak one. Confusing dissociation with concentration was recorded.
Writing dissociation equations:
- HCl → H⁺ + Cl⁻
- CH₃COOH ⇌ CH₃COO⁻ + H⁺
- Ca(OH)₂ → Ca²⁺ + 2OH⁻
Balance the ions. A recorded error wrote calcium hydroxide dissociating as “Ca⁺ + OH⁻” — it must be Ca²⁺ + 2OH⁻, because the formula contains two hydroxide groups.
Carboxylic acids are ACIDS, not neutral. A recorded error called them neutral — they are weak acids (ethanoic acid is the vinegar in your kitchen).
5. Common acids and alkalis
Acids: hydrochloric (HCl), sulfuric (H₂SO₄), nitric (HNO₃), ethanoic (CH₃COOH), citric, lactic (in sour milk), carbonic.
Alkalis: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂), aqueous ammonia (NH₃).
Lactic acid and hydrochloric acid are different substances — confusing them was recorded. Lactic acid is the weak acid in milk and muscles; hydrochloric is the strong acid in the stomach.
6. Practical points
A colour change is hard to see in an already-coloured solution. If the solution is green or blue (e.g. a copper salt), the indicator’s change is masked — a recorded question that students found difficult. Say this explicitly if asked why a titration would be unreliable.
Use a white tile or paper behind the flask to see the colour change clearly.
7. Mistakes that cost marks
Getting methyl orange’s colours backwards.
Saying red indicates alkali.
Not knowing universal indicator is green when neutral.
Using universal indicator for a titration.
Confusing strong with concentrated.
Using a single arrow for a weak acid.
Unbalanced dissociation equations.
Calling carboxylic acids neutral.
Vague pH ranges.
Giving an indicator without its colour change.
Frequently asked questions
What is an acid? A substance producing H⁺ ions in solution — or a proton donor.
What is the difference between a base and an alkali? An alkali is a SOLUBLE base.
What is the pH of a neutral solution? 7.
What colour is methyl orange in acid? Red — and yellow in alkali.
What colour is universal indicator in a neutral solution? Green.
What is a strong acid? One that is completely ionised in water.
What is a weak acid? One that is only partially ionised, shown with a reversible arrow.
Is strong the same as concentrated? No — strong is about ionisation, concentrated about amount dissolved.
Why isn’t universal indicator used in titrations? It gives a gradual range of colours rather than a sharp endpoint.
Are carboxylic acids acidic? Yes — they are weak acids.
Quick revision checklist
- I know both sets of acid/base definitions and when to use each
- I know an alkali is a soluble base
- I know the pH ranges precisely
- I know the colours of litmus, methyl orange and phenolphthalein
- I know universal indicator is green when neutral
- I know strong = completely ionised, weak = partially
- I use ⇌ for weak acids
- I know strong ≠ concentrated
- I can write balanced dissociation equations
- I know common strong and weak acids
- I know why coloured solutions make endpoints hard to see
- I state both the indicator and its colour change
These notes cover acids, bases, alkalis and pH in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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