1.4Chemical Changes and Structure
Acids and Bases — SQA National 5 Chemistry — SQA National 5 Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 4 of 4
Section 1 of 4The pH scale and ion concentrations
1Dissociation of water2 points
- Pure water slightly dissociates into hydrogen and hydroxide ions: H2O(l) ⇌ H+(aq) + OH-(aq).
- In neutral water or neutral solutions, [H+] = [OH-], pH = 7.
2Acidic vs Alkaline solutions2 points
- Acidic solutionshave higher concentration of H+ ions than OH- ions; pH < 7.
- Alkaline solutionshave higher concentration of OH- ions than H+ ions; pH > 7.
3Oxides and pH3 points
- Soluble non-metal oxides dissolve in water to produce acidic solutions (e.g. CO2, SO2, NO2).
- Soluble metal oxides dissolve in water to produce alkaline solutions (metal hydroxides, e.g. Na2O, CaO).
- Insoluble oxides do not alter the pH of water.
4Diluting solutions2 points
- Diluting an acid with water decreases [H+], causing pH to increase towards 7.
- Diluting an alkali with water decreases [OH-], causing pH to decrease towards 7.
Section 2 of 4Neutralisation reactions and spectator ions
1Neutralisation3 points
- Acid + Metal Oxide → Salt + Water
- Acid + Metal Hydroxide → Salt + Water
- Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
2Spectator ions4 points
- Ions that remain unchanged on both reactant and product sides of the chemical equation.
- ExampleReaction of hydrochloric acid with sodium hydroxide: H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l).
- Spectator ions are Na+(aq) and Cl-(aq).
- Omitting spectator ions gives the fundamental ionic neutralisation equation:
H+(aq) + OH-(aq) → H2O(l)
3Titration1 point
- An accurate quantitative technique used to find the unknown concentration of an acid or alkali using concordant titres (results within 0.2 cm3).
Section 3 of 4Key terms
| Term | Meaning |
|---|---|
| Neutralisation | The reaction between H+ ions and OH- ions producing water and a salt. |
| Spectator ion | An ion that is present in the reaction mixture but does not undergo chemical change. |
| Base | A substance that neutralises an acid (metal oxides, hydroxides, and carbonates). |
| Alkali | A base that dissolves in water to release hydroxide (OH-) ions. |
Section 4 of 4Exam technique
- Writing state symbols incorrectly for spectator ions — spectator ions are always aqueous (aq) on both sides of the equation.
- Assuming insoluble metal oxides change pH — insoluble oxides cannot release hydroxide ions into solution and therefore leave pH at 7.
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🌳 Student Search Tree & Frequently Asked Exam Questions: acids and bases
Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).
1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.
Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.
Where can I find acids and bases past paper questions by topic with the mark scheme?
On DABOURHUB every acids and bases question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.
What is the most common acids and bases exam question and why do students lose marks on it?
The repeated pattern on IGCSE papers is a short stem that asks you to apply acids and bases and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each acids and bases item here is written as a full-mark answer.
Is there an alternative to Save My Exams or Physics and Maths Tutor for acids and bases?
Yes: the acids and bases notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.
How do I revise acids and bases in two weeks before the exam?
Three passes: read the acids and bases notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.