O-Level and SEC G3 Chemistry K324
C5: Acid-Base Chemistry
Distinguish acids, bases, alkalis, pH, neutralisation, and salt preparation routes from the substances given.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Acid-base Chemistry links ions, pH, reactions, salt preparation, solubility, and the industrial manufacture of ammonia. The method depends on the reactants and desired salt, while strong versus weak describes extent of ionisation rather than concentration.
Acids, alkalis, pH and oxides
An acid produces hydrogen ions in aqueous solution, while an alkali produces hydroxide ions in aqueous solution. Universal Indicator and the pH scale compare relative acidity and alkalinity. A neutral solution has equal hydrogen and hydroxide ion concentrations. K324 does not require calculating pH from hydrogen ion concentration.
A strong acid ionises to a greater extent than a weak acid under comparable conditions; this is not the same as concentrated versus dilute. Acidic oxides are usually non-metal oxides, basic oxides are usually metal oxides, amphoteric oxides react with both acids and bases, and neutral oxides show neither behaviour. Use reaction evidence when it is supplied.
Reactions, salts and preparation choices
Acids react with suitable metals to form salt and hydrogen, with bases to form salt and water, and with carbonates to form salt, water, and carbon dioxide. Bases neutralise acids; alkalis also react with ammonium salts on warming to release ammonia. Soil acidity can be reduced using calcium hydroxide, but excess treatment can make soil too alkaline.
Choose salt preparation from solubility. Use excess insoluble metal, base, or carbonate with acid, then filter and crystallise. Use titration when both reactants are soluble and no excess can be removed by filtration. Use precipitation to make an insoluble salt from two soluble solutions, then filter, wash, and dry the solid. Apply the stated solubility rules before choosing.
Ammonia and reversible manufacture
The Haber process uses nitrogen from air and hydrogen obtained from hydrocarbons to manufacture ammonia. The reaction is reversible. Industrial conditions are a compromise among reaction rate, equilibrium yield, energy demand, equipment cost, and safety.
Interpret the supplied data rather than invoking Le Chatelier's Principle, which K324 does not require by name. Higher pressure can increase ammonia yield but raises compression and plant costs. A suitable temperature and catalyst support an economically useful production rate without claiming that a catalyst changes the equilibrium yield.
Formulae and relationships
| Relationship | Use |
|---|---|
| Represent neutralisation as the net ionic change. | |
| Represent the reversible manufacture of ammonia. |
Worked examples
Example 1: Choose a method to prepare pure dry copper(II) sulfate crystals from dilute sulfuric acid and copper(II) oxide.
- Warm the acid gently and add copper(II) oxide until some remains, showing the acid has reacted fully.
- Filter to remove excess insoluble copper(II) oxide.
- Concentrate the filtrate, cool to crystallise, then filter and dry the crystals.
Answer: Use the excess-insoluble-base method followed by filtration and crystallisation.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Distinguish acids, bases, and alkalis through particles, reactions, and pH evidence.
- Write ionic and full equations for neutralisation without cancelling species that actually react.
- Choose titration, excess-solid, precipitation, or another preparation route from the salt and reactant solubilities.
Official outcome coverage
K324 C5: 8 mapped outcomes, references C5(a), C5(b), C5(c), C5(d), C5(e), C5(f), C5(g), C5(h). Check the official K324 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
For titration or salt preparation, justify indicator choice, endpoint technique, heating, crystallisation, washing, and drying steps.
Exam traps and retrieval check
Avoid these traps
- Using strong and concentrated as if they mean the same thing.
- Choosing titration when an excess insoluble reactant can be filtered off more simply.
- Saying a catalyst increases the equilibrium amount of ammonia.
Check from memory
What ions define neutralisation?
Hydrogen ions react with hydroxide ions to form water.
How is an insoluble salt prepared?
Mix suitable soluble salts to precipitate it, then filter, wash, and dry.
Why are Haber conditions a compromise?
Yield, rate, energy, pressure cost, equipment, and safety must all be balanced.
Pure versus Combined scope
Combined Chemistry uses the same top-level topic label but a narrower outcome set. Use the Combined component checklist before removing or adding detail.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K324. Open the related IP explanation.