SEC G3 Combined Science Chemistry component K326/K328
C5: Acid-Base Chemistry
Use ions, pH, indicators, neutralisation, oxide classification, soil treatment, and ammonium-salt reactions.
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 in water to indicator evidence, neutralisation, characteristic reactions, and decisions about environmental pH.
Acidity, alkalinity, and pH
Acids produce ions in aqueous solution and alkalis produce ions. Universal Indicator is typically red to orange or yellow in acidic solutions, green near neutral, and blue to purple in alkaline solutions. Exact shades depend on indicator formulation and lighting, so interpret the supplied chart when one is given.
An acidic solution has a greater concentration than concentration and a pH below 7. An alkaline solution has a greater concentration than concentration and a pH above 7. A neutral solution has equal and concentrations and pH 7. Calculating pH from concentration is not required.
Characteristic reactions and neutralisation
Acids react with suitable metals to release hydrogen, with bases to form salt and water, and with carbonates to form salt, water, and carbon dioxide. Preparing pure salts is outside this requirement.
Neutralisation is . Bases react with acids, and an alkali warmed with an ammonium salt releases ammonia.
Soils and oxide classification
Soil pH affects nutrient availability and crop growth. Calcium hydroxide can neutralise excessive soil acidity, but adding too much can make conditions too alkaline.
Use metallic or non-metallic character to classify oxides: metallic oxides are generally basic, while non-metallic oxides are generally acidic. Amphoteric oxides show both acidic and basic behaviour, while neutral oxides show neither.
Formulae and relationships
| Relationship | Use |
|---|---|
| Represent neutralisation. |
Worked examples
Example 1: A farmer finds soil pH is too low. Name a treatment and explain it.
- Low pH means excessive acidity.
- Calcium hydroxide is a base.
- It neutralises some acid and raises the soil pH.
Answer: Apply a controlled amount of calcium hydroxide to reduce acidity.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Acidity, alkalinity, and pH with the named terms, evidence, and causal links kept distinct.
- Use Characteristic reactions and neutralisation to interpret the evidence given and justify each conclusion.
- Apply Soils and oxide classification to a new example, then check the conclusion against the information given.
Official outcome coverage
K326 C5: 7 mapped outcomes, references C5(a), C5(b), C5(c), C5(d), C5(e), C5(f), C5(g). Check the official K326 syllabus.
K328 C5: 7 mapped outcomes, references C5(a), C5(b), C5(c), C5(d), C5(e), C5(f), C5(g). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use Universal Indicator observations to compare acidity, neutrality and alkalinity, or interpret neutralisation data. Keep solution volumes and indicator amount consistent, record colour before inference, and do not calculate pH from hydrogen-ion concentration because that is outside scope.
Exam traps and retrieval check
Avoid these traps
- Calling every base an alkali.
- Saying neutralisation always gives pH 7 without considering quantities.
- Assuming every metallic oxide is only basic and overlooking an amphoteric case identified in the question.
Check from memory
Which ion characterises an aqueous acid?
.
What gas forms when acid reacts with carbonate?
Carbon dioxide.
What does an amphoteric oxide react with?
Both acids and bases.
Official Combined Science scope
This shared Combined Chemistry owner serves both K326 and K328. Acids, alkalis, pH evidence, neutralisation, characteristic reactions, soil treatment, ammonium salts, and oxide classification.