Acids, bases and the pH scale
Acids release hydrogen ions in water; bases accept them, and a soluble base is an alkali. The pH scale tells you how acidic or alkaline a solution is, from 0 to 14. At this level you reason about acids and bases qualitatively — you are not asked to calculate pH from concentration.
Key facts
| Topic | Acid-Base Chemistry |
| Acids | release H⁺ in water; pH below 7 |
| Alkalis | soluble bases; pH above 7 |
| Out of scope | calculating pH from [H⁺] |
Acids, bases and alkalis
An acid produces hydrogen ions (H⁺) in water and has a pH below 7. A base reacts with and neutralises an acid; a base that dissolves in water is an alkali, with a pH above 7. Pure water is neutral at pH 7.
Typical reactions
Acids follow reliable patterns: acid + metal → salt + hydrogen; acid + base (or metal oxide) → salt + water; acid + carbonate → salt + water + carbon dioxide. Neutralisation — acid plus alkali → salt plus water — is the reaction behind titrations and salt preparation.
The pH scale and indicators
The pH scale runs 0–14: below 7 acidic, 7 neutral, above 7 alkaline. Indicators show this — litmus (red in acid, blue in alkali), and universal indicator giving a colour across the range. Strong and weak acids differ in how fully they ionise, which you treat qualitatively at this level.
What you are — and aren't — asked to calculate
K324 keeps acids and bases largely qualitative. You should describe reactions, predict products, and interpret pH and indicator results. Calculating pH from hydrogen-ion concentration is not in the syllabus, so don't lose time on it — focus on reactions, neutralisation and observations.
Frequently asked questions
What is an acid, a base and an alkali?
An acid releases H⁺ ions in water (pH below 7); a base neutralises an acid; an alkali is a base that dissolves in water (pH above 7).
What does the pH scale show?
How acidic or alkaline a solution is, from 0 to 14 — below 7 acidic, 7 neutral, above 7 alkaline.
What are the main acid reactions to know?
Acid + metal → salt + hydrogen; acid + base/metal oxide → salt + water; acid + carbonate → salt + water + carbon dioxide.
Do I need to calculate pH from concentration?
No — calculating pH from hydrogen-ion concentration is not in the K324 syllabus. Acids and bases are treated qualitatively.
G3 Chemistry, made clear
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