GCSE Acids, Bases and Salts — Every Reaction Type With Examples

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Acids, bases and salts is one of the most content-rich topics in GCSE Chemistry. It covers the pH scale, different types of reactions producing salts, methods for preparing specific salts, and titration calculations. The key to this topic is knowing which method to use for which combination of acid and base — and that depends on whether the base is soluble or insoluble.

Acids and Bases — Definitions

At GCSE, an acid is defined as a substance that produces hydrogen ions (H⁺) when dissolved in water. A base is a substance that reacts with an acid to form a salt and water only. An alkali is a soluble base — it produces hydroxide ions (OH⁻) when dissolved in water.

The common acids you need to know: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃). The common alkalis: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂). Common insoluble bases: copper oxide (CuO), zinc oxide (ZnO), iron oxide (Fe₂O₃).

The pH Scale and Indicators

The pH scale runs from 0 to 14. pH 7 is neutral (pure water). pH below 7 is acidic — the lower the pH, the more acidic. pH above 7 is alkaline — the higher the pH, the more alkaline.

pH measures the concentration of H⁺ ions in solution. A pH of 1 has ten times the H⁺ concentration of a pH of 2. This is a logarithmic scale — each unit change represents a tenfold change in H⁺ concentration.

Strong vs Weak Acids (Higher Tier)

A strong acid completely ionises in water — every molecule releases its H⁺ ion. Hydrochloric, sulfuric and nitric acids are all strong acids. A weak acid only partially ionises — only a small fraction of molecules release H⁺ at any given time. Ethanoic acid (in vinegar) and citric acid are weak acids.

Two solutions of the same concentration but one strong acid and one weak acid will have very different pH values. The strong acid has a much lower pH because it produces far more H⁺ ions. This is the distinction between concentration (how much acid is dissolved) and strength (how completely it ionises) — they are not the same thing.

Neutralisation Reactions

When an acid reacts with a base, neutralisation occurs. The H⁺ ions from the acid react with OH⁻ ions from the base (or from water produced when an insoluble base reacts) to form water. A salt is also produced — the particular salt depends on which acid and which base are used.

Naming the salt: the first part of the salt name comes from the metal in the base. The second part comes from the acid: hydrochloric acid → chloride, sulfuric acid → sulfate, nitric acid → nitrate.

Salt Preparation Methods

The method used to prepare a salt depends on whether the base is soluble or insoluble.

Method 1: Insoluble Base + Acid (for insoluble metal oxides, hydroxides and carbonates)

1. Add excess insoluble base to the acid until no more dissolves (this ensures all acid is used up and the solution is neutral).
2. Filter off the excess insoluble base — the filtrate contains only the salt solution.
3. Evaporate the filtrate to crystallise the salt.

Method 2: Alkali + Acid (titration, for soluble bases)

1. Perform a titration to find the exact volumes of acid and alkali needed for complete neutralisation.
2. Repeat with no indicator (indicators contaminate the salt).
3. Evaporate the neutral solution to obtain the pure salt.

Method 3: Acid + Carbonate (produces CO₂ gas — the fizzing reaction)

Carbonates react with acids to produce a salt, water and carbon dioxide gas. The same excess solid method applies: add excess carbonate until fizzing stops, filter, evaporate.

Reactions of Metals with Acids

Metals above hydrogen in the reactivity series react with dilute acids to produce a salt and hydrogen gas. The hydrogen can be tested with a burning splint — it burns with a squeaky pop.

Metals below hydrogen in the reactivity series (copper, silver, gold) do not react with dilute acids.

The full acids and bases specification for AQA is at the AQA GCSE Chemistry specification page.