What you'll learn
Reactions of acids are central to GCSE Chemistry and appear in many exam questions and practicals. In this guide you will learn what acids and alkalis are, the pH scale, how acids react with metals, bases (metal oxides and hydroxides) and carbonates, how to write balanced equations for these reactions, the difference between strong and weak acids, and how to make a soluble salt by neutralisation. These reactions also underpin the required practical on making salts.
Key terms and definitions
Acid — a substance that produces hydrogen ions (H⁺) in aqueous solution; has pH less than 7.
Alkali — a soluble base that produces hydroxide ions (OH⁻) in solution; has pH greater than 7.
Base — a substance that neutralises an acid (e.g. metal oxides, hydroxides, carbonates).
Neutralisation — the reaction between an acid and a base to form a salt and water.
pH scale — a scale from 0 to 14 measuring acidity or alkalinity.
Salt — the compound formed when the hydrogen of an acid is replaced by a metal (or ammonium) ion.
Core concepts
Acids, alkalis and the pH scale
Acids release hydrogen ions (H⁺) in water; alkalis release hydroxide ions (OH⁻). The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral, above 7 is alkaline. pH can be measured with universal indicator (a colour change) or, more accurately, a pH probe/meter. In neutralisation, H⁺ ions react with OH⁻ ions to form water: H⁺ + OH⁻ → H₂O.
Acid + metal
Acids react with reactive metals to form a salt and hydrogen:
acid + metal → salt + hydrogen
For example, hydrochloric acid + magnesium → magnesium chloride + hydrogen (Mg + 2HCl → MgCl₂ + H₂). The hydrogen gas can be tested with a lighted splint, which gives a squeaky pop.
Acid + base (metal oxide or hydroxide)
Acids react with bases such as metal oxides and metal hydroxides to form a salt and water:
acid + base → salt + water
For example, hydrochloric acid + copper oxide → copper chloride + water; sulfuric acid + sodium hydroxide → sodium sulfate + water.
Acid + carbonate
Acids react with carbonates to form a salt, water and carbon dioxide:
acid + carbonate → salt + water + carbon dioxide
For example, hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide. The carbon dioxide can be tested with limewater, which turns milky/cloudy.
Naming salts
The salt's name comes from the acid used: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates. The metal part comes from the metal, base or carbonate.
Strong and weak acids
A strong acid (hydrochloric, sulfuric, nitric) is completely ionised in water — all its molecules release H⁺ ions. A weak acid (ethanoic, citric, carbonic) is only partially ionised. For the same concentration, a strong acid has a lower pH. Note: concentration (amount of acid per volume) is different from strength (degree of ionisation). As pH decreases by 1 unit, the H⁺ concentration increases by a factor of 10.
Making a soluble salt (required practical)
To make a soluble salt, add an excess of an insoluble base (e.g. copper oxide) to warm acid until no more reacts, filter off the excess solid, then crystallise the salt solution by evaporating some water and leaving it to form crystals.
Worked examples
Example 1: Acid + metal
Write a word equation for sulfuric acid reacting with zinc.
Sulfuric acid + zinc → zinc sulfate + hydrogen. (Zn + H₂SO₄ → ZnSO₄ + H₂.)
Example 2: Acid + carbonate
What are the products of nitric acid reacting with calcium carbonate?
Calcium nitrate + water + carbon dioxide. The salt is a nitrate because nitric acid is used.
Example 3: Strong vs weak
Two acids both have concentration 0.1 mol/dm³, but acid A has pH 1 and acid B has pH 4. Which is the strong acid?
Acid A is the strong acid: a lower pH at the same concentration means it is fully ionised, releasing more H⁺ ions.
Common mistakes and how to avoid them
Forgetting carbon dioxide with carbonates. Acid + carbonate gives salt + water + CO₂, unlike acid + base.
Confusing strength and concentration. Strength = degree of ionisation; concentration = amount per volume. A dilute strong acid is still strong.
Naming the wrong salt. Match the acid to the salt: hydrochloric→chloride, sulfuric→sulfate, nitric→nitrate.
Mixing up the gas tests. Hydrogen → squeaky pop with a lighted splint; carbon dioxide → limewater turns milky.
Skipping "excess" and "filter" in the salt practical. Excess base ensures all acid reacts; filtering removes the leftover solid.
Exam technique for Reactions of Acids
Learn the three general equations (metal, base, carbonate) and their products.
Name salts correctly from the acid used.
Quote the gas tests precisely for hydrogen and carbon dioxide.
Distinguish strength from concentration, and recall the factor-of-10 pH relationship.
Describe the salt practical in order: react with excess base, filter, crystallise.
Quick revision summary
Acids release H⁺ ions (pH < 7); alkalis release OH⁻ ions (pH > 7); neutralisation is H⁺ + OH⁻ → H₂O. Three key reactions: acid + metal → salt + hydrogen (tested by a squeaky pop); acid + base → salt + water; and acid + carbonate → salt + water + carbon dioxide (tested by limewater turning milky). The salt is named from the acid — hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate. A strong acid is fully ionised (hydrochloric, sulfuric, nitric); a weak acid is only partially ionised (ethanoic, citric) — different from concentration, and a fall of one pH unit means ten times more H⁺. To make a soluble salt, react acid with excess insoluble base, filter off the excess, then crystallise. Know the general equations, name salts from the acid, quote the gas tests, separate strength from concentration, and recall the practical method.