Acid: A substance that releases hydrogen ions (H⁺) when dissolved in water. Acids have a pH below 7.
Alkali: A base that dissolves in water, releasing hydroxide ions (OH⁻). Alkalis have a pH above 7.
Base: A substance that reacts with an acid to neutralise it. All alkalis are bases, but not all bases are alkalis (some bases are insoluble).
pH: A scale from 0 to 14 that measures how acidic or alkaline a solution is. pH 7 = neutral; below 7 = acidic; above 7 = alkaline.
Indicator: A substance that changes colour depending on whether a solution is acidic or alkaline. Universal indicator shows a range of colours across the pH scale.
Neutral: A solution with pH 7, in which the concentration of H⁺ ions equals the concentration of OH⁻ ions. Pure water is neutral.
This definitions covers Key Definitions within Acids and Alkalis for GCSE Chemistry. Revise Acids and Alkalis in Chemical Changes for GCSE Chemistry with 25 exam-style questions and 21 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 6 of 12 in this topic. Make sure you can use the exact wording confidently, because definition marks are often lost through vague language.
Practice questions for Acids and Alkalis
Which ion do acids produce when dissolved in water?
Explain the difference between a strong acid and a concentrated acid.