This definitions covers Key Definitions within Acids and Alkalis for GCSE Chemistry. Revise Acids and Alkalis in Chemical Changes for GCSE Chemistry with 20 exam-style questions and 20 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. 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.
Topic position
Section 6 of 12
Practice
20 questions
Recall
20 flashcards
📖 Key Definitions
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.