Acids & Alkalis — GCSE Chemistry Revision
Revise Acids & Alkalis for GCSE Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Making SaltsWhat is Acids & Alkalis?
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. Alkalis are soluble bases that produce hydroxide ions (OH⁻) in water. The pH scale, from 0 to 14, is used to measure the acidity or alkalinity of a solution.
Board notes: Acids and alkalis are a core topic for all exam boards. You need to know the definitions, the pH scale, the use of indicators, and the general equations for acid reactions. The difference between strong and weak acids is a higher-tier concept.
Step-by-step explanationWorked example
Hydrochloric acid (HCl) is a strong acid because it fully ionises in water to produce H⁺ and Cl⁻ ions. Sodium hydroxide (NaOH) is a strong alkali because it fully dissociates in water to produce Na⁺ and OH⁻ ions.
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Common mistakes
- 1Confusing the terms alkali and base. All alkalis are bases, but not all bases are alkalis. A base is any substance that neutralises an acid, while an alkali is a soluble base.
- 2Thinking that a strong acid is the same as a concentrated acid. Strength refers to how much the acid ionises in water, while concentration is about the amount of acid dissolved in a certain volume.
- 3Forgetting the products of common acid reactions. Acid + Metal → Salt + Hydrogen; Acid + Base → Salt + Water; Acid + Carbonate → Salt + Water + Carbon Dioxide.
Acids & Alkalis exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Acids & Alkalis
Core concept
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. Alkalis are soluble bases that produce hydroxide ions (OH⁻) in water. The pH scale, from 0 to 14, is used to measure the a…
Frequently asked questions
What is a neutral solution?
A neutral solution is one that is neither acidic nor alkaline, with a pH of 7. Pure water is the most common example.
What is an indicator?
An indicator is a substance that changes colour depending on the pH of the solution it is in. Litmus and universal indicator are common examples used in school labs.