This topic summary covers Knowledge Organiser: Acids and Alkalis 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 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Acids and Alkalis
Key Terms
- Acid: releases H⁺, pH < 7
- Alkali: releases OH⁻, pH > 7
- Base: reacts with acid; alkalis are soluble bases
- pH 7: neutral (equal H⁺ and OH⁻)
- Indicator: shows pH by colour change
- Concentration: amount dissolved per volume
Must-Know Facts
- Three lab acids: HCl, H₂SO₄, HNO₃
- Two lab alkalis: NaOH, KOH
- Litmus: red in acid, blue in alkali
- Phenolphthalein: colourless → pink (acid → alkali)
- Methyl orange: red → yellow (acid → alkali)
- HT: strong acids fully ionise; weak acids partially ionise
Key Equations
- HCl → H⁺ + Cl⁻ (strong acid — fully ionises)
- NaOH → Na⁺ + OH⁻ (strong alkali — fully dissociates)
- H⁺ + OH⁻ → H₂O (neutralisation ionic equation)
Common Mistakes
- Confusing concentration and strength: A dilute acid can be strong (fully ionised); a concentrated acid can be weak (partially ionised) — strength and concentration are different
- Saying all bases are alkalis: Alkalis are soluble bases — copper oxide is a base but NOT an alkali because it doesn't dissolve in water
- Getting indicator colours wrong: Litmus is red in ACID and blue in ALKALI — learn both colours, not just one
- Forgetting pH scale goes 0–14: pH 0 = very strong acid; pH 7 = neutral; pH 14 = very strong alkali
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.
Quick recall flashcards
What is universal indicator?
An indicator that shows a range of colours across the pH scale (rainbow of colours from red to purple)
Name two common alkalis
Sodium hydroxide (NaOH), Potassium hydroxide (KOH)