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
Memory Aid
- Litmus: Red in acid, Blue in alkali — remember "Red = Acidic, Blue = Alkaline"
- Phenolphthalein: Colourless → Pink in alkali — "Phenol turns Pink in base"
- Methyl orange: Red in acid → Yellow in alkali — "Methyl goes RED when angry (acidic)"