Acids & Bases (Cambridge (CIE) AS Chemistry): Revision Note

Common Acids

• An acid is a substance that neutralises a base forming a salt and water:

acid + base salt + water

2HCl (aq) + CaO (s) CaCl₂ (aq) + H₂O (l)

• Acids are also substances that release hydrogen ions when they dissolve in water:

HCl(g) → H⁺(aq) + Cl^- (aq)

Acid dissociation

Common acids

• Hydrochloric acid

  • HCl forms H⁺ + Cl^– in water

• Nitric acid

  • HNO₃ forms H⁺ + NO₃^– in water

• Sulfuric acid

  • H₂SO₄ forms H⁺ + SO₄^2– in water

• Ethanoic acid

  • CH₃COOH forms H⁺ + CH₃COO^– in water

• Monoprotic inorganic acids, such as hydrochloric acid, fully dissociate into their ions

• Organic acids, such as carboxylic acids, do not fully dissociate into their ions

  • Only some of the hydrogen atoms can form ions 

Common Alkalis

• A base is a compound that neutralises an acid forming a salt and water

acid + base salt + water

2HCl (aq) + CaO (s) CaCl₂ (aq) + H₂O (l)

• A base is a substance that accepts hydrogen ions or a compound that contains oxide or hydroxide ions

  • For example, when the base ammonia is added to water, the ammonium ion and hydroxide ions are formed:

NH₃ (g) + H₂O (l) → NH₄⁺ (aq) + OH^– (aq)

• For example, when sodium hydroxide is dissolved in solution, sodium ions and hydroxide ions are formed:

NaOH (s) + aq → Na⁺ (aq) + OH^– (aq)

• A base that is soluble in water is called an alkali

Common alkalis table

• Sodium hydroxide

  • NaOH forms Na⁺ + OH^– in water

• Potassium hydroxide

  • KOH forms K⁺ + OH^–  in water

• Aqueous ammonia

  • NH₃ forms NH₄⁺ + OH^– in water