Alkalis & Bases (Oxford AQA IGCSE Combined Science Double Award)

Alkalis & Bases

• On the pH scale, alkalis and bases are above pH 7

• An alkali is a soluble (can dissolve in water) base

• Many bases are insoluble in water but the ones that do dissolve in water are called alkalis

• They thus form an alkaline solution

Examples of Alkalis and Bases

Neutralisation Reactions

• Acids react with bases to form salts in a neutralisation reaction

• When an acid reacts with a base, a neutralisation reaction occurs

  • These reactions are exothermic

• Examples of alkalis are soluble metal hydroxides such as NaOH and Ca(OH)₂

• In all acid-base neutralisation reactions, salt and water are produced:

acid + base ⟶ salt + water

• Where the base can be a hydroxide or a metal oxide

acid + hydroxide ⟶ salt + water

acid + metal oxide ⟶ salt + water

• If the base is a metal carbonate, carbon dioxide is also produced:

acid + carbonate ⟶ salt + water + carbon dioxide

Limewater

• A solution of calcium hydroxide which is sparingly soluble in water known as limewater

Formation of limewater

• Carbon dioxide is bubbled through the solution of limewater to form calcium carbonate

• The solution turns milky or cloudy as a suspension (precipitate in solution) of calcium carbonate is formed

calcium hydroxide + carbon dioxide → calcium carbonate + water

Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)

Bubbling carbon dioxide through limewater

Ammonium Salts

• Ammonia gas, NH₃, is an alkali so is soluble in water

• It will react with acids to form an ammonium salt

  • For example, it will react with hydrogen chloride gas to form fine white solid particles of ammonium chloride

NH₃ (g) + HCl (g) ⟶ NH₄Cl (s)

• The ammonium ion has the formula NH₄⁺ and will form salts with negatively charged ions (anions)

• All ammonium salts are soluble in water

• Ammonium salts and nitrates are commonly used as fertilisers

• Common fertiliser compounds include:

  • Ammonium nitrate, NH₄NO₃

  • Ammonium phosphate, (NH₄)₃PO₄