Alkalis & BasesOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is a base?
A base is a substance that can neutralise an acid, forming a salt and water. Metal oxides and metal hydroxides are both classed as bases.
- Metal oxides: e.g. copper oxide, magnesium oxide
- Metal hydroxides: e.g. sodium hydroxide, calcium hydroxide
Bases are usually solids that do not dissolve in water (with some exceptions below).
Section 2
What is an alkali?
An alkali is simply a base that dissolves in water. All alkalis are bases, but not all bases are alkalis (most metal oxides, for instance, are insoluble).
- Soluble hydroxides such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) are alkalis
- Alkalis release hydroxide ions, OH⁻(aq), when dissolved in water
- Alkaline solutions have a pH greater than 7
Students often use 'base' and 'alkali' interchangeably. An alkali must be soluble — a base does not have to be.
Section 3
Ammonia as a base
Ammonia (NH₃) is a gas, but it dissolves readily in water to produce an alkaline solution. This aqueous ammonia solution is used industrially and in the laboratory to produce ammonium salts, which are important as fertilisers.
- Ammonia + water → ammonium hydroxide solution (alkaline)
- Ammonia reacting with an acid produces an ammonium salt, e.g. ammonia + hydrochloric acid → ammonium chloride
NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq) — ammonium nitrate, a common nitrogen fertiliser.
Section 4
Limewater and carbon dioxide
Limewater is a solution of calcium hydroxide in water — it is one of the few metal hydroxides that is (weakly) soluble, so it counts as an alkali.
When carbon dioxide gas is bubbled through limewater, it reacts to produce calcium carbonate, which is insoluble and turns the solution cloudy (a milky-white precipitate).
- Calcium hydroxide + carbon dioxide → calcium carbonate + water
- This reaction is the standard laboratory test for carbon dioxide gas
Examiners want the precise observation 'limewater turns cloudy/milky', not just 'it reacts'.
Must Know
- Metal oxides and metal hydroxides are bases
- Alkalis are bases that dissolve in water, releasing OH⁻(aq) ions
- Ammonia gas dissolves in water to form an alkaline solution, used to make ammonium salts (fertilisers)
- Limewater is a solution of calcium hydroxide
- Carbon dioxide turns limewater cloudy by forming insoluble calcium carbonate
- This cloudiness is the standard test for CO₂
That's the notes covered.
Carry on to the next subtopic.