2.7.4Inorganic Chemistry
Bases — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 41 of 52
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 4Bases and Alkalis
A base neutralises an acid, while an alkali is a base that dissolves in water.
Base
- A base is any substance that neutralises an acid to form a salt and water, and it is usually a metal oxide, hydroxide or carbonate.
Alkali
- An alkali is a base that is soluble in water, and its solution has a pH above 7.
Relationship
- All alkalis are bases, but not all bases are alkalis, because insoluble bases do not form alkaline solutions.
Indicator test
- Red litmus paper turns blue in an alkali, which is a simple test for alkaline conditions.
Part 2 of 4Common Alkalis
Each alkali releases hydroxide ions into solution, which is the source of its alkaline properties.
Common Alkalis
Sodium hydroxide
- NaOH(s) → Na⁺(aq) + OH⁻(aq), a strong alkali that is used widely in industry.
Potassium hydroxide
- KOH(s) → K⁺(aq) + OH⁻(aq), which behaves like sodium hydroxide in neutralisation reactions.
Aqueous ammonia
- NH₃(g) dissolves in water to form ammonia solution, where NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Limewater
- Calcium hydroxide solution is slightly soluble, and it is used to test for carbon dioxide.
| Name | Formula | Ions formed in water |
|---|---|---|
| Sodium hydroxide | NaOH | Na⁺ and OH⁻ |
| Potassium hydroxide | KOH | K⁺ and OH⁻ |
| Aqueous ammonia | NH₃ (with H₂O) | NH₄⁺ and OH⁻ |
Part 3 of 4Ammonia Terminology
Careful terminology is needed because ammonia, ammonium and ammonia solution are different.
1
Ammonia is the gas, NH₃, which has a sharp choking smell and is a base.
2
Ammonia dissolves in water to form ammonia solution, which is alkaline because it produces hydroxide ions.
3
Ammonium is the positive ion, NH₄⁺, found in ammonium compounds such as ammonium chloride.
4
Ammonium hydroxide is another name for aqueous ammonia, so the two terms describe the same solution.
Part 4 of 4Exam Checklist & Tips
2.7.4 – Key Points for the Exam
- Base A substance that neutralises an acid, usually a metal oxide, hydroxide or carbonate.
- Alkali A base that dissolves in water to form a solution with a pH above 7.
- Hydroxide ion An OH⁻ ion, which makes a solution alkaline.
- Ammonium ion The NH₄⁺ ion present in ammonium compounds.
Common Mistakes
- Calling all bases alkalis; insoluble bases such as copper(II) oxide do not form alkaline solutions.
- Confusing ammonia, NH₃, with the ammonium ion, NH₄⁺.
- Stating that red litmus turns red in an alkali; it turns blue.
- Describing aqueous ammonia as an acid because it contains nitrogen and hydrogen.
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