Introduction to Bases
Bases and alkalis: definitions, aqueous hydroxide ions, neutralisation and the ammonium-ion reaction.
On this page
“Base” and “alkali” are related but not identical. Keeping their definitions separate makes later reaction and solubility questions much clearer.
1. Definition
A. Base
A base accepts hydrogen ions from an acid. Common examples react with acids to form salts. A base does not have to dissolve in water: an insoluble solid can react with hydrogen ions at its surface.
B. Alkali
An alkali is a base that is soluble in water and produces hydroxide ions, OH⁻(aq), in solution.
2. Key Ideas
- Bases neutralise acids to form salts. Many common bases at O-Level also form water (metal oxides/hydroxides), but ammonia is an exception.
- Alkalis are bases, but not all bases are alkalis (many bases are insoluble).
- The particles that make solutions alkaline are hydroxide ions, OH⁻(aq).
3. Detailed Explanations
- Base: neutralises an acid by accepting H + (aq).
- Alkali: soluble base that produces OH⁻(aq) in water.
- Neutralisation ionic equation: H + (aq) + OH⁻(aq) → H₂O(l).
- Ammonia is a base but can form ammonium salts without water (e.g., NH₃ + HCl → NH₄Cl).
A. Base vs alkali (do not confuse these)
Most bases you meet at O-Level are metal oxides and metal hydroxides.
- Many metal oxides/hydroxides are bases because they neutralise acids.
- A soluble base that produces hydroxide ions in water is an alkali. Some oxides react with water to form a hydroxide rather than simply dissolving.
| Category | Examples (O-Level) |
|---|---|
| Insoluble bases | CuO(s), Cu(OH)₂(s) |
| Soluble bases (alkalis) | NaOH(aq), KOH(aq), Ca(OH)₂(aq), NH₃(aq) |
Calcium hydroxide is sparingly soluble: only a small amount dissolves. Its clear solution, limewater, is alkaline. Do not confuse an alkaline solution with a solid being completely soluble.
Sodium hydroxide already contains ions in its solid lattice. Dissolving it frees the ions:
NaOH(s) → Na + (aq) + OH⁻(aq)
Ammonia is molecular. It produces hydroxide ions by reacting partially with water:
NH₃(aq) + H₂O(l) ⇌ NH₄ + (aq) + OH⁻(aq)
Write aqueous ammonia as NH₃(aq): hydroxide ions arise from its partial reaction with water. The solution contains mainly ammonia molecules, with some ammonium and hydroxide ions.
Most “alkali properties” questions (pH, indicators, conductivity, neutralisation) are covered in:
Physical & Chemical Properties of Alkalis
.
B. Neutralisation (acid + base)
Neutralisation is when an acid reacts with a base to form a salt (and often water).
For an acid reacting with hydroxide ions, the net ionic equation is: H + (aq) + OH⁻(aq) → H₂O(l)
Examples: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) H₂SO₄(aq) + ZnO(s) → ZnSO₄(aq) + H₂O(l)
Do not write “base + acid → salt + water” as an absolute rule.
Ammonia is a base, but it forms an ammonium salt (no water):
NH₃(aq) + HCl(aq) → NH₄Cl(aq)
C. Bases reacting with ammonium salts
When an ammonium salt is warmed with an alkali, ammonia gas is released:
NH₄ + (aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
The ammonia turns damp red litmus paper blue. This is the prescribed test for the ammonium ion.
4. Common Mistakes
- Defining a base as “produces OH⁻ in water” (that is the definition of an alkali, not a base).
- Writing “base + acid → salt + water” as an absolute rule (ammonia forms ammonium salts without water).
- Forgetting to warm the ammonium salt with aqueous sodium hydroxide.
- Testing ammonia with dry rather than damp red litmus paper.
5. Exam Tips
- A base neutralises an acid by accepting H + aq ions (forming a salt).
- An alkali is a soluble base that produces OH ^ -aq in water.
- For an acid and a hydroxide solution, write H + (aq) + OH⁻(aq) → H₂O(l). For an insoluble oxide or ammonia, use the actual reacting particles rather than inventing hydroxide ions in the reactants.
- If the question asks you to classify a substance as an alkali, mention soluble in water + produces OH⁻(aq).
6. Worked Examples
Modelled example 1
Classify base vs alkali
Problem
Classify each as base only or alkali: CuO, NaOH and NH₃(aq).
Study the worked solution
Classify copper(II) oxide
Method
Describe CuO as a base but not an alkali.Reason
It neutralises acids but is insoluble in water.Working
CuO: base only.Classify sodium hydroxide
Method
Describe NaOH as an alkali.Reason
It is a soluble base that produces OH⁻(aq) ions.Working
NaOH(s) → Na + (aq) + OH⁻(aq).Classify aqueous ammonia
Method
Describe NH₃(aq) as an alkali.Reason
It is a soluble base and produces some OH⁻(aq) through partial reaction with water.Working
NH₃(aq): alkali.
Guided practice 2
Neutralisation ionic equation
Problem
Write the ionic equation for neutralisation between an acid and an alkali.
Keep only reacting ions
Hints
Hint 1: identify reacting ions
Acids supply H + aq and alkalis supply OH ^ -aq.
Hint 2: combine charges
One + 1 ion and one -1 ion form neutral water.
View solution step by step
Select the reacting species
Method
Use H + (aq) and OH⁻(aq).Reason
Other ions from the acid and alkali are spectators in the net change.Working
Reactants: H + (aq), OH⁻(aq).Write the balanced change
Method
Combine the ions in a 1:1 ratio to form water.Reason
Atoms and total charge are balanced.Working
H + (aq) + OH⁻(aq) → H₂O(l).
Common misconception 3
Spot the overclaim
Learner claim
“Bases dissolve in water to form hydroxide ions.” What is overclaimed, and how should the sentence be corrected?
Separate the broader and narrower classes
View solution step by step
Locate the overclaim
Method
Reject “all bases dissolve”.Reason
Some bases, including CuO, are insoluble in water.
Working
CuO is a base but not an alkali.Correct the definition
Method
Replace “bases” with “alkalis”.Reason
Alkali is the term for a soluble base producing OH ^ -aq.
Working
“Alkalis are soluble bases that produce OH ^ -aq ions in water.”
Challenge 4
Test an ammonium salt
Procedure transfer
Describe how to test a solution for ammonium ions, including reagent, condition, observation and conclusion.
Build the complete test
Hints
Hint 1: release ammonia
Ammonium ions react with hydroxide ions when warmed.
Hint 2: test the basic gas
Use damp red litmus paper for the evolved ammonia.
View solution step by step
Add reagent and warm
Method
Add aqueous sodium hydroxide to the sample and warm gently.Reason
Hydroxide ions release ammonia gas from ammonium ions.Working
NH₄ + (aq) + OH⁻(aq) → NH₃(g) + H₂O(l).Test the gas
Method
Expose damp red litmus paper to the gas.Reason
Ammonia is alkaline only when it dissolves in the moisture on the paper.Working
Damp red litmus turns blue, confirming ammonium ions.
7. Mind Stretchers
Mind stretcher 1: Ammonia trapExtension
Question: A student writes: “Neutralisation always forms water.” Use ammonia to prove the sentence is wrong.
Show answer
Ammonia is a base but reacts with hydrochloric acid to form ammonium chloride only: NH₃(aq) + HCl(aq) → NH₄Cl(aq) No water is formed, so “always forms water” is wrong.
Mind stretcher 2: Particle logicExtension
Question: Explain why aqueous sodium hydroxide conducts electricity.
Show answer
It contains mobile ions, Na + (aq) and OH⁻(aq), which carry charge through the solution.
Try independently: A black solid is insoluble in water but reacts with dilute acid to form a salt and water. Can you classify it as a base? Does that establish that it is an alkali? Explain both decisions.
Show answer and reasoning
It acts as a base because it neutralises the acid. It is not an alkali: it is insoluble in water. Acid can react with a solid at its surface, so insolubility does not prevent neutralisation.
8. Quiz
Practise the ideas in the Acid–Base Chemistry topic check.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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