Reactions of Ions in Aqueous Solution

A concise A-Level Chemistry study page covering what this inorganic chemistry topic means, what students are expected to know, and the key practical observations used to identify ions.

Topic Overview

In A-Level inorganic chemistry, reactions of ions in aqueous solution focuses on how metal ions behave when dissolved in water and how they can be identified using simple test-tube reactions.

        Students should understand the formation of metal-aqua ions, their acidity, and the visible changes
        that occur when reagents such as hydroxide ions, ammonia, and carbonate ions are added.
      

Aqua ions Precipitates Ligand behavior Qualitative analysis

Core Ideas Students Must Know

Metal ions in water form metal-aqua complexes.
Typical examples include [Fe(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺, [Al(H₂O)₆]³⁺, and [Fe(H₂O)₆]³⁺.
3+ metal-aqua ions are more acidic than 2+ metal-aqua ions because their higher charge and similar size give a greater charge-to-size ratio.
Some hydroxides are amphoteric, meaning they dissolve in both acids and bases. Aluminium hydroxide is the key example.
Students must describe and explain reactions with OH⁻, NH₃, and CO₃²⁻ and identify ions from the observations.

Key Aqueous Ion Reactions and Observations

Ion	Reagent	Observation	What it shows
Fe²⁺	NaOH / NH₃	Green precipitate, often turns brown in air	Formation of iron(II) hydroxide, then oxidation
Fe³⁺	NaOH / NH₃	Red-brown precipitate	Formation of iron(III) hydroxide
Cu²⁺	NaOH	Pale blue precipitate	Formation of copper(II) hydroxide
Cu²⁺	Excess NH₃	Deep or dark blue solution	Ligand exchange and complex formation
Al³⁺	NaOH	White precipitate, soluble in excess	Amphoteric hydroxide behavior
Al³⁺	NH₃	White precipitate, insoluble in excess	Helps distinguish Al³⁺ from some other ions

Practical Skills Emphasized

This topic strongly supports practical work. Students are expected to carry out test-tube reactions with reagents such as sodium hydroxide, aqueous ammonia, and sodium carbonate to identify unknown ions.

Observe precipitate colour
Check whether the precipitate dissolves in excess reagent
Recognize gas evolution where relevant
Use observations to identify the ion present

Summary

The syllabus area is essentially about using visible chemical reactions to identify aqueous ions. It combines theory and practice: students learn why aqua ions are acidic, how hydroxides behave, and how test-tube observations reveal the identity of metal ions.

Reference

Official AQA specification: https://www.aqa.org.uk/subjects/chemistry/a-level/chemistry-7405/specification/subject-content/inorganic-chemistry