Identification of ions and gases

O Level / IGCSE Chemistry · Experimental techniques and chemical analysis. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic free All O Level / IGCSE Chemistry topics

The idea

A salt is identified in two parts: a test for the positive ion (cation) and a test for the negative ion (anion). Most metal cations are found by adding aqueous sodium hydroxide or aqueous ammonia, a little at first and then in excess. Note the colour of the precipitate and whether it dissolves in excess. Some cations are found by a flame test instead.

Anions are found with a different reagent for each one. The solution is often acidified with dilute nitric acid first, to remove ions such as carbonate that would give a false result.

Each common gas also has its own simple test. Learn every test as three things: reagent, observation, conclusion.

Rules to remember

Carbonate: add dilute acid, effervescence, gas turns limewater milky. Sulfite: turns acidified aqueous potassium manganate(VII) from purple to colourless
Halides: add dilute nitric acid, then aqueous silver nitrate. Chloride white, bromide cream, iodide yellow precipitate
Sulfate: dilute nitric acid, then aqueous barium nitrate, white precipitate. Nitrate: warm with aluminium foil and aqueous sodium hydroxide, ammonia given off
With aqueous sodium hydroxide: Cu²⁺ light blue, Fe²⁺ green, Fe³⁺ red-brown, Ca²⁺ white precipitate, all insoluble in excess. Al³⁺ and Zn²⁺ white, Cr³⁺ green, all soluble in excess. NH₄⁺ gives ammonia on warming
With aqueous ammonia: Zn²⁺ white precipitate, soluble in excess. Al³⁺ white, insoluble. Cu²⁺ light blue, soluble in excess to a dark blue solution. Cr³⁺ green, Fe²⁺ green, Fe³⁺ red-brown, all insoluble. Ca²⁺ no precipitate or a very slight white one
Gases: ammonia turns damp red litmus blue. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus. Hydrogen pops with a lighted splint. Oxygen relights a glowing splint. Sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless
Flame tests: Li⁺ red, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Ba²⁺ light green, Cu²⁺ blue-green

Common mistake

Students swap the two nitrate reagents. Silver nitrate is for the halides (chloride, bromide, iodide); barium nitrate is for sulfate.

Worked example

A solution of salt X gives a white precipitate with aqueous sodium hydroxide that dissolves in excess. With aqueous ammonia it gives a white precipitate that does not dissolve in excess. With dilute nitric acid and aqueous barium nitrate it gives a white precipitate. What is X?

White precipitate soluble in excess sodium hydroxide: the cation is Al³⁺ or Zn²⁺.
The precipitate with aqueous ammonia does not dissolve in excess, so it is Al³⁺ (Zn²⁺ would dissolve).
White precipitate with acidified barium nitrate: the anion is sulfate, SO₄²⁻.
Combine the ions: 2Al³⁺ and 3SO₄²⁻ give Al₂(SO₄)₃.

Answer: X is aluminium sulfate, Al₂(SO₄)₃.

Practice questions

Try each one, then open the answer.

1. Aqueous sodium hydroxide is added to a solution. A green precipitate forms which does not dissolve in excess. Which cation is present?

A
Fe³⁺
B
Fe²⁺
C
Cu²⁺
D
Cr³⁺

Show answer

Answer: B. Iron(II) hydroxide is green and insoluble in excess. Chromium(III) gives a green precipitate that dissolves in excess; Fe³⁺ gives red-brown and Cu²⁺ light blue.

2. Aqueous sodium hydroxide is added to a solution of a metal salt. A white precipitate forms which dissolves in excess sodium hydroxide. Which ion could be present?

A
Cu²⁺
B
Zn²⁺
C
Fe³⁺
D
Ca²⁺

Show answer

Answer: B. Zinc and aluminium hydroxides are white and dissolve in excess NaOH. Calcium hydroxide is white but insoluble in excess; copper gives blue, iron(III) red-brown.

3. A solution is warmed with aluminium foil and aqueous sodium hydroxide. A gas is given off that turns damp red litmus paper blue. Which anion is present?

A
chloride, Cl⁻
B
sulfate, SO₄²⁻
C
carbonate, CO₃²⁻
D
nitrate, NO₃⁻

Show answer

Answer: D. Aluminium reduces nitrate ions to ammonia in alkaline conditions. Ammonia is the only common gas that turns damp red litmus blue.

Read the full notes

Keep going

← Separation and purification All rules on one page Stuck? Ask a question