7.12 - Tests for Cations

The flame test

• A cation is a positively charged ion.
• When a metal compound is heated in a flame, it gives off a characteristic colour that identifies the metal ion.

Carrying out the test

1. Clean the wire. Dip a nichrome or platinum wire loop in hydrochloric acid, then hold it in a blue Bunsen flame until it burns with no colour. Any residue left from an earlier test would mask the sample's colour. Hydrochloric acid is used because chloride ions give no flame colour. A fresh damp wooden splint can be used instead of the wire and discarded after one test.

2. Pick up the sample. Dip the clean wire loop or damp splint into the solid sample so a little clings to it.

3. Hold it in the flame and record the colour. Use the roaring blue flame (air hole open), which is the hottest part and won't add any yellow colour of its own.

Flame colours

Where a question describes lithium's flame as crimson, a flame it calls simply red is calcium's.

What the test cannot do

• A flame test only works reliably on a single metal ion.
• If a sample contains a mixture of ions, the colours blend and a strong colour masks a weaker one (e.g. sodium's yellow completely masks potassium's lilac).

Precipitates with sodium hydroxide

• Adding sodium hydroxide solution to a solution of a metal compound identifies the positive ion (cation) it contains.
• Most transition metal hydroxides are insoluble in water. When mixed, the insoluble metal hydroxide forms as a solid suspended in the liquid - a precipitate.
• Method - Dissolve the sample in water, add sodium hydroxide solution a few drops at a time, and record the colour of the precipitate.

• The colour belongs to the solid precipitate, not the solution.
• This test cannot identify Group 1 metals. Lithium, sodium, and potassium hydroxides are soluble in water, so no precipitate forms. These are identified using flame tests instead.

The test for the ammonium ion

The ammonium ion, NH₄⁺, is a non-metal cation. It doesn't give a flame colour or a hydroxide precipitate.

The test

1. Add sodium hydroxide solution to the sample in a test tube.
2. Warm the mixture gently.
3. Hold damp red litmus paper over the mouth of the tube (without touching the sides or liquid).

If ammonium ions are present, ammonia gas is released:

Ammonia gas turns damp red litmus paper blue

The litmus paper must be damp so ammonia gas can dissolve in the water to form alkaline hydroxide ions.

The reaction

Hydroxide ions from sodium hydroxide react with ammonium ions, transferring a proton to release ammonia gas and water:

NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l)

Full equation with ammonium sulfate:

2NaOH(aq) + (NH₄)₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2NH₃(g)

Ionic equations for the hydroxide precipitates

An ionic equation shows only the reacting ions that form the precipitate, omitting unchanged spectator ions.

Constructing an ionic equation

1. Full balanced equation:
   • CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)
2. Split dissolved aqueous compounds into individual ions:
   • Cu²⁺(aq) + SO₄^2-(aq) + 2Na⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s) + 2Na⁺(aq) + SO₄^2-(aq)
3. Cancel spectator ions (Na⁺ and SO₄^2-):
   • Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s)

Common ionic equations

The number of OH^- ions always equals the charge on the metal cation so charges cancel to zero.