8.4 - 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.

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

3. Hold the loop 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

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 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.

Telling the white precipitates apart

Add sodium hydroxide solution until it is in excess.

• Only aluminium hydroxide dissolves in excess sodium hydroxide. Calcium and magnesium precipitates remain unchanged.
• 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.

Equations for the hydroxide precipitates

When sodium hydroxide solution reacts with a soluble metal salt, an insoluble metal hydroxide precipitate and a soluble sodium salt form:

Metal salt + sodium hydroxide → metal hydroxide + sodium salt

Writing the balanced equation

Example using copper(II) sulfate:

1. Reactants - CuSO₄(aq) and NaOH(aq).
2. Products - Cu(OH)₂(s) and Na₂SO₄(aq).
3. Formula of hydroxide - Cu²⁺ requires two OH^- ions to balance the charge → Cu(OH)₂.
4. Balanced equation:
   • CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)

Other hydroxide precipitate equations

The number of hydroxide ions and NaOH units matches the magnitude of the metal ion's charge (2+ ions need 2NaOH; 3+ ions need 3NaOH).

Worked example - Equation for iron(III) hydroxide

Sodium hydroxide solution is added to a solution of iron(III) chloride, and a brown precipitate of iron(III) hydroxide forms. Write the balanced symbol equation for the reaction.

Worked example - Identifying a compound from tests

A white solid is thought to be a single ionic compound. Four tests are carried out. Name the compound and give its formula.

1. A flame test on the solid gives no colour.
2. Adding sodium hydroxide solution to a solution of the solid gives a green precipitate.
3. Adding dilute hydrochloric acid to a fresh portion of the solid produces no bubbles.
4. Acidifying a fresh portion of the solution with dilute hydrochloric acid, then adding barium chloride solution, gives a white precipitate.