Chemistry

12.8 - Tests for Cations

5 min

note 1 of 3

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

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

Metal ion	Flame colour
Lithium, Li⁺	crimson (red)
Sodium, Na⁺	yellow (yellow-orange)
Potassium, K⁺	lilac
Calcium, Ca²⁺	orange-red (brick red)
Barium, Ba²⁺	light green (green or apple green)
Copper(II), Cu²⁺	blue-green

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

Six wire loops side by side, each burning with a different flame colour: crimson, yellow, lilac, orange-red, light green, and blue-green, labelled lithium, sodium, potassium, calcium, barium, and copper(II).

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).

note 2 of 3

Precipitates with sodium hydroxide and ammonia

Adding aqueous sodium hydroxide or aqueous ammonia to a metal ion solution forms an insoluble metal hydroxide precipitate.
Testing with both reagents, first a few drops and then in excess, allows cations to be distinguished by their precipitate colour and solubility.

Cation	With aqueous sodium hydroxide	With aqueous ammonia
Chromium(III), Cr³⁺	green precipitate, dissolves in excess to a dark green solution	green precipitate, does not dissolve in excess
Copper(II), Cu²⁺	light blue precipitate, does not dissolve in excess	light blue precipitate, dissolves in excess to a dark blue solution
Iron(II), Fe²⁺	green precipitate, does not dissolve in excess	green precipitate, does not dissolve in excess
Iron(III), Fe³⁺	red-brown precipitate, does not dissolve in excess	red-brown precipitate, does not dissolve in excess
Aluminium, Al³⁺	white precipitate, dissolves in excess to colourless solution	white precipitate, does not dissolve in excess
Calcium, Ca²⁺	white precipitate, does not dissolve in excess	no precipitate (or very slight white)
Magnesium, Mg²⁺	white precipitate, does not dissolve in excess	white precipitate, does not dissolve in excess
Zinc, Zn²⁺	white precipitate, dissolves in excess to colourless solution	white precipitate, dissolves in excess to colourless solution

On standing in air, the green iron(II) hydroxide precipitate slowly turns brown at the surface as it oxidises to iron(III) hydroxide.

A grid of tubes: columns for chromium(III), copper(II), iron(II), iron(III), aluminium, calcium, magnesium and zinc; rows for sodium hydroxide, excess sodium hydroxide, ammonia and excess ammonia, showing each precipitate's colour, and where it dissolves, the solution left: dark green for chromium(III) in excess sodium hydroxide, dark blue for copper(II) in excess ammonia, colourless for the others.

Distinguishing key pairs

Chromium(III) vs iron(II) - Both give green precipitates; only chromium(III) hydroxide dissolves in excess sodium hydroxide.
Aluminium vs zinc - Both give white precipitates that dissolve in excess sodium hydroxide; only zinc hydroxide dissolves in excess ammonia.
Calcium vs magnesium - In aqueous ammonia, calcium gives no precipitate (or a faint trace), whereas magnesium produces a distinct white precipitate.
Always use a fresh sample portion for each reagent.

note 3 of 3

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

Add sodium hydroxide solution to the sample in a test tube.
Warm the mixture gently.
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)

Four stages left to right: a tube of sample, add aqueous sodium hydroxide, heat gently over a Bunsen, ammonia gas produced, and damp red litmus held at the mouth turning blue, positive result.

key terms

TermDefinition

CationA positively charged ion, which moves towards the cathode during electrolysis.

PrecipitateAn insoluble solid that forms in a solution during a chemical reaction, usually when two solutions are mixed.

end of notes

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