Inorganic Salts: Properties, Preparation and Identification of Ions

Lessons

Senior 2 · Chemistry

Inorganic Salts: Properties, Preparation and Identification of Ions

5 lessons

Lesson 5 of 5

Identifying Cations and Anions

25 min

Why this matters

Water-testing laboratories check drinking water for dissolved ions. Agricultural inspectors check that a bag labelled "potassium chloride" really contains potassium and chloride. Forensic scientists identify unknown white powders. All of them rely on qualitative analysis: simple tests that show which ions a substance contains, by a colour, a precipitate or a gas.

By the end of this lesson you will be able to carry out and interpret flame tests, the sodium hydroxide and ammonia tests for cations, and the tests for carbonate, chloride, sulfate and nitrate ions.

Flame tests

Some metal ions give a characteristic colour to a flame.

Clean a nichrome or platinum wire loop by dipping it in concentrated hydrochloric acid and holding it in a hot, blue (non-luminous) Bunsen flame until it gives no colour.
Dip the loop in the acid again, then into the solid sample.
Hold the sample at the edge of the flame and observe the colour.

Ion	Flame colour
Na⁺	Bright yellow-orange (lasts a long time)
K⁺	Lilac (pale purple)
Ca²⁺	Brick-red (orange-red)
Cu²⁺	Blue-green
Li⁺	Crimson (deep red)
Ba²⁺	Pale (apple) green

Testing cations with sodium hydroxide and aqueous ammonia

Dissolve the sample in distilled water. Add a few drops of sodium hydroxide solution, then add it in excess (until no further change). Repeat with a fresh sample using aqueous ammonia. Most metal hydroxides are insoluble, so a precipitate forms, and its colour and behaviour in excess identify the ion.

Cation	Sodium hydroxide: a few drops	Sodium hydroxide: excess	Aqueous ammonia: a few drops	Aqueous ammonia: excess
Cu²⁺	Pale blue precipitate	Insoluble	Pale blue precipitate	Dissolves to give a deep blue solution
Fe²⁺	Green precipitate (turns brown on standing)	Insoluble	Green precipitate	Insoluble
Fe³⁺	Red-brown precipitate	Insoluble	Red-brown precipitate	Insoluble
Zn²⁺	White precipitate	Dissolves (colourless solution)	White precipitate	Dissolves (colourless solution)
Al³⁺	White precipitate	Dissolves	White precipitate	Insoluble
Pb²⁺	White precipitate	Dissolves	White precipitate	Insoluble
Ca²⁺	White precipitate	Insoluble	No precipitate (or very slight)	No precipitate
NH₄⁺	No precipitate. On warming with sodium hydroxide, ammonia gas is given off: it has a sharp smell and turns damp red litmus paper blue.

Al³⁺ and Pb²⁺ give the same results with both reagents. To tell them apart, add potassium iodide solution: Pb²⁺ gives a bright yellow precipitate of lead(II) iodide, and Al³⁺ gives none.

The precipitates are hydroxides, for example:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)

NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Testing anions

Carbonate, CO₃²⁻. Add dilute hydrochloric acid. Effervescence (fizzing) shows a gas; bubble it through limewater. If the limewater turns milky, the gas is carbon dioxide and the sample contains carbonate.

CO₃²⁻ + 2H⁺(aq) → H₂O(l) + CO₂(g) then Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Chloride, Cl⁻. Acidify the solution with dilute nitric acid, then add silver nitrate solution. A white precipitate of silver chloride shows chloride. (Bromide gives a cream precipitate and iodide a yellow one.)

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Sulfate, SO₄²⁻. Acidify with dilute hydrochloric acid (or nitric acid), then add barium chloride (or barium nitrate) solution. A white precipitate of barium sulfate shows sulfate.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why acidify first? Carbonate ions also give white precipitates with silver ions and with barium ions. The acid destroys any carbonate (as carbon dioxide) first, so a white precipitate can only mean chloride or sulfate. Never acidify with hydrochloric acid before a chloride test: it would add the very chloride ions you are testing for.

Nitrate, NO₃⁻. The brown ring test (a teacher demonstration, because it uses concentrated acid): add freshly made iron(II) sulfate solution to the sample, then carefully pour concentrated sulfuric acid down the side of the tilted test tube so that it forms a layer underneath. A brown ring where the two layers meet shows nitrate. An alternative is to warm the sample with sodium hydroxide solution and a little aluminium foil: nitrate is reduced to ammonia, which turns damp red litmus blue.

Worked example 1

A blue solid dissolves in water. With sodium hydroxide it gives a pale blue precipitate that does not dissolve in excess. After acidifying with dilute hydrochloric acid, barium chloride gives a white precipitate. Identify the salt.

Solution. The pale blue hydroxide, insoluble in excess sodium hydroxide, shows Cu²⁺. The white precipitate with acidified barium chloride shows SO₄²⁻. The salt is copper(II) sulfate.

Worked example 2

A white powder gives a lilac flame. It fizzes with dilute hydrochloric acid, and the gas turns limewater milky. Identify it.

Solution. Lilac flame: K⁺. Carbon dioxide with acid: CO₃²⁻. The salt is potassium carbonate, K₂CO₃.

Tip

Colours first: blue = copper(II), green = iron(II), red-brown ("rusty") = iron(III).

Then the white precipitates: zinc dissolves in excess of both reagents; aluminium and lead(II) dissolve only in excess sodium hydroxide; calcium dissolves in neither and gives nothing with ammonia.

Anions: "Acid first, then the silver or barium."

✦

Practice

Try each item before you read the solution.

A solution gives a white precipitate with a few drops of sodium hydroxide, which dissolves in excess. With aqueous ammonia it gives a white precipitate that does not dissolve in excess. Which two cations could be present, and how could you tell them apart?

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Solution: Al³⁺ or Pb²⁺. Add potassium iodide solution: a yellow precipitate shows Pb²⁺; no precipitate shows Al³⁺.

A green solution gives a green precipitate with sodium hydroxide that slowly turns brown at the surface. Name the cation and explain the colour change.

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Solution: Iron(II), Fe²⁺. The green iron(II) hydroxide is slowly oxidised by oxygen in the air to red-brown iron(III) hydroxide.

How would you show that a fertiliser contains ammonium ions?

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Solution: Warm a sample with sodium hydroxide solution. If a gas with a sharp smell is given off that turns damp red litmus paper blue, it is ammonia, so ammonium ions are present: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l).

Byiringiro adds silver nitrate to a solution without acidifying it, sees a white precipitate and concludes that chloride is present. Why might he be wrong?

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Solution: Carbonate ions also give a white precipitate (silver carbonate) with silver nitrate. He should first add dilute nitric acid to remove any carbonate; only a white precipitate that forms after acidifying proves chloride.

Summary

Flame tests: Na yellow-orange, K lilac, Ca brick-red, Cu blue-green, Li crimson, Ba pale green.
With NaOH and NH₃: Cu²⁺ pale blue (deep blue solution in excess NH₃); Fe²⁺ green; Fe³⁺ red-brown; Zn²⁺ white, dissolves in both excesses; Al³⁺ and Pb²⁺ white, dissolve only in excess NaOH; Ca²⁺ white with NaOH only. NH₄⁺ gives ammonia on warming with NaOH.
Carbonate: acid gives CO₂, which turns limewater milky. Chloride: HNO₃ then AgNO₃ gives white AgCl. Sulfate: HCl then BaCl₂ gives white BaSO₄. Nitrate: brown ring test.
Always acidify before the silver nitrate and barium chloride tests to remove carbonate.

Sources and credits

Physical Sciences Grade 10, 18.4 Precipitation reactions (tests for chloride, sulfate and carbonate ions) by Siyavula Education and volunteers, Siyavula, licensed under CC BY 3.0. Adapted by Ganzaa.
Qualitative inorganic analysis (hydroxide precipitates and anion tests, including the brown ring test), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Chemistry 2e, 18.1: Periodicity (flame colours of the alkali metals) by Paul Flowers, Klaus Theopold, Richard Langley and William R. Robinson, OpenStax, via LibreTexts, licensed under CC BY 4.0. Adapted by Ganzaa.
Image: Flammenfärbung Calcium (Ca).jpg by S k y r, via Wikimedia Commons, licensed under CC BY 4.0.
The worked examples, Practice items and exercises were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why this matters

2Flame tests

3Testing cations with sodium hydroxide and aqueous ammonia

4Testing anions

Worked example 1

Worked example 2

Practice

Why this matters

Flame tests

Testing cations with sodium hydroxide and aqueous ammonia

Testing anions