Tests for ions

Metal ions can be found with flame tests and by adding sodium hydroxide solution to see the colour of the precipitate. Negative ions have their own tests: carbonates fizz with acid, halides give coloured precipitates with silver nitrate, and sulfates give a white precipitate with barium chloride. Using one test for each ion identifies a salt.

Key facts

  • Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
  • With sodium hydroxide solution: copper(II) blue, iron(II) green, iron(III) brown precipitates; aluminium, calcium and magnesium white. Only aluminium hydroxide dissolves in excess.
  • Carbonates: add dilute acid; the gas (carbon dioxide) turns limewater milky.
  • Halides: add dilute nitric acid then silver nitrate: chloride white, bromide cream, iodide yellow precipitate.
  • Sulfates: add dilute hydrochloric acid then barium chloride: a white precipitate.
  • Flame emission spectroscopy is an instrumental method: it identifies metal ions in mixtures and measures their concentrations.

Worked examples

Example 1

What colour flame do lithium ions give in a flame test?

  1. Lithium ions give a crimson flame.

Answer: crimson

Example 2

Why is the wire for a flame test cleaned in acid first?

  1. Old compounds on the wire would add their own colours.
  2. Cleaning (dipping in acid and heating until no colour shows) avoids this.

Answer: to remove any metal ions left from earlier tests

Example 3

A compound gives a yellow flame in a flame test. Which metal ion does it contain?

  1. Sodium ions give a yellow flame.

Answer: sodium ions

Example 4

Why can a flame test be hard to read for a mixture of metal ions?

  1. Some flame colours are much stronger than others.
  2. For example, yellow from sodium can hide the lilac of potassium.

Answer: one colour can hide another

Example 5

What colour is the precipitate when sodium hydroxide solution is added to a solution of copper(II) ions?

  1. Copper(II) ions give a blue precipitate.

Answer: blue

Example 6

Sodium hydroxide solution is added to solutions P, Q and R. P gives a blue precipitate, Q a brown one and R a white one that dissolves in excess. Which metal ions are present?

  1. Blue: copper(II) hydroxide, so P contains Cu²⁺. Cu²⁺ + 2OH⁻ → Cu(OH)2
  2. Brown: iron(III) hydroxide, so Q contains Fe³⁺. Fe³⁺ + 3OH⁻ → Fe(OH)3
  3. White, dissolving in excess: aluminium hydroxide, so R contains Al³⁺.

Answer: P copper(II), Q iron(III), R aluminium

Example 7

A solution gives a cream precipitate with dilute nitric acid and silver nitrate solution. Which negative ion is present?

  1. Bromide ions give a cream precipitate with dilute nitric acid and silver nitrate solution.

Answer: bromide ions

Example 8

A white solid gives a lilac flame. Its solution gives a cream precipitate with dilute nitric acid and silver nitrate. Name the solid.

  1. A lilac flame shows potassium ions, K⁺.
  2. A cream precipitate with silver nitrate shows bromide ions, Br⁻ (silver bromide). Ag⁺ + Br⁻ → AgBr
  3. So the solid is potassium bromide, KBr.

Answer: potassium bromide

Example 9

Why is hydrochloric acid added before barium chloride when testing for sulfate ions?

  1. Carbonate ions also form a white precipitate with barium ions (barium carbonate).
  2. The acid reacts with any carbonate and removes it as carbon dioxide.
  3. Hydrochloric acid is used, not sulfuric acid, because sulfuric acid contains sulfate ions itself.

Answer: to remove carbonate ions, which would also give a white precipitate

Example 10

Why is flame emission spectroscopy better than a flame test?

  1. A spectroscope splits the light into a line spectrum for each metal ion.
  2. It can identify several ions in a mixture, where flame colours would mask each other.
  3. The intensity of the lines gives the concentration.

Answer: it can identify ions in mixtures and measure their concentration