top of page

12.4 Tests for Positive Ions

GCSE Chemistry · AQA · Chapter 12: Chemical Analysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: flame tests for metal ions and the sodium hydroxide test for metal hydroxide precipitates. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how to carry out a flame test and recall the flame colours of Li⁺, Na⁺, K⁺, Ca²⁺ and Cu²⁺
  • Recall the colours of the precipitates formed when sodium hydroxide solution is added to solutions of Al³⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺ and Fe³⁺
  • Tell aluminium, calcium and magnesium ions apart
  • Write balanced symbol equations, and (Higher tier) ionic equations, for the formation of metal hydroxide precipitates

Key words

  • Positive ion: An ion with a positive charge (a cation). Most are metal ions, e.g. Na⁺, Cu²⁺.
  • Flame test: A test in which a compound is held in a hot flame. Some metal ions give a characteristic colour.
  • Precipitate: An insoluble solid that forms when two solutions are mixed.
  • Hydroxide ion: OH−. Sodium hydroxide solution supplies these ions.
  • Excess: More than is needed to react; here, adding lots more sodium hydroxide solution.

Flame tests

Method: (1) Clean a nichrome wire loop by dipping it in dilute hydrochloric acid and holding it in a hot (blue) Bunsen flame until it gives no colour. (2) Dip the loop in the acid, then into the solid sample. (3) Hold it at the edge of the blue flame and note the colour.

Key fact: If a sample contains a mixture of metal ions, one colour can hide (mask) another. The bright yellow of sodium hides most others, so flame tests work best for a single compound. Flame emission spectroscopy (section 12.6) can identify ions in mixtures.

Metal hydroxide precipitates

Add a few drops of dilute sodium hydroxide solution to a solution of the compound. Many metal ions form an insoluble hydroxide as a coloured or white precipitate.

  • copper(II), Cu²⁺; Precipitate colour blue; Precipitate formed copper(II) hydroxide, Cu(OH)₂
  • iron(II), Fe²⁺; Precipitate colour green; Precipitate formed iron(II) hydroxide, Fe(OH)₂
  • iron(III), Fe³⁺; Precipitate colour orange-brown (rusty brown); Precipitate formed iron(III) hydroxide, Fe(OH)₃
  • aluminium, Al³⁺; Precipitate colour white – dissolves in excess NaOH; Precipitate formed aluminium hydroxide, Al(OH)₃
  • calcium, Ca²⁺; Precipitate colour white – stays in excess; Precipitate formed calcium hydroxide, Ca(OH)₂
  • magnesium, Mg²⁺; Precipitate colour white – stays in excess; Precipitate formed magnesium hydroxide, Mg(OH)₂

Sodium, potassium and lithium ions give no precipitate, because their hydroxides are soluble. Use a flame test for these.

Telling white precipitates apart

  • Add more sodium hydroxide (an excess): only aluminium hydroxide dissolves to give a colourless solution.
  • Calcium and magnesium both stay white. Do a flame test: calcium gives orange-red; magnesium gives no colour.

Exam tip: Add sodium hydroxide a few drops at a time. If you pour in a lot at once, aluminium hydroxide may dissolve before you ever see it.

Writing the equations

The metal ion joins with hydroxide ions to form the insoluble hydroxide. The number of OH− ions equals the charge on the metal ion.

Worked example: copper(II) sulfate with sodium hydroxide

CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)
(Higher tier) Ionic equation: Cu²⁺(aq) + 2OH−(aq) → Cu(OH)₂(s)

Answer: a blue precipitate of copper(II) hydroxide

Worked example: iron(III) chloride with sodium hydroxide

Fe³⁺ needs three OH− ions, so the hydroxide is Fe(OH)₃.
FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq)
(Higher tier) Ionic equation: Fe³⁺(aq) + 3OH−(aq) → Fe(OH)₃(s)

Answer: an orange-brown precipitate

Worked example: aluminium chloride with sodium hydroxide

Al³⁺ needs three OH− ions. A white precipitate of Al(OH)₃ forms, then dissolves if more NaOH is added.
AlCl₃(aq) + 3NaOH(aq) → Al(OH)₃(s) + 3NaCl(aq)
(Higher tier) Ionic equation: Al³⁺(aq) + 3OH−(aq) → Al(OH)₃(s)

Answer: a white precipitate that dissolves in excess

Exam tip: In an ionic equation, leave out the spectator ions (here Na⁺ and Cl−), which stay dissolved and do not change. Check that the atoms and the charges balance, and give the precipitate the state symbol (s).

Common mistakes

  • Mixing up iron(II) and iron(III): Fe²⁺ gives green; Fe³⁺ gives orange-brown. (A green iron(II) precipitate slowly turns brown at the surface as air oxidises it.)
  • Writing “red” for calcium. Lithium is crimson; calcium is orange-red.
  • Forgetting to clean the wire loop, so the colour from the last sample contaminates the next.
  • Thinking all white precipitates are the same. Use excess NaOH and then a flame test.
  • Writing Cu(OH) or Fe(OH)₂ for iron(III). Match the number of OH− ions to the charge on the metal ion.

Revision questions

Try these, then check your answers below each one.

Question 1. What is seen when sodium hydroxide solution is added to a solution containing Fe³⁺ ions? (Choose from: An orange-brown precipitate, A green precipitate, A blue precipitate, A white precipitate.)

Answer: An orange-brown precipitate

Working: Iron(III) hydroxide, Fe(OH)₃, is an orange-brown precipitate.

Question 2. A white precipitate forms with sodium hydroxide solution and then dissolves when excess sodium hydroxide is added. Which ion is present? (Choose from: Al³⁺, Ca²⁺, Mg²⁺, Cu²⁺.)

Answer: Al³⁺

Working: Aluminium hydroxide dissolves in excess sodium hydroxide solution; calcium and magnesium hydroxides do not.

Question 3. True or false? A flame test can be used to tell calcium ions apart from magnesium ions.

Answer: True

Working: Calcium ions give an orange-red flame; magnesium ions give no flame colour.

Related lessons

Continue in the full course

This lesson is section 12.4 (AGCCH1204) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

bottom of page