AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical analysis › Identification of ions
Practise Metal hydroxides. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Adding sodium hydroxide solution identifies some metal ions from the colour of the hydroxide precipitate. This is in GCSE Chemistry only (not Combined Science). Expect colour recall, how to tell the white precipitates apart, and balanced equations.
Key facts
Cu2+ → blue precipitate
Fe2+ → green precipitate
Fe3+ → brown precipitate
Al3+, Ca2+, Mg2+ → white precipitate
Only Al(OH)3 dissolves in excess sodium hydroxide solution
Ca2+ or Mg2+? Flame test: calcium gives orange-red
CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4
Notes
The test
Add a few drops of sodium hydroxide solution to a solution of the compound. Many metal ions form an insoluble metal hydroxide, seen as a precipitate.
Then add excess sodium hydroxide solution and see whether the precipitate dissolves.
Results
diagram
Copper(II), Cu2+: blue precipitate.
Iron(II), Fe2+: green precipitate.
Iron(III), Fe3+: brown precipitate.
Aluminium Al3+, calcium Ca2+ and magnesium Mg2+: white precipitates.
Only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution, giving a colourless solution.
To tell calcium from magnesium, do a flame test: calcium ions give an orange-red flame; magnesium ions give no distinctive colour.
A green iron(II) hydroxide precipitate slowly turns brown in air as it is oxidised, so record the colour as soon as it forms.
Colour of the hydroxide precipitate first; for white, add excess, then a flame test.
Equations
The hydroxide has one OH− for each positive charge on the metal ion: Cu(OH)2, Fe(OH)2, Fe(OH)3, Al(OH)3, Mg(OH)2, Ca(OH)2.
Add sodium hydroxide solution a few drops at a time, then in excess.
If the white precipitate dissolves in excess, the ion is aluminium.
If it does not dissolve, do a flame test: orange-red means calcium; no colour suggests magnesium.
Example. Three unlabelled solutions contain aluminium chloride, calcium chloride and magnesium chloride. All three give a white precipitate with a few drops of sodium hydroxide solution. Describe how you could identify each solution. Give the results.
Show the model answerHide the model answer
Add excess sodium hydroxide solution to each precipitate (1). The precipitate that dissolves is from aluminium chloride (1). Do a flame test on the other two solutions (1). Calcium chloride gives an orange-red flame; magnesium chloride gives no distinctive colour (1).
Don’t lose marks
Writing just 'iron'. Say iron(II) or iron(III), because they give different colours.
Saying calcium or magnesium hydroxide dissolves in excess. Only aluminium hydroxide does.
Writing 'turns blue' instead of 'blue precipitate'. Say that a precipitate (a solid) forms.
Wrong formulae such as FeOH3 or Al(OH)2. Use brackets, with the number of OH equal to the charge.
More tips
Memory tricks
Rust is brown and contains iron(III), so iron(III) hydroxide is brown; iron(II) is the green one.
Copper(II) sulfate solution is blue, and so is copper(II) hydroxide.
'Aluminium Always dissolves' in excess; calcium and magnesium don't.
Number of OH in the formula = charge on the metal ion.
Exam technique
Always use the word 'precipitate' and give its colour.
To tell all three white precipitates apart you need BOTH excess sodium hydroxide AND a flame test.
Put (s) after the metal hydroxide in equations: it is an insoluble solid.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Recall the coloured hydroxide precipitatesCopper(II) blue, iron(II) green, iron(III) brown.
Grade 4
Name the ions giving white precipitatesAluminium, calcium and magnesium ions all give a white precipitate.
Grade 5
Use excess sodium hydroxide to identify aluminiumOnly aluminium hydroxide dissolves in excess sodium hydroxide solution.
Grade 5
Distinguish calcium ions from magnesium ionsBoth precipitates stay in excess, so do a flame test: calcium gives orange-red.
Required practical:Identifying ions (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A solution forms a blue precipitate with sodium hydroxide solution. Name the metal ion in the solution.
copper(II)
Copper(II) sulfate solution reacts with sodium hydroxide solution to form a precipitate. Name the precipitate.
copper(II) hydroxide
Name the ion that forms a brown precipitate with sodium hydroxide solution.
iron(III)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Sodium hydroxide solution can be used to identify some metal ions.
(a) What colour precipitate forms when sodium hydroxide solution is added to a solution containing iron(III) ions? Tick (✓) one box.[1]
Blue
Brown
Green
White
(b) A solution forms a blue precipitate with sodium hydroxide solution. Name the metal ion in the solution.[1]
(c) Give two metal ions that form a white precipitate with sodium hydroxide solution.[2]
Show the answer and mark scheme
(a)Answer: Brown
(b)Answer: copper(II)
copper(II) / copper / Cu2+
(c)
aluminium / Al3+
calcium / Ca2+
magnesium / Mg2+
Question 2Medium5 marks
A student has three unlabelled solutions. One contains aluminium ions, one contains calcium ions and one contains magnesium ions. When a few drops of sodium hydroxide solution are added to each solution, a white precipitate forms.
(a) Describe how the student could use sodium hydroxide solution to identify the solution containing aluminium ions. Give the result.[2]
(b) Sodium hydroxide solution cannot be used to tell apart the solutions containing calcium ions and magnesium ions. Explain why. Name another test that could identify the calcium ions and give the result.[3]
Show the answer and mark scheme
(a)
add excess sodium hydroxide solution (to each precipitate)
only the aluminium hydroxide precipitate dissolves (to give a colourless solution)
(b)
both form white precipitates that do not dissolve in excess sodium hydroxide solution
flame test
calcium ions give an orange-red flame (magnesium ions give no flame colour)