AQA Chemistry (8462) 4.8.3.2 · Classify
Metal hydroxides
A few drops of sodium hydroxide, and a solid appears in the tube. Can its colour name the metal ion, and what do you do when the solid is white?
Triple Science · Higher tier
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Blue solution in. Blue solid out.
Copper(II) sulfate solution is clear and blue. Add a few drops of colourless sodium hydroxide solution and a pale blue solid appears, swirls, and slowly sinks to the bottom of the tube.
If you had to guess, is the blue solid a new substance, or copper sulfate coming back out of the solution?
Sodium hydroxide solution tests for some metal ions (cations). Its hydroxide ions join the metal ions to make metal hydroxides that are insoluble, so they come out of solution as a precipitate. Some precipitates have a colour of their own; a white one needs a second observation before it names anything.
Report the colour of the precipitate, never the solution's: copper(II) sulfate solution is blue before you add a drop.
Required practical 7 · chemistry only · the sodium hydroxide part
A few drops first. Then excess.
- Put about 1 cm³ of the solution in a clean test tube.
- Add dilute sodium hydroxide solution a few drops at a time. Record the colour of any precipitate.
- Keep adding sodium hydroxide, shaking, until it is in excess. Record whether a white precipitate dissolves.
Risks. Sodium hydroxide is an irritant and especially harmful to eyes: wear eye protection. Copper compounds are harmful: avoid skin contact and wash your hands.
The precipitate bench
Copper(II) sulfate · call it, then add the sodium hydroxide
0 of 6 solutions tested
Commit first. A few drops of sodium hydroxide go in. What forms?
Think again · excess at once
“I tipped in lots of sodium hydroxide straight away. No precipitate appeared, so the solution cannot contain aluminium ions.”
Spot the flaw
ExplainWhat is wrong with that conclusion?
Equations · writing the reaction
metal salt + sodium hydroxide → metal hydroxide + sodium salt
One OH for each positive charge on the metal ion, then balance the sodium.
M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s)
M³⁺(aq) + 3OH⁻(aq) → M(OH)₃(s)
The ionic equation keeps only the ions that make the precipitate. Sodium ions and the salt's other ion stay dissolved and unchanged, so they are left out.
Equation forge
Watch one built. Then build two.
0 of 2 balanced
Worked · copper(II) sulfate
- Copper(II) is Cu²⁺, so its hydroxide needs two OH: Cu(OH)₂.
Your turn · iron(III) chloride
FeCl₃ + __NaOH → __ + __NaCl
Iron(III) hydroxide
Number in front of NaOH
Number in front of NaCl
Your turn · iron(II) sulfate
FeSO₄ + __NaOH → __ + __
Iron(II) hydroxide
Number in front of NaOH
The sodium salt
Ionic equation
Build the ionic equation for copper(II).
Cu²⁺(aq) + __OH⁻(aq) → __
How many OH⁻?
The product
Command words in this lesson
- Identify
- Name the ion from the results given. No reason needed unless asked.
- Complete
- Fill the gaps in an equation: the missing formula and the balancing numbers.
- Explain
- Say what you add, what you see, and what that tells you, linked in order.
- Plan
- A sequence of tests that would tell every sample apart, with the result each one gives.
Key fact
Three coloured precipitates and three white ones. Of the white ones, only aluminium hydroxide dissolves in excess sodium hydroxide.
Key note · AQA 4.8.3.2 · RP7
Metal hydroxides
- NaOH test for metal ions: Cu²⁺ = blue, Fe²⁺ = green, Fe³⁺ = brown, Al³⁺ = white (dissolves in excess), Ca²⁺ = white (stays), Mg²⁺ = white (stays).
- Only aluminium hydroxide dissolves in excess sodium hydroxide solution.
- Add sodium hydroxide a few drops at a time first, then in excess.
- Calcium or magnesium? A flame test decides: calcium gives an orange-red flame.
- CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄: one OH for each positive charge on the metal ion.
- M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s)
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Two white precipitates and not sure which is which?
The bench draws each precipitate in one idealised shade and does not model volumes or concentrations. Real colours vary: iron(II) hydroxide turns brown at the surface as air oxidises it, and calcium's precipitate can be faint.