GCSE · Chemistry · AQA · Spec 8462

Metal hydroxide precipitate tests

Add sodium hydroxide to an unknown solution and a solid appears. Its colour is your first clue to the ion.

Chemistry · Identifying metal ions

Walk the test: colour first, then excess only if it's white

Pick the colour you'd see. Before each ending opens, say which ion you think it is.

Colour of the precipitate → White only: keep adding sodium hydroxide

What's happening: the metal ions join up with hydroxide ions, OH⁻, from the sodium hydroxide. The metal hydroxide they make is insoluble, so instead of staying dissolved it shows up as a solid. Metal ions are positive ions, also called cations.

5 possible endings.

On Add sodium hydroxide solution to the unknown solution. 4 branches to choose from.

Six ions, but only five endings. One ending holds two ions, calcium and magnesium, and sodium hydroxide on its own can't split them.

Watch out: Only the white branch needs the excess step. Blue, green or brown has already named the ion.

Which would you trust?

Four statements, one of them right

A classmate is revising the sodium hydroxide test and has written these four statements in their notes. Only one of them is right.

Which statement is closest to what you think right now?

If you add enough sodium hydroxide, any white precipitate will dissolve.A white precipitate that stays in excess means the solution contains magnesium ions.Iron gives the same colour precipitate whichever iron ion is in the solution.A white precipitate that stays in excess could be calcium or magnesium ions, and this test can't say which.

How sure are you?

I'm sureI think soI'm guessing

If you add enough sodium hydroxide, any white precipitate will dissolve.

This is a common misconception

Only one white precipitate does that: aluminium hydroxide. Calcium hydroxide and magnesium hydroxide stay, however much sodium hydroxide you add. That difference is the whole reason the excess step exists. If every white precipitate dissolved, the step would tell you nothing.

A white precipitate that stays in excess means the solution contains magnesium ions.

This is a common misconception

It might, but calcium ions give exactly the same result: a white precipitate that stays in excess. Naming magnesium alone claims more than the test showed. The honest conclusion is 'calcium or magnesium ions'.

Iron gives the same colour precipitate whichever iron ion is in the solution.

This is a common misconception

Iron has two ions in this test, and they give two different colours. Iron(II) ions, Fe²⁺, give a green precipitate; iron(III) ions, Fe³⁺, give a brown one. The Roman numeral in the name is the charge, and the colour tells you which one you've got.

A white precipitate that stays in excess could be calcium or magnesium ions, and this test can't say which.

This matches the evidence

Exactly right. Both hydroxides are white and neither dissolves in excess, so the two ions give an identical result. Stopping at 'calcium or magnesium' isn't giving up. It's reading the result correctly.

Only aluminium hydroxide dissolves in excess. White-and-stays means calcium or magnesium. And iron's colour depends on its charge: green for iron(II), brown for iron(III).

Writing the equation

Let the charge do the counting

Iron(III) chloride solution, FeCl₃, reacts with sodium hydroxide solution, NaOH. A brown precipitate of iron(III) hydroxide forms, and sodium chloride, NaCl, stays dissolved. Write the balanced equation with state symbols.

Start with the charge on the metal ion. The (III) in iron(III) means the ion is Fe³⁺.
missing step

Which line is step 2?

Each hydroxide ion is OH⁻, so it takes three of them to cancel the 3+ charge. The precipitate is Fe(OH)₃.One hydroxide ion goes with each metal ion, so the precipitate is FeOH.The 3 belongs to the iron, so the precipitate is Fe₃OH.Iron's hydroxide is Fe(OH)₂, so the precipitate is Fe(OH)₂.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Add sodium hydroxide solution and watch the solid that forms. Its colour does most of the detective work.

What you need to know

Sodium hydroxide solution can be used to identify some metal ions (cations) from the precipitate it forms.
Aluminium, calcium and magnesium ions form white precipitates with sodium hydroxide solution. Only aluminium hydroxide dissolves in excess sodium hydroxide.
Copper(II) ions form a blue precipitate, iron(II) ions a green precipitate and iron(III) ions a brown precipitate.
A white precipitate that does not dissolve in excess means calcium or magnesium ions. This test alone can't tell which.
Balanced equations show how each insoluble hydroxide forms. The charge on the metal ion decides how many hydroxide ions it needs, and so how much sodium hydroxide.

The big picture

Sodium hydroxide solution can identify some metal ions from the precipitate it forms. Copper(II), iron(II) and iron(III) ions give blue, green and brown precipitates, so the colour alone names the ion. Aluminium, calcium and magnesium ions all give white precipitates, and only aluminium hydroxide dissolves in excess sodium hydroxide, so a white precipitate that stays means calcium or magnesium ions, which this test cannot tell apart. You can write balanced equations for forming the insoluble hydroxides, using the charge on the metal ion to decide how many hydroxide ions it needs.

Key points

1Colour first. Blue, green or brown names the ion straight away.

2White is the only colour that needs a second question: what happens in excess?

3Dissolves in excess: aluminium. Stays in excess: calcium or magnesium.

4Iron(II) is green and iron(III) is brown. Same metal, different charge, different colour.

5A metal hydroxide has one OH⁻ for each unit of charge on the metal ion. Mg²⁺ has a 2+ charge, so magnesium hydroxide is Mg(OH)₂.

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Worked example

Problem

Two bottles have lost their labels. One holds a solution of aluminium ions and the other a solution of magnesium ions. How can sodium hydroxide solution tell you which bottle is which?

⚠ Watch out

Writing 'magnesium ions' (or 'calcium ions') for a white precipitate that stays in excess. Both ions give that result, so the correct answer is 'calcium or magnesium ions'.

🧠

Memory hook

Colour first, excess second. Copper's blue; iron two is green; iron three is brown. White? Keep adding: if it vanishes, it's aluminium; if it stays, it's calcium or magnesium.

✓

Check yourself

Cover the page and redraw the test as a key. What do you look at first, when do you need the second step, and which ending holds two ions?

Flashcards

(10)

What is a precipitate?

An insoluble solid that forms in a solution.

In the sodium hydroxide test, what is the precipitate made of?

A metal hydroxide: the metal ions join with hydroxide ions (OH⁻) from the sodium hydroxide, and the hydroxide formed is insoluble.

Which three metal ions give a white precipitate with sodium hydroxide solution?

Aluminium (Al³⁺), calcium (Ca²⁺) and magnesium (Mg²⁺).

Which white precipitate dissolves in excess sodium hydroxide?

Only aluminium hydroxide.

What colour precipitate do copper(II) ions give with sodium hydroxide?

Blue (copper(II) hydroxide).

Iron(II) or iron(III): which gives a green precipitate and which a brown one?

Iron(II), Fe²⁺, gives green. Iron(III), Fe³⁺, gives brown.

Why can't sodium hydroxide tell calcium ions from magnesium ions?

Both give a white precipitate, and neither hydroxide dissolves in excess. Their results are identical.

When do you need to add sodium hydroxide in excess?

Only when the precipitate is white. Blue, green or brown has already named the ion.

How does the charge on a metal ion set the formula of its hydroxide?

One OH⁻ for each unit of positive charge: Cu²⁺ gives Cu(OH)₂, Fe³⁺ gives Fe(OH)₃.

Which state symbol does the precipitate get in an equation?

(s), because it's a solid, even though it forms in a solution.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Metal hydroxide precipitate tests

Walk the test: colour first, then excess only if it's white

Six ions, four questions

Four statements, one of them right

Let the charge do the counting

More AQA GCSE Chemistry topics