Revision notes for AQA GCSE Chemistry Metal hydroxides. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Metal hydroxides
What you'll learn
- What a precipitate is, and why metal hydroxides often form one.
- How sodium hydroxide solution is used to identify some metal ions.
- The precipitate colours for copper(II), iron(II), iron(III), aluminium, calcium and magnesium ions.
- How to write balanced equations for making insoluble metal hydroxides.
The big idea
In this topic, you use a simple chemical test to identify some metal ions in solution. The test is: add sodium hydroxide solution and observe what happens.
Definition
Cation
A cation is a positively charged ion. Metal ions are usually cations, for example Cu2+, Fe2+, Fe3+, Mg2+ and Al3+.
The word aqueous, shown by the state symbol (aq), means a substance is dissolved in water. In this test, the metal ions are in aqueous solution.
Sodium hydroxide solution
Sodium hydroxide, NaOH, is an alkali. An alkali is a soluble base that releases hydroxide ions, OH−, in water.
So when sodium hydroxide solution is added to a solution containing metal ions, the useful reacting particle is often the hydroxide ion:
NaOH(aq) contains Na+(aq) and OH−(aq)
The sodium ions usually stay dissolved in solution. The hydroxide ions can react with metal ions to form a metal hydroxide.
Definition
Precipitate
A precipitate is an insoluble solid that forms when two solutions react. In this topic, the precipitates are insoluble metal hydroxides.
Making metal hydroxides
A metal hydroxide is a compound made from a metal ion and hydroxide ions, OH−.
The formula depends on the charge of the metal ion:
- A 2+ ion needs two hydroxide ions: Cu(OH)2, Fe(OH)2, Mg(OH)2, Ca(OH)2
- A 3+ ion needs three hydroxide ions: Fe(OH)3, Al(OH)3
Key Idea
Charge balance
The total positive charge must balance the total negative charge. Hydroxide is OH−, so a 2+ metal ion needs two OH− ions.
Common Mistake
Forgetting the brackets
Write Mg(OH)2, not MgOH2. The brackets show that there are two whole hydroxide ions, meaning two O atoms and two H atoms.
Example
Writing a metal hydroxide formula
Work out the formula of aluminium hydroxide.
- Aluminium forms Al3+ ions, so the total positive charge is 3+.
- Each hydroxide ion is OH−, so three hydroxide ions are needed to give a total negative charge of 3−.
- Combine one Al3+ with three OH− ions, giving Al(OH)3.
The sodium hydroxide test
To carry out the test:
- Put a small amount of the unknown metal ion solution in a test tube.
- Add sodium hydroxide solution dropwise and shake gently.
- Observe the colour of any precipitate.
- If the precipitate is white, add excess sodium hydroxide solution and observe whether it dissolves.
Definition
Excess
Excess means more than enough to react with the substance being tested. Here, it means adding more sodium hydroxide solution after the precipitate has first formed.
The overall test path looks like this:
The observations you need to know
| Ion in solution | Observation when sodium hydroxide solution is added | Extra observation |
|---|---|---|
| Aluminium, Al3+ | White precipitate | Dissolves in excess sodium hydroxide solution |
| Calcium, Ca2+ | White precipitate | Does not dissolve in excess sodium hydroxide solution |
| Magnesium, Mg2+ | White precipitate | Does not dissolve in excess sodium hydroxide solution |
| Copper(II), Cu2+ | Blue precipitate | Colour identifies it for this topic |
| Iron(II), Fe2+ | Green precipitate | Colour identifies it for this topic |
| Iron(III), Fe3+ | Brown precipitate | Colour identifies it for this topic |
Why the colours matter
Some metal hydroxides have distinctive colours:
- Cu(OH)2(s) is blue
- Fe(OH)2(s) is green
- Fe(OH)3(s) is brown
These colours let you identify copper(II), iron(II) and iron(III) ions quickly.
The white precipitates are trickier because aluminium, calcium and magnesium ions all give white precipitates at first. That is why the test continues with excess sodium hydroxide solution.
Key Idea
White precipitates need a second step
If the precipitate is white, you cannot identify the ion from the first observation alone. Add excess sodium hydroxide solution: aluminium hydroxide dissolves, but calcium hydroxide and magnesium hydroxide do not.
Common Mistake
Claiming calcium and magnesium are fully distinguished
Sodium hydroxide solution alone separates aluminium from calcium/magnesium, but it does not clearly distinguish Ca2+ from Mg2+ in this test because both give white precipitates that remain in excess sodium hydroxide.
Example
Identifying a cation from observations
A solution forms a white precipitate when sodium hydroxide solution is added. The precipitate dissolves when excess sodium hydroxide solution is added. Identify the metal ion.
- A white precipitate rules out Cu2+, Fe2+ and Fe3+, because they give blue, green and brown precipitates.
- The possible ions are now Al3+, Ca2+ and Mg2+.
- The precipitate dissolves in excess sodium hydroxide solution, which is the result for aluminium ions only, so the ion is Al3+.
Writing equations for precipitate formation
You need to be able to write balanced equations for producing insoluble hydroxides.
A general pattern is:
metal salt solution + sodium hydroxide solution → metal hydroxide precipitate + sodium salt solution
For example:
CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)
Notice the state symbols:
- (aq) means dissolved in water
- (s) means solid
- The precipitate is the insoluble solid, so it has state symbol (s)
You can also write ionic equations. These show only the particles that actually change:
Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
The sodium ions and sulfate ions are not included because they stay dissolved and unchanged.
Tip
Fast formula check
For a 2+ metal ion, the hydroxide formula usually ends in (OH)2. For a 3+ metal ion, it usually ends in (OH)3.
Example
Writing a precipitation equation
Write a balanced equation for magnesium chloride solution reacting with sodium hydroxide solution.
- Magnesium ions are Mg2+, so the insoluble hydroxide formed is Mg(OH)2(s).
- The other product is sodium chloride, NaCl(aq), because the sodium ions pair with chloride ions in solution.
- Start with the word pattern: magnesium chloride + sodium hydroxide → magnesium hydroxide + sodium chloride.
- Write the formula equation: MgCl2(aq) + NaOH(aq) → Mg(OH)2(s) + NaCl(aq).
- Balance the equation by putting 2 in front of NaOH and NaCl: MgCl2(aq) + 2NaOH(aq) → Mg(OH)2(s) + 2NaCl(aq).
Key equations to recognise
You do not need to memorise every possible salt, but you should be comfortable with the pattern.
Examples include:
- CuCl2(aq) + 2NaOH(aq) → Cu(OH)2(s) + 2NaCl(aq)
- FeCl2(aq) + 2NaOH(aq) → Fe(OH)2(s) + 2NaCl(aq)
- FeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq)
- AlCl3(aq) + 3NaOH(aq) → Al(OH)3(s) + 3NaCl(aq)
- CaCl2(aq) + 2NaOH(aq) → Ca(OH)2(s) + 2NaCl(aq)
- MgCl2(aq) + 2NaOH(aq) → Mg(OH)2(s) + 2NaCl(aq)
For aluminium hydroxide dissolving in excess sodium hydroxide, you only need the observation for GCSE Chemistry. You are not expected to write the equation for the product formed in excess sodium hydroxide.
Practical notes
This is a test-tube reaction, so your observation is the important evidence.
Use clean equipment and add sodium hydroxide solution carefully. Sodium hydroxide can damage skin and eyes, so eye protection is needed in the lab.
Common Mistake
Observation words matter
Do not just write “goes cloudy” if a precipitate forms. In exam answers, say “a precipitate forms” and give its colour.
Exam technique
In the exam
- If sodium hydroxide is added and a coloured precipitate forms, match the colour: blue = Cu2+, green = Fe2+, brown = Fe3+.
- If a white precipitate forms, look for the excess sodium hydroxide result: dissolves = Al3+; remains = Ca2+ or Mg2+.
- In equations, balance the hydroxide ions and include state symbols, especially (s) for the metal hydroxide precipitate.
Self review
Check yourself
- What colour precipitate forms when sodium hydroxide solution is added to a solution containing Fe2+ ions?
- Why does a white precipitate not immediately identify the metal ion?
- Write a balanced equation for FeCl3(aq) reacting with NaOH(aq) to form Fe(OH)3(s).
Identification of ions by chemical and spectroscopic means (chemistry only)
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