Flame Tests and Metal Hydroxides

This lesson covers flame tests and the reactions of metal ions with sodium hydroxide solution to form coloured precipitates, as required by the AQA GCSE Chemistry specification (8.1.2). These tests are used to identify the metal ions present in a compound. You need to know the flame colours for specific metal ions and the precipitate colours produced when sodium hydroxide is added to solutions of metal ion compounds.

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Flame Tests

A flame test is used to identify certain metal ions in a compound by the characteristic colour they produce when heated in a flame.

How to Carry Out a Flame Test

1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a blue Bunsen burner flame until it produces no colour.
2. Dip the clean wire loop into the concentrated hydrochloric acid again.
3. Dip the wire loop into the sample (solid or solution) to be tested.
4. Hold the wire loop in the blue Bunsen burner flame.
5. Observe the colour of the flame produced and compare with known flame colours.

Why Clean the Wire?

If the wire is not cleaned, residues from a previous test could contaminate the new sample, giving a false colour. The wire must be cleaned until it produces no colour in the flame before testing a new sample.

Exam Tip: When describing the flame test method, always include the cleaning step. State that the wire loop is dipped in concentrated hydrochloric acid and heated in a blue flame until no colour is produced. This shows the examiner you understand the importance of avoiding contamination.

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Flame Test Colours

You must memorise these flame colours for the following metal ions:

Metal Ion	Symbol	Flame Colour
Lithium	Li\u207a	Crimson (dark red)
Sodium	Na\u207a	Yellow
Potassium	K\u207a	Lilac (light purple)
Calcium	Ca\u00b2\u207a	Orange-red
Copper	Cu\u00b2\u207a	Green

Memory Aid

A helpful way to remember the flame colours:

Metal	Colour	Memory Aid
Lithium	Crimson	"Li-Cri" — Lithium Crimson
Natrium (Sodium)	Yellow	Sodium streetlights glow yellow
K (Potassium)	Lilac	"K for the King's Lilac robes"
Calcium	Orange-red	"Ca-Orange" — similar starting sounds
Cu (Copper)	Green	Copper roofs turn green (verdigris)

Exam Tip: The most commonly confused flame colours are lithium (crimson) and calcium (orange-red). Crimson is a deep, dark red; orange-red is lighter and more orange. If the question gives you options, choose carefully. Also, sodium's yellow flame is very strong and can mask other colours — this is why potassium's lilac flame is sometimes hard to see.

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Flame Test Identification Flowchart

graph TD
    A["Perform Flame Test"] --> B{"What colour is the flame?"}
    B -->|Crimson / dark red| C["Lithium ion Li+"]
    B -->|Yellow| D["Sodium ion Na+"]
    B -->|Lilac| E["Potassium ion K+"]
    B -->|Orange-red| F["Calcium ion Ca2+"]
    B -->|Green| G["Copper ion Cu2+"]

    style A fill:#e74c3c,color:#fff
    style C fill:#8b0000,color:#fff
    style D fill:#f1c40f,color:#2c3e50
    style E fill:#9b59b6,color:#fff
    style F fill:#e67e22,color:#fff
    style G fill:#27ae60,color:#fff

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Metal Hydroxide Precipitates

When a few drops of sodium hydroxide solution (NaOH) are added to a solution containing certain metal ions, a coloured precipitate (an insoluble metal hydroxide) is formed. The colour of the precipitate identifies the metal ion.

How to Carry Out the Test

1. Place a small amount of the solution to be tested in a test tube.
2. Add a few drops of sodium hydroxide solution (NaOH).
3. Observe the colour of the precipitate formed.
4. Compare the colour with known results.

Metal Hydroxide Precipitate Colours

Metal Ion	Symbol	Colour of Precipitate	Metal Hydroxide Formed
Calcium	Ca\u00b2\u207a	White	Ca(OH)\u2082
Magnesium	Mg\u00b2\u207a	White	Mg(OH)\u2082
Aluminium	Al\u00b3\u207a	White (dissolves in excess NaOH)	Al(OH)\u2083
Copper(II)	Cu\u00b2\u207a	Blue	Cu(OH)\u2082
Iron(II)	Fe\u00b2\u207a	Green (sometimes called "dirty green")	Fe(OH)\u2082
Iron(III)	Fe\u00b3\u207a	Brown (rust-coloured)	Fe(OH)\u2083

Distinguishing the Three White Precipitates

Three metal ions produce white precipitates with NaOH, so you need to distinguish between them:

Metal Ion	Precipitate Colour	Distinguishing Test
Ca\u00b2\u207a	White	Does NOT dissolve in excess NaOH. Use flame test (orange-red)
Mg\u00b2\u207a	White	Does NOT dissolve in excess NaOH. No distinctive flame colour
Al\u00b3\u207a	White	Dissolves in excess NaOH to form a colourless solution

Exam Tip: The key to distinguishing aluminium from calcium and magnesium is that aluminium hydroxide dissolves in excess NaOH. If the question says the white precipitate dissolves when more NaOH is added, the metal ion is Al\u00b3\u207a. This is because aluminium hydroxide is amphoteric — it can react with both acids and bases.

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Ionic Equations for Precipitate Formation

You should be able to write ionic equations for these precipitation reactions:

Reaction	Ionic Equation
Copper(II) ions + NaOH	Cu\u00b2\u207a(aq) + 2OH\u207b(aq) \u2192 Cu(OH)\u2082(s)
Iron(II) ions + NaOH	Fe\u00b2\u207a(aq) + 2OH\u207b(aq) \u2192 Fe(OH)\u2082(s)
Iron(III) ions + NaOH	Fe\u00b3\u207a(aq) + 3OH\u207b(aq) \u2192 Fe(OH)\u2083(s)
Aluminium ions + NaOH	Al\u00b3\u207a(aq) + 3OH\u207b(aq) \u2192 Al(OH)\u2083(s)
Calcium ions + NaOH	Ca\u00b2\u207a(aq) + 2OH\u207b(aq) \u2192 Ca(OH)\u2082(s)
Magnesium ions + NaOH	Mg\u00b2\u207a(aq) + 2OH\u207b(aq) \u2192 Mg(OH)\u2082(s)

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Combining Flame Tests and NaOH Tests

In practice, you may need to use both tests together to identify a metal ion:

1. Perform a flame test first — this can quickly identify Li\u207a, Na\u207a, K\u207a, Ca\u00b2\u207a, or Cu\u00b2\u207a.
2. If the flame test is inconclusive (e.g., no colour), use the NaOH precipitate test to identify Mg\u00b2\u207a, Al\u00b3\u207a, Fe\u00b2\u207a, or Fe\u00b3\u207a.
3. If a white precipitate forms, add excess NaOH to check if it dissolves (Al\u00b3\u207a) or remains (Ca\u00b2\u207a or Mg\u00b2\u207a).

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Summary