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4.3.2Electrochemistry

Flame Tests — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 4 of 20 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Method of the Flame Test

4.3.2 Flame Tests

The loop must be clean before each test, or a previous ion can give a false colour.

1
Dip a clean wire loop, usually nichrome or platinum, into dilute hydrochloric acid to remove any ions left from earlier tests.
2
Hold the loop in the blue flame of a Bunsen burner until it produces no colour, which shows the loop is clean.
3
Dip the loop into the solid or solution being tested so that some of the sample sticks to it.
4
Place the loop at the edge of the blue flame and record the colour observed before cleaning the loop again.

Part 2 of 5Flame Colours of Metal Ions

Each metal ion emits a characteristic colour when it is heated in the flame.

Lithium, Li⁺
  • Crimson flame, which is a deep red colour.
Sodium, Na⁺
  • Yellow flame, which is very intense and can hide other colours if the loop is dirty.
Potassium, K⁺
  • Lilac flame, which is a pale purple colour.
Calcium, Ca²⁺
  • Red flame, which is a brick red colour that is different from lithium.
Barium, Ba²⁺
  • Green flame, which is a pale apple-green colour.
Copper(II), Cu²⁺
  • Blue-green flame, which is a useful colour for identifying copper compounds.

Part 3 of 5Why the Method Matters

Why the Method Matters
Blue flame
  • The blue flame is hot and does not emit its own colour, so it does not mask the colour of the metal ion.
Dilute acid
  • Dipping the loop in acid first removes ions left from a previous test, which could otherwise mix with the new colour.
Masking
  • A strong yellow from sodium can hide other colours, so the loop must be cleaned between every test.
Identification
  • An unknown is identified by matching its flame colour to a known colour in the table.

Part 4 of 5Worked Example

Question

  • Five samples, A to E, gave these flame colours: A lilac, B yellow, C crimson, D green, E red.
  • Identify the metal ion in each sample.

  1. Match lilac to potassium, so sample A contains K⁺.
  2. Match yellow to sodium, so sample B contains Na⁺.
  3. Match crimson to lithium, so sample C contains Li⁺;
    • match green to barium for D, Ba²⁺, and red to calcium for E, Ca²⁺.

Answer A: K⁺; B: Na⁺; C: Li⁺; D: Ba²⁺; E: Ca²⁺

Part 5 of 5Exam Checklist & Tips

4.3.2 – Key Points for the Exam

  • Flame test A test in which a metal ion gives a characteristic colour when a sample is heated in a flame.
  • Nichrome wire An unreactive metal wire used to hold a sample in the flame without adding its own colour.
  • Blue flame A hot, non-luminous flame from a Bunsen burner, used to carry out flame tests.
  • Masking When one strong colour hides another colour in the same flame.

Common Mistakes

  • Using a yellow, luminous flame, which can hide the colours of other metal ions.
  • Failing to clean the wire in dilute acid between tests, so the colours mix or mask each other.
  • Confusing the flame colours of lithium (crimson) and barium (green) or calcium (red).
  • Identifying the anion in the sample from a flame test, when flame tests identify only metal cations.

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