Flame tests
Lajoy Tucker
Teacher
Contents
What Are Flame Tests?
Flame tests are a simple method used to identify metal ions (cations) in compounds.
Many metal ions produce distinctive flame colours when heated strongly.
These colours can be used as a quick, qualitative test.
How a Flame Test Works
1. A nichrome wire loop is cleaned in hydrochloric acid and then placed in a blue Bunsen flame until the flame does not change colour
2. The loop is dipped into the sample (solid or solution).
3. The loop is held in the flame again.
4. The colour observed indicates which metal ion is present.
The flame colour is produced when electrons in the metal ions absorb heat, jump to higher energy levels, and then release light when they fall back again.
Flame Colours for Common Metal Ions
Metal Ion (Cation) | Flame Colour |
Lithium (Li⁺) | Crimson red |
Sodium (Na⁺) | Yellow |
Potassium (K⁺) | Lilac |
Calcium (Ca²⁺) | Orange-red |
Copper (Cu²⁺) | Green |
Mixed Samples
Cleaning the wire with hydrochloric acid is important to remove any other ions that may be present which may make the flame colour difficult to interpret.
If a sample contains more than one metal ion, some flame colours may be hidden (masked) or unclear.
Further tests may be needed (e.g. flame emission spectroscopy or precipitation reactions using sodium hydroxide solution).
Question 1
A flame test on a sample produces a crimson flame. Which ion is present?
Answer:
Lithium (Li⁺).
Question 2
A student tests a sample containing sodium ions and calcium ions. Only a bright yellow flame is observed.
Explain why.
Answer:
Sodium’s very intense yellow flame masks the orange-red flame of calcium