Flame tests: identifying metal ions
Dip, flame, identify. Use the colours metal ions give a Bunsen flame to identify unknown salts — the same physics that puts colour in fireworks.
WHAT YOU'LL LEARN
- ✓The characteristic flame colours of Li⁺, Na⁺, K⁺, Ca²⁺ and Cu²⁺
- ✓Why heated electrons emit specific colours (energy levels, not magic)
- ✓Clean technique: why a contaminated wire ruins everything
ON YOUR BENCH
- □Bunsen burner
- □Nichrome wire loop
- □Concentrated hydrochloric acid (for cleaning the loop)
- □Watch glasses with salt samples: LiCl, NaCl, KCl, CaCl₂, CuCl₂
- □Two unknowns, labelled A and B
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Set a roaring flame
Open the air hole fully for a blue, non-luminous flame. A yellow safety flame is itself sodium-coloured and will mask every result.
- 02
Clean the loop
Dip the nichrome loop in concentrated HCl, then hold it in the flame until it adds no colour. Repeat until the flame stays blue — this is the step everyone rushes and regrets.
- 03
Test each known salt
Dip the clean loop in HCl, touch it to the salt so a few crystals stick, and hold it at the edge of the blue flame. Record the colour immediately — some flashes are brief.
- 04
Clean between every sample
Re-clean the loop with HCl and flame until colourless before each new salt. Sodium contamination is notorious: a trace of NaCl swamps everything with yellow.
- 05
Identify the unknowns
Test unknowns A and B with the same technique and match their colours against your recorded knowns.
Expected flame colours
- →Persistent yellow that you can't clean away means sodium contamination, not sodium in your sample.
- →Potassium's lilac is faint — view it through blue cobalt glass to filter out any sodium yellow.
| Ion | Flame colour |
|---|---|
| Lithium, Li⁺ | Crimson red |
| Sodium, Na⁺ | Intense yellow-orange |
| Potassium, K⁺ | Lilac |
| Calcium, Ca²⁺ | Brick red |
| Copper, Cu²⁺ | Blue-green |
⚠ BEFORE YOU START
- •Concentrated HCl is corrosive — keep it in the fume-cupboard side of the bench and handle with the loop only.
- •Tie back hair and secure loose sleeves before lighting the Bunsen.
- •The wire loop stays hot long after it stops glowing. Rest it on the heatproof mat, not the bench.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.