Qualitative analysis: flame colours of metal cations
Spray chloride salts into a flame: each cation emits its own colour and the palette Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺ becomes a qualitative test.
Goal
Build a colour table for seven cations and use it to identify an unknown salt; recognise the masking effect of sodium.
Apparatus and reagents
Bunsen burner, clean platinum or nichrome wire, concentrated HCl, chloride salts of Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺, cobalt glass.
Procedure
- Cycle the metal selector through all seven salts and record the flame colour of each.
- Identify Li⁺ (crimson), Na⁺ (intense yellow) and K⁺ (lilac) first — the most distinctive.
- Compare the reds of Ca²⁺ (brick) and Sr²⁺ (carmine), then the greens of Ba²⁺ and Cu²⁺.
- Explain why a sodium impurity can hide the lilac of potassium, and how a cobalt-blue glass reveals it.
What to observe
- Each metal gives a distinct, reproducible colour; the yellow of sodium dominates any mixture because its 589 nm line is extremely intense.
- Colours fade as the salt is consumed; a larger amount of sample prolongs the colour without changing it.
Explanation
Heat excites outer electrons to higher orbitals; when they fall back they emit light at fixed wavelengths — Na⁺ 589 nm, Sr²⁺ ≈ 606/640 nm, Cu²⁺ ≈ 510 nm. A flame is a crude emission spectrometer; real atomic emission (AES/ICP) resolves the same lines quantitatively.
History of the experiment
Chemists behind it
Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.