Science 7–10 · Year 10

Flame tests: identifying metals by the colour of their light

Periodic table and atomic structure

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Each element gives out light of its own colours when its electrons drop back to lower energy levels, so a flame colour identifies the metal in a compound.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Bunsen flame; splints can flare
copper(II) chloride is harmful
lithium chloride is an irritant

Controls

eye protection
splints held with the wet end only in the flame and dropped into a beaker of water afterwards
hair tied back, bench clear

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson. The wooden-splint method avoids the nichrome wire and concentrated acid of older methods.

What you need

wooden splints soaked in water well before the lesson, one per test
chloride solutions of lithium, sodium, potassium, calcium and copper(II), 0.5 mol/L, a few mL each in labelled test tubes (the RSC method)
Bunsen burner set to a roaring blue flame, heat-resistant mat, blue cobalt glass (optional, to view potassium)
an unknown chloride solution issued by the teacher; a hand-held spectroscope (optional)

How to do it

Dip the soaked end of a splint into the first solution and hold it at the edge of the blue flame; record the colour at once, before the wood itself burns.
Use a fresh splint for each solution. View the potassium flame through blue cobalt glass to remove any sodium yellow.
Test the unknown and match its colour to the reference set.
Explain each colour with a diagram of an electron gaining energy in the flame and releasing it as light of a fixed colour.
Optional: view the sodium and lithium flames through a hand-held spectroscope and sketch the bright lines, the evidence that led to the shell model of the atom.

What you should see

Lithium gives crimson-red, sodium a strong yellow-orange, potassium lilac (clearest through cobalt glass), calcium orange-red and copper(II) green to blue-green. The sodium colour is so intense that any sodium contamination masks other colours, which is why fresh splints matter. Through a spectroscope the sodium and lithium flames each show a few bright lines rather than a continuous rainbow, and sodium's yellow is a close pair of lines near 589 nm; calcium and copper(II) flames give broader bands, and the glowing splint adds a faint continuous background. The learner knows it worked when the unknown is matched to one reference and the colour is seen again on a second splint.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The colour comes from the salt burning like a fuel.
All metals give the same colour because all flames are yellow.
The colour belongs to the chloride, not the metal.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-PRT-01 SC5-WS-01
Australian Curriculum v9AC9S10U06AC9S10H02

Sources

The pages the author read to write this activity.

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