Methanol Flame Colors

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Atoms, Combustion, Elements and Periodic Table

Alternative titles: Rainbow Flames with Metal Salts

Summary

Metal salts are heated in burning methanol to produce vivid flame colors that correspond to electronic transitions in their ions.

Procedure

Clear a flame-resistant bench and place five heat-safe borosilicate Petri dishes on ceramic squares, spaced at least 8 cm apart.
Add 5–7 g of a different dry chloride salt to each dish: sodium chloride, strontium chloride, copper(II) chloride, lithium chloride, and potassium chloride.
Pipette about 7–10 mL methyl alcohol to each dish so the crystals are just wetted. Cap the alcohol bottle immediately and move it well away from the area.
Dim the lights. Ignite each dish with a long-neck lighter and observe the characteristic colors as the alcohol burns and dissolves the salts.
After observation, smother each dish with its lid or an inverted 600 mL beaker to extinguish.
Allow all glassware to cool completely before touching or repeating the display.

Links

Rainbow Flame! Coloured Fire Experiment! - Thoioi2:

The rainbow flame demonstration - Royal Society of Chemistry:

📄 Methyl Alcohol Flame Tests - Flinn Scientific: https://www.flinnsci.ca/api/library/Download/841f94471d9a4cabb2651a5c48f5dfd5

Variations

Prepare 1.0 M aqueous salt solutions and do splint flame tests at a Bunsen burner as a lower-risk alternative.
View flames through a simple spectroscope or diffraction grating to see line spectra, then estimate dominant wavelengths.
Add borax, barium chloride or calcium chloride in additional dishes to expand the color set.

Safety Precautions

Wear safety glasses.
Keep the methyl alcohol bottle capped and away from the demo once dispensing is finished. Do not add alcohol to a hot or lit dish.
Be aware that methyl alcohol flames can be nearly invisible. Confirm extinguishment by covering the dish; never blow on flames.
Work on a flame-resistant surface in good ventilation. Remove all other combustibles from the area.
Use only intact borosilicate glass Petri dishes. Do not use watch glasses.
Space dishes several inches apart to prevent fire spread. Have a fire extinguisher ready.
Some salts are harmful, check their material data sheets before handling.
Do not immediately repeat. Allow dishes and salts to cool to room temperature before any reset or disposal.

Questions to Consider

Which ion produced each observed flame color, and how reliably can color be used to identify an unknown?
How do electron excitations and relaxations in ions create discrete emission lines rather than a continuous spectrum?