Gas burner flame colors

Written by julie durr
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Gas burner flame colors
The hottest part of a flame is the tip of the inner core flame. (Nick Koudis/Photodisc/Getty Images)

In a simple flame test, certain ionic compounds introduced into a gas-heated Bunsen burner flame emit characteristic flame colours. The colours emitted aid in the identification of metallic ions in a salt compound. The heat of the flame provides energy to the atom or ion's electrons. The electrons absorb the heat energy and enter an excited energy state. However, the electrons quickly return to their ground state. As they do, the electrons of the metallic ions or atoms lose energy. This released energy is in the visible light spectrum range as their characteristic colours.

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Blue

To conduct a flame test, dip a clean loop or platinum ring into the analyte. Insert the loop into the hottest part of the flame, which is the tip of the inner core flame. The flame of the Bunsen burner when hot is blue. A cooler flame with too little air to gas ratio is yellow. Analyse and document any flame colours emitted. Halide copper compounds, selenium, indium and arsenic impart a blue colour. Selenium has a characteristic odour of rotting radishes and is an azure blue colour. Likewise, arsenic has a characteristic odour of garlic.

Red

Lithium minerals, strontium carbonates and sulphates emit a crimson red colour when ignited. Calcium imparts a yellowish-orange reddish colour.

Yellow

Sodium compounds such as sodium chloride and sodium bromide impart an intense bright yellow to orange-yellow colour. Iron imparts a golden yellow colour when introduced into a flame.

Violet-Purple

Violet and violet-purple flame colours correlate with potassium, rubidium and caesium. Sodium with a prominent yellow can mask these colours.

Green

Bright bluish-green streaks in the flame indicate the presence of zinc. Copper also imparts a bright green colour and may appear white or emerald green. A pale green flame colour is indicative of tellurium, antimony, manganese and lead, and is not decisive. Manganese shoots out incandescent flashes. A pale bluish-green, which is also not decisive, is phosphorus. A yellowish-green flame correlates with molybdenum in the oxide or sulphide forms. Other compounds that emit a greenish flame are boron, barium and thallium compounds.

No Color

Magnesium, aluminium and cobalt do not have a characteristic flame colour. However, cobalt when introduced to the flame shoots out white, incandescent flashes with the burning gas. Cobalt glass acts as a filter to improve observations and results. The pale violet flame produced by potassium viewed through cobalt glass, appears reddish.

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