Sheds light on an elemental colour

10th March 2006 at 00:00
The bright yellow street lamps used on every major road rely on glowing sodium vapour as their light source. This element was discovered to be present in the Sun by Josef von Fraunhofer in 1814, who made a detailed study of sunlight. It was Fraunhofer who first suspected a connection between the lines in the solar spectrum and the yellow glow of sodium on earth.

Children may have noticed the same colour when a saucepan of boiling salted water overflows on to a gas flame. Common salt is a sodium compound, sodium chloride. When the salt is vaporised in a flame it gives the familiar yellow colour. We can use this effect in flame tests to identify particular elements by their flame colours. Sodium is a good choice since its flame colour is very strong and even small amounts will impart a recognisable colour to a gas flame.

For flame tests, pupils will need to use a support material that does not itself produce a flame colour. Possible choices include pencil leads which are made of graphite or metal wires. The best wire is unreactive platinum but cheaper alternatives such as nichrome will do.

For test samples try common salt, sea salt, bicarbonate of soda and powders scraped from the surface of crisps and pretzels. Locate the hottest part of the gas flame by slowly lowering a horizontal test wire into the flame from the top. It will glow red hot in the hottest region of the flame. Heat the wire or pencil lead in the flame to clean it, dip it into the sample and return to the hottest part of the flame. A flash of yellow confirms the presence of a sodium compound.

This investigation can be extended. Lewis Carroll's Alice famously went swimming in a pool of her own tears. Tears and sweat contain sodium chloride and give a positive flame test. Even rubbing a clean test wire with your fingers transfers enough salt to give a flame colour. Many foods contain salt, and an ordinary glass rod, soda glass, also turns the flame yellow.

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