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Stories About Chemistry

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79. Light and Colour

On each major Soviet holiday you hear the Moscow Radio announcer say: “Order of the Minister of Defence... ‘In Honour of... I order a salute shall be fired in the capital of our country, Moscow, in the capitals of the Union Republics and in the Hero cities’...”

The sky is magnificent during the salute with yellow, green, and red lights flashing through it under the thunder of the salvoes.

The tradition of celebrating holidays by salvoes and fireworks is very old. The art of pyrotechnics was known in China as far back as two thousand B.C. But only comparatively recently did it occur to scientists to utilize coloured flames for chemical analysis.

Just over a hundred years ago the German physicist Kirchhoff noticed that the salts of different metals coloured a colourless gas-burner flame differently. Sodium salts make the flame yellow, calcium salts colour it carmine-red, barium salts—green, etc.

Kirchhoff soon realized that this offered a quick and reliable method of detecting chemical elements in substances. However, his glee was premature. All was well as long as pure salts were used. But if salts of, say, sodium and potassium were mixed, the violet colour of potassium was practically indiscernible against the background of the bright yellow burner flame (due to the presence of sodium).

The physicist Bunsen came to the aid of the chemist Kirchhoff. He suggested examining the burner flame with the mixture of salts introduced into it through a special instrument called a spectroscope. The main element of this instrument is a prism which resolves the white light passed through in into a spectrum, i.e., into its components. The name “spectroscope” means “to observe a spectrum.”

The idea was a great success. Unlike other sources of light, the gas-burner flame into which the salt under test was introduced, gave a linear spectrum instead of a continuous one, and the lines of the spectrum were always in strictly constant positions Thus, a sodium salt introduced into the flame caused two closely spaced intense yellow lines to appear in the spectrum. Potassium salts gave rise to one red and two violet lines, etc.

Kirchhoff and Bunsen found that the lines of any particular chemical element always appeared in exactly the same positions in the spectrum. No matter whether sodium was introduced into the flame as the chloride, the sulphate, the carbonate, or the nitrate, the positions of the sodium lines were always the same. Even mixing the sodium salts with other salts, say, of potassium, copper, iron, strontium, or barium, did not affect the positions of the sodium lines.

Inspired by their discovery Kirchhoff and Bunsen worked indefatigably. They tested a very large number of elements and compounds “in the flame”. After some time they drew up a list of the chemical elements with the characteristics of their lines in the spectrum. Now scientists could unerringly analyse many complex mixtures of substances.

Thus was born spectroscopic or spectrum analysis. It proved not only an excellent method for determining various known chemical elements qualitatively in mixtures. It also helped to discover new elements: rubidium, cesium, indium, and gallium. And when it was found that the intensity (brightness) of the lines depends on the amount of substance present in the mixture, spectrum analysis took up an honourable position among quantitative methods.


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by Ian Ellis
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