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QUESTION CORNER
Turning black in colour
PHOTO S. THANTHONI
Why does a silver plate turn black in colour when a boiled egg is placed on it?
Subhadra
Hyderabad
Silver is a well known precious metal used in jewellery and coinage. Its scientific uses are extensive.
Silver has the highest electrical and thermal conductives of all known materials. Even though it is one among the noble metals, its nobility gets tarnished when it comes into contact with certain oxidising substances such as halogens, sulphur, hydrogen sulphide, nitric acid, etc.
A shining silver metal, when in contact with a boiled egg, turns grey or black. A boiled egg has certain levels of hydrogen sulphide generated while boiling due to hydrolysis of some sulphur proteins in the albumin and yolk.
This hydrogen sulphide removes an electron from each silver atom when it comes into contact with silver while hydrogen gas and silver sulphide are formed.
During boiling of eggs, some trace amounts of sulphur are also obtained and are hidden in colloidal state in the wetness of the boiled eggs.
Elemental sulphur, especially in decimated state, like in colloidal form, also oxidises silver to give silver sulphide. In other words, the hydrogen sulphide and elemental sulphur present in boiled eggs oxidise a portion of metallic silver to form a layer of silver sulphide.
The natural colour of silver sulphide is dirty black. Hence, a silver plate turn black when a boiled egg is placed on it.
PROF. A. RAMACHANDRAIAH
Department of Chemistry
National Institute of Technology, Warangal
Andhra Pradesh
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