Silver Reaction Causes Black Tarnish

Silver objects often turn black due to a chemical process called tarnishing. This occurs when the metal undergoes a chemical reaction with specific substances present in its environment.

Identifying the Substance Causing Blackening

The primary cause of silver tarnishing is its reaction with sulphur or sulphur-containing compounds. Hydrogen sulfide ($H_2S$), a gas commonly found in the atmosphere, is a frequent culprit.

The Chemical Reaction

When silver ($Ag$) comes into contact with sulphur ($S$), they react to form silver sulfide ($Ag_2S$). This compound is insoluble in water and possesses a distinct black color.

The chemical equation for this reaction can be represented as:

$2Ag(s) + S(s) \rightarrow Ag_2S(s)$

In the presence of hydrogen sulfide and oxygen, the reaction is more complex but still results in the formation of black silver sulfide.

Analysis of Options

• Magnesium (Mg), Zinc (Zn), and Carbon (C) do not directly cause silver to form a black tarnish through common reactions.
• Sulphur is the element that chemically reacts with silver to produce the black coating.

Therefore, sulphur is the substance with which silver reacts to turn black.