A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining. Silver articles turn black when kept in the open for a few days because of ________.

Hint: Silver in presence of oxygen present in forms silver oxide. Also, in presence of oxygen and hydrogen sulphide present in the air, it forms a layer of silver sulphide. Both silver sulphide and silver oxide are black in colour. Toothpaste contains calcium carbonate, aluminium hydroxide which can remove a layer of silver sulphide.

Complete step by step solution:
-Silver in presence of air, reacts with the oxygen present in the air, a layer of silver oxide is formed on the articles. Silver oxide is black in colour. The silver undergoes oxidation in presence of air.
- Finally, after sufficient time, when silver reacts with oxygen and hydrogen sulphide present in the air, it forms Silver sulphide. Silver sulphide is also black in colour. This process is termed as tarnishing of silver or corrosion.
$4Ag+{{O}_{2}}+{{H}_{2}}S\to 2A{{g}_{2}}S+2{{H}_{2}}O$
-Tarnishing is limited to outer few layers.
-Toothpaste contains substances like calcium carbonate, aluminium hydroxide which can remove the layer of silver sulphide.
\[3A{{g}_{2}}S+2Al\to 6Ag+A{{l}_{2}}{{S}_{3}}\]
Silver stars shining after rubbing with toothpaste. Using hot water and sodium bicarbonate the cleaning process can be improved.
-Tarnishing can be prevented by applying a thin coat of polish.
-It can be removed by rubbing the surface with sandpaper.
-silverwares can be wrapped in a piece of aluminium foil in a pot of boiling water with a small amount of baking soda.
Silver articles turn black when kept in the open for a few days because of the formation of the layer of silver sulphide.

Note: Calcium carbonate is a gentler abrasive present in toothpaste. It is used to clean tarnished silver as it does not damage or scratch silver and does not leave unwanted residues. Tarnish is a thin layer of corrosion which also forms over copper, brass, aluminium etc. tarnishing is a surface phenomenon.