Chemical Reactivity of Silver
Silver (Ag) is a lustrous, white transition metal known for its excellent electrical and thermal conductivity. Positioned in Group 11 and Period 5 of the periodic table, it is considered a relatively noble metal due to its low chemical reactivity compared to many other metals.
Reaction with Water
Silver does not react with water or steam under normal environmental conditions, nor does it react at elevated temperatures. This stability in aqueous environments is one reason for its historical use in coinage and jewellery.
Reaction with Air
Pure silver does not react with oxygen in clean air, even when heated. However, silver readily tarnishes when exposed to air containing trace amounts of hydrogen sulfide (H₂S) gas. Hydrogen sulfide is a natural component of polluted air, often resulting from industrial emissions or the decay of organic matter. The reaction forms a thin, black layer of silver sulfide (Ag₂S) on the surface of the silver.
$$ 2\text{Ag(s)} + \text{H}_2\text{S(g)} \rightarrow \text{Ag}_2\text{S(s)} + \text{H}_2\text{ (g)} $$
This phenomenon is commonly observed on silver ornaments, utensils, and religious artifacts like diyas or idols in Indian households, which gradually lose their shine and develop a dark coating over time.
Toxicity
Elemental silver is generally considered non-toxic. However, soluble silver compounds, such as silver nitrate (AgNO₃), can be toxic if ingested or absorbed in large quantities. Prolonged or excessive exposure to silver compounds can lead to a condition called argyria, where silver particles deposit in the skin and mucous membranes, causing a permanent grey or bluish-grey discoloration. This condition is primarily cosmetic and typically not life-threatening.
Radioactivity
Naturally occurring silver is not radioactive. It consists of two stable isotopes: Silver-107 ($^{107}\text{Ag}$) and Silver-109 ($^{109}\text{Ag}$). Various radioactive isotopes of silver have been synthesized in laboratories for scientific and medical applications, but these are not found naturally.
Flammability
Silver is a metal and is not flammable. It does not burn in air or react combustibly under normal conditions. It possesses a high melting point of 961.8 °C and a boiling point of 2162 °C.
Famous Chemical Reaction: Formation of Silver Halides
One of the most well-known chemical reactions involving silver is its interaction with halogens, particularly the formation of silver halides like silver chloride (AgCl), silver bromide (AgBr), and silver iodide (AgI). These compounds are characteristically light-sensitive.
A prominent example is the precipitation reaction that occurs when a solution containing silver ions (Ag⁺) is mixed with a solution containing chloride ions (Cl⁻), such as sodium chloride (common salt). A white precipitate of silver chloride forms immediately:
$$ \text{AgNO}_3\text{(aq)} + \text{NaCl(aq)} \rightarrow \text{AgCl(s)} + \text{NaNO}_3\text{(aq)} $$
Or, in its ionic form:
$$ \text{Ag}^+\text{(aq)} + \text{Cl}^-\text{(aq)} \rightarrow \text{AgCl(s)} $$
This reaction, particularly with silver bromide, formed the fundamental basis of traditional black-and-white photography. Silver halide crystals embedded in photographic film or paper undergo chemical changes upon exposure to light, forming a latent image that can then be developed into a visible photograph.