Silver iodide
Other names	Silver(I) iodide
Identifiers
CAS number	7783-96-2
Properties
Molecular formula	AgI
Molar mass	234.773 g/mol
Appearance	yellow, crystalline solid
Density	5.675 g/cm3, solid
Melting point	552°C
Boiling point	1506°C
Solubility in water	3x10-7g/100mL (20 °C)
Thermochemistry
Std enthalpy of formation ΔfHo298	-62.4 kJ/mol at 1 atm
Hazards
EU classification	not listed
NFPA 704	0 2 0
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Silver iodide (AgI) is a chemical compound used in photography and as an antiseptic in medicine. Silver iodide is highly insoluble in water and has a crystalline structure similar to that of ice, allowing it to induce freezing (heterogeneous nucleation) in cloud seeding for the purpose of rainmaking.

The crystalline structure adopted by silver iodide changes with temperature. The following phases are known:¹

• Up to 420K (147 °C), AgI exists in the β-phase, which has a wurtzite structure. It is known as the mineral iodargyrite.
• Above 420K (147 °C), AgI undergoes a transition to the α-phase, which has a body-centered cubic structure and has the silver ions distributed randomly between 2-, 3-, and 4-coordinate sites.
• A metastable γ-phase also exists below 420K, which has a zinc blende structure.

Silver iodide as a fast ion conductor

The transition between the β and α forms represents the melting of the silver (cation) sublattice. The entropy of fusion (melting) for α-AgI is approximately half that for sodium chloride (a typical ionic solid). This can be rationalised by noting that the AgI crystalline lattice has essentially already partly melted in the transition between α and β forms. Adding the entropy of transition from α-AgI to β-AgI to the entropy of fusion gives a value that is much closer to the entropy of fusion for sodium chloride.

References

In Media

• In the television series The Batman, Poison Ivy used silver iodide to create rain so that she could populate Gotham City with her new breed of plants that assisted her in taking over the city.

• In the anime and manga One Piece a green powder similar to silver iodide called Dance Powder was used to make rain when burnt and its use in the desertic land of Arabasta caused a war.

External links

Wikimedia Commons has media related to:

Silver iodide

• A commercial site (Vista Chemicals) regarding silver iodide

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