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Gold is insoluble in nitric acid , but soluble in aqua regia to produce gold acid H[AuCl 4 ]
.
Au+HNO 3 +4HC1=H[AuCl 4 ]+NO+2H 2 O
After concentration by evaporation, yellow HAuCl 4 ·4H 2 O was precipitated
.
[AuCl .
4 ] - a square structure
.
[AuCl .
Au 2 O 3 (brown) is unstable and decomposes at about 160°C
.
2Au 2 O 3 =4Au+3O 2
The oxidation number of gold in the combined state is mainly +3
.
Compounds with oxidation number +1 are unstable and can easily be converted to compounds with oxidation number +3
The only Au(V) compound is AuF (red), which has poor stability and decomposes at 60°C
.
The stability of Au(Ⅲ) halide decreases as the halide ion radius increases
.
AuFs (orange-yellow) is stable, sublimates at 300°C but does not decompose; AuCl 3 (red) decomposes above 160°C; AuBrs (red-brown) decomposes at about 160°C
Gold reacts with chlorine at 200°C to obtain diamagnetic AuCl 3
.
No matter in the solid state or in the gas state, the compound is a dimer with a planar structure
AuCl 3 decomposes into AuCl and Cl 2 above 160°C
.
AuCl 3 =AuCl+Cl 2
Cs[AuCl 4 ] is partially decomposed to obtain black CsAuCl 3 , the structure of which is Cs 2 [AuCl 2 ][AuCl 4 ], Au is not in oxidation state +2, but oxidation state +1 and +3.
This structure supports CsAuCl 3 is the experimental result of diamagnetism
The halides of Au(I) are not very stable.
AuCI (yellow) decomposes at 289°C, AuBr (dark yellow) decomposes at 165°C, and Aul (yellow green) decomposes at 120°C