Metal +dilute/conc. Nitric acid=??

Nitric acid 係oxidizing agent, dilute 會reduce 做NO (g), conc. 會reduce 做NO2 (g).

Dilute HNO3

Reduction half equation : NO3- (aq) + 4H+ (aq) + 3e- ==> NO(g) + 2H2O (l)
Oxidation half equation : Cu (s) ==> Cu2+ (aq) + 2e-
Overall equation : 2NO3- (aq) + 8H+ (aq) + 3Cu (s) ==> 2NO(g) + 4H2O (l) + 3Cu2+ (aq)

如果同copper react, copper 會被oxidize 做Cu2+ ions.
同時間test tube 口會有brown fumes, 係因為NO (g) 同Oxygen further react, forming NO2(g), 並不是nitric acid reduced to NO2 (g).

Concentrated HNO3

Reduction half equation: NO3- (aq) + 2H+ (aq) + e- ==> NO2 (g) + H2O(l)
Oxidation half equation : Cu (s) ==> Cu2+ (aq) + 2e-
Overall equation : 2NO3- (aq) + 4H+ (aq) + Cu (s) ==> 2NO2 (g) + 2H2O(l) + Cu2+ (aq)

同樣地, copper 都係被oxidize 做Cu2+ ions.
今次test tube 口D brown fumes 就真係nitric acid 直接reduce 成為NO2(g).

[如果都係唔明可以睇番Chem. 書, Redox reactions o個part 應該有寫]