Why does nitric oxide become broen when released inn air?

Nitric oxide becomes brown when released in air. Explain why ?

Metals lose lustre when placed in air why?

Why does iron become passive when dipped in conc. HNO_3 ?

Why does food chain become short?

Why does a metallic piece become very hot, when it is surrounded by a coil carrying high frequency alternating current?

Why copper becomes green if left open in air ?