Question

What happens when:
A Silver article is kept in the open for a few days
Copper nitrate undergoes heating
Aluminium is added to dilute hydrochloric acid

Answer 1

Heya my INDIAN friend!!
Hope you are doing good!

So I'll give your answer now:

When a silver article is kept in open for a few days, it gets tarnished and turns black.
Why?
Because it reacts with gaseous pollutants of air like hydrogen sulphide and forms a thin black layer of silver sulphide (Ag2S) on its surface.

When you heat copper nitrate, a black substance (CuO) is formed, a reddish brown gas (NO2) evolves, and a colourless gas (O2) evolves which rekindles a glowing splinter.
Reaction:
2Cu(NO3)2 ----∆----> 2CuO + 4NO2 + O2

When aluminium is added to dilute HCl, a colourless gas (H2) evolves, which burns with a pop sound in air.
Reaction:
2Al + 6HCl -----> 2AlCl3 + 3H2

Thanks!