[junbumkim]

so ammonia (NH3) in solution forms NH₄OH,

so in a reaction such as NH₃ + ZnI₂,
the products or Zn(OH)₂ + NH4⁺ right?

does that mean, even before NH3 is added to ZnI₂, ammonia solution is in the form of NH₄OH ?

[Yggdrasil]

Here's how I would write the equations for the reaction:

NH_{3 (l)} + H₂O _(l) --> NH₄OH _(aq)

2NH₄OH _(aq) + ZnI_{2 (aq)} --> Zn(OH)_{2 (s)} + 2NH₄I _(aq)

with the net ionic equation for the second reaction being:

2OH^- _(aq) + Zn²⁺ _(aq) --> Zn(OH)_{2 (s)}

You could combine the first two reactions to write:

2NH_{3 (l)} + 2H₂O _(l) + ZnI_{2 (aq)} --> Zn(OH)_{2 (s)} + 2NH₄I _(aq)

but separating them into two equations makes it more clear what is happening.