ammoium hydroxide係唔係 soluble?

First of all, there is NO compound called ammonium hydroxide.
Ammonium hydroxide does not exist as a pure compound.

When a solution containing ammonium ions and hydroxide ions is vaporized to dryness, ammonia ions and hydroxide ions would decomposed to ammonia and water. Ammonium hydroxide would never exist as a pure compound.
NH4^+(aq) + OH^-(aq) → NH3(g) + H2O(g/l)

The term ammonium hydroxide appeared in Chemistry textbook 30 years ago. The term is used to describe the resulting solution of dissolving ammonia gas in water. Nowadays, such solution is known as ammonia solution instead.

係, 所有ammonium compound都係soluble in water