Answer:
There is reaction but it's a series of equilibria we have to consider, not a simple complete change.
First the ions separate:
NH4+Cl-(s) = NH4+(aq) + Cl-(aq)
Ammonia is a weak base and water contains hydroxide ions from its dissociation:
H2O = H+(aq) + OH-(aq)
So some ions react:
NH4+(aq) + OH-(aq) = NH4OH(aq)
This means that more water splits up to replace the lost hydroxide ions. This results in a build up of hydrogen ions and this is why the solution is acidic.
