Although the dissolution of ammonium chloride in water is an endothermic reaction, it is spontaneous because:

$∆\mathrm{G}=∆\mathrm{H}-\mathrm{T}∆\mathrm{S}$

when NH₄Cl is dissolved in water number of ions increases and $∆\mathrm{S} = +\mathrm{ve}$

when $∆\mathrm{H}=+\mathrm{ve}$ and $∆\mathrm{S} = +\mathrm{ve}$ the process is spontaneous ($∆\mathrm{G}=-\mathrm{ve}$) only when magnitude of $∆\mathrm{H}<\mathrm{T}∆\mathrm{S}$