Here's why:
* HCl is a strong acid and NH₄Cl is the salt of a weak base (ammonia, NH₃) and a strong acid (HCl).
* When a strong acid is added to the salt of a weak base, the acid will protonate the weak base, but the equilibrium lies heavily towards the products. In this case, the reaction would be:
* NH₄Cl + HCl ⇌ NH₄⁺ + Cl⁻ + H⁺ + Cl⁻
* However, ammonium ions (NH₄⁺) are already present in solution due to the dissociation of ammonium chloride. Adding more H⁺ ions from HCl will simply increase the concentration of H⁺ ions in solution.
* In other words, the reaction will not proceed to a significant extent.
Essentially, the solution will become more acidic but no new chemical species are formed.
Note: If you were to heat the solution, you might see some decomposition of ammonium chloride, but that's a different reaction altogether.
