The white residue that forms on bottles of concentrated HCl and NH4OH when placed next to each other is ammonium chloride (NH4Cl).

Here's why:

* HCl (Hydrochloric Acid) is a strong acid, and NH4OH (Ammonium Hydroxide) is a weak base.

* When the vapors of these two compounds come into contact, they react to form ammonium chloride (NH4Cl), a white solid.

* The reaction is an acid-base neutralization reaction:

HCl (g) + NH4OH (g) → NH4Cl (s) + H2O (g)

This reaction occurs because the acidic hydrogen ions (H+) from HCl react with the basic hydroxide ions (OH-) from NH4OH, forming water (H2O) and ammonium ions (NH4+). The ammonium ions then combine with chloride ions (Cl-) to form ammonium chloride (NH4Cl), which precipitates out as a white solid.