This is an acid-base reaction (neutralization): NH4Cl is an acid, Ca(OH)2 is a base.
Reactants:
NH4Cl – Ammonium chloride
Other names: Sal ammoniac, Salmiac, Nushadir saltSal armagnac, Salt armoniack, Salmiak, Ammonium chloride fume, Amchlor, Ammoneric, Ammonium muriate, Darammon, Sal ammonia, Salammonite, [NH4]Cl, NH4Cl, Ammoniumchlorid, Azanium chloride, Ammonium muriate fume, Sal ammoniac fume
Appearance: White solid, hygroscopic ; Odourless colourless-to-white hygroscopic solid in various forms ; Finely divided, odorless, white particulate dispersed in air.
Appearance: White powder ; Colourless crystals or white powder ; White, odorless powder. [Note: Readily absorbs CO2 from the air to form calcium carbonate.]