This is an acid-base reaction (neutralization): CaO is a base, HCl is an acid.
CaO – Calcium oxide
Other names: Quicklime, Burnt lime, Unslaked lime
Appearance: White to pale yellow/brown powder ; White hygroscopic crystalline powder ; White or gray, odorless lumps or granular powder.
HCl – Chlorane, Hydrogen chloride
Other names: Hydrochloric acid, M, Uriatic acid
Appearance: Colorless, transparent liquid, fumes in air if concentrated ; Colorless gas ; Colourless compressed liquefied gas with pungent odour ; Colorless to slightly yellow gas with a pungent, irritating odor. [Note: Shipped as a liquefied compressed gas.]
H2O – Water, oxidane
Other names: Water (H2O), Hydrogen hydroxide (HH or HOH), Hydrogen oxide
Appearance: White crystalline solid, almost colorless liquid with a hint of blue, colorless gas
CaCl2 – Calcium chloride
Other names: Calcium(II) chloride, Calcium dichloride, E509
Appearance: White powder, hygroscopic ; Odourless hygroscopic colourless or white crystals