This is an acid-base reaction (neutralization): CaO is a base, H2SO4 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.
H2SO4 – Sulfuric acid
Other names: Oil of vitriol, Sulfuric acid, concentrated (> 51% and , H2SO4
Appearance: Clear, colorless liquid ; Odourless colourless oily hygroscopic liquid ; Colorless to dark-brown, oily, odorless liquid. [Note: Pure compound is a solid below 51°F. Often used in an aqueous solution.]
Names: Calcium sulfate, Plaster of Paris, Drierite
Appearance: White solid ; White hygroscopic powder or crystalline powder ; Odorless, white powder or colorless, crystalline solid. [Note: May have blue, gray, or reddish tinge.]
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