When CaO (calcium oxide) reacts with HCl (hydrochloric acid), it forms calcium chloride (CaCl2) and water (H2O). The chemical reaction between CaO and HCl can be represented as follows:

CaO + 2HCl → CaCl2 + H2O

In this reaction, calcium oxide (CaO) reacts with hydrochloric acid (HCl) to produce calcium chloride (CaCl2) and water (H2O).