Q. Is CaO 2HCl CaCl2 H2O a redox reaction

No, the reaction CaO + 2HCl → CaCl2 + H2O is not a redox reaction—it is an acid-base neutralization reaction classified as a double displacement reaction. In this reaction, calcium oxide (a basic oxide) reacts with hydrochloric acid to produce calcium chloride salt and water, but no electrons are transferred between atoms and no oxidation states change. Calcium remains at +2 oxidation state throughout (it's +2 in both CaO and CaCl2), chlorine remains at -1 (in both HCl and CaCl2), oxygen remains at -2, and hydrogen remains at +1.

Redox reactions are defined by electron transfer and changes in oxidation states, which simply don't occur here. Instead, this is a classic example of a base reacting with an acid: the oxide ion (O2-) from CaO acts as a base and accepts protons from HCl, ultimately forming water, while the calcium and chloride ions form the salt. This type of metal oxide plus acid reaction always produces a salt and water without any redox chemistry involved. The absence of oxidation state changes distinguishes this from reactions like Zn + 2HCl → ZnCl2 + H2, where zinc goes from 0 to +2 (oxidation) and hydrogen goes from +1 to 0 (reduction), making that reaction both a single replacement and a redox process. Understanding the difference between acid-base chemistry (proton transfer) and redox chemistry (electron transfer) is fundamental to correctly classifying reactions and predicting their behavior.