Q. Is Ca(OH)2 H2SO4 → CaSO4 2H2O a single replacement reaction

No, the reaction Ca(OH)2 + H2SO4 → CaSO4 + 2H2O is not a single replacement reaction—it is a double displacement reaction (also called a double replacement or metathesis reaction). In this reaction, the positive and negative ions of two different compounds exchange partners to form two entirely new compounds. Specifically, the calcium ion (Ca2+) from calcium hydroxide swaps with the hydrogen ions (H+) from sulfuric acid, resulting in calcium sulfate and water.

Single replacement reactions follow the pattern A + BC → AC + B, where one element replaces another in a compound, leaving one element by itself. This acid-base neutralization follows the pattern AB + CD → AD + CB, where both compounds exchange ions. The distinguishing feature is that in double displacement reactions like this one, you start with two compounds and end with two different compounds, with no free elements involved. This reaction type is characteristic of neutralization reactions where acids and bases combine to form salts and water, making it one of the most common double displacement reactions encountered in chemistry labs and industrial processes.