Q. What type of reaction is Ca(OH)2 HCl → CaCl2 H2O

The reaction Ca(OH)2 + 2HCl → CaCl2 + 2H2O is classified as a double displacement reaction (double replacement reaction), and more specifically, it's an acid-base neutralization reaction. In this reaction, the calcium ion (Ca2+) from calcium hydroxide exchanges places with the hydrogen ions (H+) from hydrochloric acid, forming calcium chloride salt and water. The characteristic pattern of double displacement reactions—where positive and negative ions from two compounds swap partners—is clearly evident here.

As a neutralization reaction, this represents the classic interaction between a base (calcium hydroxide) and an acid (hydrochloric acid) to produce a salt and water, which is a subset of double displacement reactions with particular significance in chemistry. The reaction occurs readily at room temperature, is exothermic (releases heat), and is commonly used in laboratories and industrial processes for pH control, waste treatment, and acid spill cleanup. When balancing this equation, you need a coefficient of 2 before HCl and H2O to account for the two hydroxide ions in Ca(OH)2, resulting in the balanced equation Ca(OH)2 + 2HCl → CaCl2 + 2H2O. Understanding this as both a double displacement and a neutralization reaction helps predict that the products will be a salt plus water and that the reaction will neutralize both the basic and acidic properties of the reactants.