Slaking of lime is considered a chemical change because it involves the formation of a new substance with different properties from the original reactants. Here's a breakdown:

The Reaction:

Slaking of lime is the reaction of quicklime (calcium oxide, CaO) with water (H2O) to form slaked lime (calcium hydroxide, Ca(OH)2) and releasing heat:

CaO (s) + H2O (l) → Ca(OH)2 (s) + Heat

Why it's a Chemical Change:

* New Substance: Calcium hydroxide (Ca(OH)2) is a different compound than calcium oxide (CaO). It has different chemical properties and physical properties.

* Chemical Bonds Broken and Formed: The reaction involves breaking the bonds in both CaO and H2O molecules and forming new bonds to create Ca(OH)2. This is a key characteristic of a chemical change.

* Irreversible: Slaking of lime is generally considered an irreversible reaction. While it's possible to dehydrate calcium hydroxide back to calcium oxide, this requires a separate chemical process with different conditions.

* Heat Release: The reaction releases heat, indicating an energy change and a chemical transformation.

In contrast to physical changes, where the substance's composition remains the same, chemical changes alter the molecular structure and result in new substances with different properties.