Observation:
- When water is added to solid calcium oxide, a vigorous reaction takes place.
- The solid calcium oxide initially appears as a white powder.
- As water is added, the solid starts to heat up and a hissing sound is heard.
- A white, powdery substance is formed.

Formula for the Product:
The product formed when water is added to solid calcium oxide is calcium hydroxide. The chemical equation for the reaction is:
CaO (s) + H2O (l) → Ca(OH)2 (aq)

Explanation:
- When water is added to solid calcium oxide, a highly exothermic reaction occurs. This is because calcium oxide is a highly reactive substance that readily reacts with water to form calcium hydroxide.
- The reaction between calcium oxide and water is a classic example of an acid-base reaction. In this case, calcium oxide acts as a base, while water acts as an acid.
- The formation of calcium hydroxide is a white, powdery substance that is also known as slaked lime. This compound is commonly used in various applications, such as in the construction industry for mortar and plaster.
- The reaction between calcium oxide and water is also known as a hydration reaction, as water molecules are added to the calcium oxide to form calcium hydroxide.
- Overall, the addition of water to solid calcium oxide results in the formation of calcium hydroxide, a useful compound with various industrial applications.