Observation:
- When water is added to solid calcium oxide, a vigorous reaction takes place.
- Heat is evolved during the reaction.
- A white, powdery substance is formed.

Formula for the product formed:
- The product formed when water is added to solid calcium oxide is calcium hydroxide, which has the chemical formula Ca(OH)2.

Explanation:
- When water is added to solid calcium oxide (CaO), it undergoes a chemical reaction to form calcium hydroxide (Ca(OH)2). This reaction is highly exothermic, meaning it releases a large amount of heat.
- The reaction can be represented by the following chemical equation:
CaO + H2O → Ca(OH)2
- In this reaction, solid calcium oxide reacts with water to produce calcium hydroxide. Calcium hydroxide is a white, powdery substance that is slightly soluble in water.
- Calcium hydroxide is commonly known as slaked lime and is used in various applications such as in the preparation of mortar, as a pH regulator in agriculture, and in the manufacturing of chemicals.

Importance:
- The reaction between calcium oxide and water to form calcium hydroxide is an important chemical reaction with various industrial applications.
- Understanding this reaction is crucial in industries such as construction, agriculture, and chemical manufacturing.