What happens when you mix calcium oxide and water

When you mix calcium oxide and water, an immediate and highly exothermic chemical reaction occurs, producing calcium hydroxide and releasing intense heat—enough to bring water to boiling temperatures. The reaction CaO + H2O → Ca(OH)2 is so energetic that it can cause the water to steam, the mixture to bubble and froth, and the solid quicklime to expand, crack, and break apart into smaller particles. If you add a large amount of CaO to water at once, the heat generated can be dangerous, potentially causing the hot alkaline mixture to splatter or even explode from rapid steam formation.

The resulting calcium hydroxide (slaked lime) is a fine white powder or creamy paste, depending on the amount of water used, and it forms a strongly alkaline solution (pH above 12) that can cause chemical burns to skin. This slaking process is fundamental to numerous industries: in construction, slaked lime is mixed into mortar and plaster; in agriculture, it's spread on fields to reduce soil acidity; in water treatment, it's used to adjust pH and remove impurities. The reaction must be controlled carefully in industrial settings by adding water to quicklime gradually while stirring, or by adding small amounts of quicklime to excess water, allowing time for heat dissipation. Safety precautions are essential—workers must wear protective clothing, gloves, and eye protection, and ensure adequate ventilation to handle any steam produced. The dramatic transformation from lumps of white quicklime to a hot, caustic paste demonstrates why CaO is considered a hazardous material that requires respect and proper handling procedures.