What Happens When Water is Poured on Calcium Oxide?

CaO(s) + H₂O(l) → Ca(OH)₂(s) + heat

Word equation: Calcium oxide + Water → Calcium hydroxide + Heat

This equation is already balanced — one mole of CaO reacts with one mole of water to give one mole of calcium hydroxide.

With excess water, Ca(OH)₂ partially dissolves: • Lime water — clear solution of Ca(OH)₂ in excess water • Milk of lime — white suspension when more Ca(OH)₂ is added

This reaction is classified as:

1. Combination (Synthesis) Reaction — two substances combine to form a single product.
2. Exothermic Reaction — large amount of heat is released. Temperature can rise above 150°C.
3. Non-redox Reaction — oxidation states of Ca, O, and H do not change.

Calcium oxide (CaO) is called quicklime. Adding water to quicklime is called slaking of lime, giving slaked lime Ca(OH)₂.

When water is poured on calcium oxide:

1. A loud hissing sound is produced.
2. Steam or vapour is evolved due to intense heat.
3. The solid swells, expands, and crumbles to a white powder.
4. Temperature of the mixture rises sharply.
5. A white powder (slaked lime) is formed with a small amount of water.
6. A milky suspension (milk of lime) forms with excess water.

The product Ca(OH)₂ has many important applications:

1. Construction — making mortar and plaster.
2. Whitewashing — dilute Ca(OH)₂ solution used to coat walls.
3. Agriculture — neutralises acidic soil.
4. Water treatment — softens hard water.
5. Making bleaching powder: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
6. Manufacture of glass and cement.