Calcium Hydroxide
Formula: Ca(OH)₂ — Slaked lime, pickling lime, hydrated lime
Appearance: White powder
Hazard: Irritant · Corrosive
Properties
White powder, slightly soluble in water (the solution is called limewater). Strong base but safer to handle than sodium hydroxide. Made by adding water to quicklime (calcium oxide). Used in construction (mortar), food processing (nixtamalization of corn), and water treatment. Absorbs CO₂ from air over time, converting back to calcium carbonate.
Historical Context
The lime cycle - limestone to quicklime to slaked lime and back - has been known for at least 12,000 years. Ancient buildings at Göbekli Tepe (9000 BCE) used lime mortar. The chemistry involves burning limestone (Cite: CaCO₃ → CaO + CO₂), adding water (CaO + H₂O → Ca(OH)₂), and allowing the hydroxide to absorb atmospheric CO₂ and harden back to calcium carbonate.
Limewater’s ability to turn cloudy white with carbon dioxide was noted by Joseph Black in 1756, leading to his discovery of “fixed air” (carbon dioxide) - one of the first demonstrations that gases were distinct substances. This observation helped overturn phlogiston theory and establish modern chemistry.
The lime cycle remains essential to construction. Portland cement, concrete, and mortar all rely on the carbonation reaction. In agriculture, slaked lime raises soil pH and provides calcium. In food processing, it’s used for nixtamalization - the ancient Mesoamerican process that makes corn more nutritious and easier to grind for tortillas.
Experiments
CO₂ Detection: Mix with water to make limewater - it turns cloudy white when CO₂ is bubbled through it, forming calcium carbonate. Perfect for testing exhaled breath or combustion products. This demonstrates gas detection and precipitation reactions.
pH Indicator: Strong base (pH ~12.4) useful for demonstrating acid-base neutralization. Mix with phenolphthalein indicator and watch the pink color disappear as you add acid. Can also be used to show the pH scale with universal indicator.
Experiments using this chemical:
- Limewater CO₂ Test - Detect CO₂ in breath
Safety
Moderate hazard — corrosive; strongly alkaline.
Incompatible with: Strong acids (vigorous exothermic neutralisation); ammonium salts (releases ammonia gas); CO₂ in air (slow conversion to calcium carbonate); maleic anhydride