Calcium Hydroxide
Slaked Lime, Lime Water, and Bleaching Powder.
Calcium Hydroxide ($Ca(OH)_2$), commercially known as Slaked Lime, is a white amorphous powder. It is an inexpensive, sparingly soluble strong base that forms the backbone of the construction industry (mortar) and is crucial in qualitative chemical analysis.
1. Preparation & The "Lime Water" Trap
Slaked lime is prepared by adding a controlled amount of water to quick lime ($CaO$) in a highly exothermic process called slaking.
Calcium hydroxide is only sparingly soluble in water. This limited solubility leads to two very distinct, highly tested mixtures:
- Lime Water: The clear, filtered, aqueous solution of Calcium Hydroxide in water. It is fully dissolved and transparent. Used primarily for testing $CO_2$ gas.
- Milk of Lime: A thick, milky suspension of solid Calcium Hydroxide particles in water. It is opaque. Used for whitewashing walls and manufacturing bleaching powder.
2. Analytical Chemistry: The $CO_2$ Test
Lime water is the standard laboratory reagent used to test for the presence of Carbon Dioxide ($CO_2$) gas. This reaction occurs in two distinct, sequential stages.
Stage 1: Turning Milky
When $CO_2$ is initially passed through clear lime water, it reacts to form Calcium Carbonate ($CaCO_3$). Because $CaCO_3$ is insoluble in water, it precipitates out as fine white particles, turning the solution milky.
Stage 2: Clearing Up (Excess $CO_2$)
If excess $CO_2$ gas continues to be passed through the milky suspension, the precipitate dissolves. The insoluble Calcium Carbonate reacts with water and the extra $CO_2$ to form Calcium Bicarbonate, which is highly soluble, making the solution clear again.
This exact same chemistry explains how caves and stalactites are formed in limestone ($CaCO_3$) mountains by rainwater containing dissolved $CO_2$.
3. Reaction with Chlorine: Bleaching Powder
One of the most important industrial uses of slaked lime is the manufacture of Bleaching Powder. This is a very high-yield reaction for JEE Advanced.
The Reaction:
When Chlorine gas ($Cl_2$) is passed over dry slaked lime powder, a complex reaction occurs producing bleaching powder.
While simplified textbooks write the formula of bleaching powder as $CaOCl_2$, the actual industrial bleaching powder is a complex auto-complexed salt mixture. The true, accurate composition is:
*The active bleaching agent in this mixture is Calcium Hypochlorite: $Ca(OCl)_2$.*
4. Industrial & Everyday Uses
- Building Material (Mortar): A mixture of slaked lime, sand, and water forms mortar. As the mortar sets, the slaked lime slowly reacts with atmospheric $CO_2$ to form a hard, solid mass of Calcium Carbonate, binding the bricks together.
- Whitewashing: A suspension of slaked lime (Milk of Lime) is applied to walls. Over a few days, it reacts with $CO_2$ in the air to form a thin, shiny white layer of $CaCO_3$.
- Water Treatment: It is used in the Clarke's process to remove temporary hardness from water by precipitating out soluble bicarbonates as insoluble carbonates.
- Manufacture of Sugar: It is used to purify sugar solutions from sugarcane juice.
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