Calcium Hydroxide ($Ca(OH)_2$)
A fundamental alkaline earth metal hydroxide. Universally recognized as slaked lime, it is a cornerstone of the construction industry, water treatment facilities, and qualitative analytical chemistry.
Quick Facts
1. Introduction and Chemical Significance
Calcium hydroxide ($Ca(OH)_2$), traditionally known as slaked lime, is an inorganic compound resulting from the hydration of calcium oxide (quicklime). It is a moderately strong base belonging to the Group 2 alkaline earth metals.
Throughout human history, it has been an indispensable material. Ancient civilizations used it to create durable mortar and plaster for buildings. Today, its utility spans massive industrial sectors: it is a primary flocculant in water and sewage treatment, an acidity regulator in the food industry, and an essential reagent in the paper-making Kraft process. In chemistry laboratories, its aqueous solution ("limewater") is the classic qualitative test for carbon dioxide gas.
2. Molecular Structure and Bonding
Calcium hydroxide is an ionic compound composed of calcium cations ($Ca^{2+}$) and polyatomic hydroxide anions ($OH^-$).
- Ionic Lattice: In the solid state, it adopts a hexagonal crystal structure characterized by a polymeric layer-lattice (brucite-type structure).
- Layered Bonding: Layers of calcium ions are sandwiched between layers of hydroxide ions. The bonds within the layers are strongly ionic, but the adjacent layers are held together by much weaker intermolecular forces (hydrogen bonding between the $OH$ groups). This layered structure accounts for its soft, powdery physical nature.
- Dissociation: When it dissolves in water, it dissociates into $Ca^{2+}$ and two $OH^-$ ions, making the solution strongly basic, though its total alkalinity is limited by its relatively low solubility.
3. Physical Properties and Retrograde Solubility
Calcium hydroxide exhibits a highly unusual and frequently tested physical property regarding its solubility in water.
| Property | Value / Description | Scientific Context |
|---|---|---|
| Physical State | Solid | A fine, soft, white, odourless powder. |
| Thermal Stability | Decomposes at $\approx 580^\circ\text{C}$ | It does not melt. Upon heating, it decomposes back into quicklime ($CaO$) and water vapour. |
| Solubility in Water | Retrograde Solubility | Unlike most solid salts, $Ca(OH)_2$ becomes less soluble as the temperature of the water increases. (Solubility is $\approx 1.89\text{ g/L}$ at $0^\circ\text{C}$, but drops to $\approx 0.66\text{ g/L}$ at $100^\circ\text{C}$). This is due to the highly exothermic nature of its dissolution process. |
4. Chemical Properties and Reactions
A. The Limewater Test for $CO_2$
This is arguably the most famous reaction of calcium hydroxide. When carbon dioxide gas is passed through clear limewater (a saturated aqueous solution of $Ca(OH)_2$), the solution turns milky or cloudy due to the precipitation of insoluble calcium carbonate.
The "Excess $CO_2$" Phenomenon: If you continue to bubble $CO_2$ through the milky solution for a prolonged period, the milkiness eventually disappears, and the solution becomes clear again. This occurs because the insoluble calcium carbonate reacts with the excess $CO_2$ and water to form calcium bicarbonate, which is soluble.
B. Synthesis of Bleaching Powder
When dry slaked lime is treated with chlorine gas, it produces bleaching powder. This is a critical industrial reaction used for producing water disinfectants and textile bleaches.
Note for advanced students: The actual composition of commercial bleaching powder is more complex, represented as $Ca(OCl)_2 \cdot CaCl_2 \cdot Ca(OH)_2 \cdot 2H_2O$. However, $CaOCl_2$ (Calcium oxychloride) is the standard pedagogical formula used in most high school curricula.
C. Reaction with Acids
As a strong base, it readily neutralizes acids to form calcium salts and water. This property is exploited in agriculture to neutralize overly acidic soils.
5. Preparation: The Slaking of Lime
Calcium hydroxide is prepared commercially on a massive scale by adding water to calcium oxide (quicklime). This hydration process is called slaking. It is a highly exothermic reaction, releasing so much heat that the added water can boil and generate steam.
$\Delta H = -63.7\text{ kJ/mol}$ (Highly Exothermic)
If a limited, stoichiometric amount of water is added, a dry white powder (slaked lime) is obtained. If an excess of water is added, a thick paste or suspension (milk of lime) is produced.
6. Important Uses and Applications
- Construction (Mortar and Plaster): A traditional mixture of slaked lime, sand, and water is used as mortar. Over time, the $Ca(OH)_2$ slowly absorbs $CO_2$ from the atmosphere to form a hard, solid mass of calcium carbonate ($CaCO_3$), binding the bricks together.
- Water Treatment: Used as a flocculant in water and sewage treatment. It raises the pH of the water, causing heavy metals and suspended particles to precipitate out so they can be filtered.
- Agriculture: Applied to agricultural fields as an inexpensive soil amendment to neutralize soil acidity and provide a source of calcium for plants.
- Food Industry: Used in the traditional Mesoamerican preparation of maize (corn) called nixtamalization. Boiling corn in limewater releases bound niacin (Vitamin B3), making the corn far more nutritious and easier to grind into masa for tortillas.
- Whitewashing: A suspension of slaked lime is painted on walls. As the water evaporates, the $Ca(OH)_2$ reacts with atmospheric $CO_2$ to form a bright white, shiny, protective layer of $CaCO_3$.
📝 High-Yield Notes for JEE, NEET & CBSE: The Lime Lexicon
Examiners frequently test your knowledge of the different forms and names of calcium compounds. You must know these distinctions flawlessly:
- Quicklime: Calcium Oxide ($CaO$).
- Slaked Lime: Solid Calcium Hydroxide ($Ca(OH)_2$ powder).
- Limewater: A clear, saturated aqueous solution of $Ca(OH)_2$. Used to test for $CO_2$.
- Milk of Lime: An opaque aqueous suspension of solid $Ca(OH)_2$ particles in water. Used for whitewashing and industrial bulk reactions.
- Limestone / Chalk / Marble: Calcium Carbonate ($CaCO_3$).
Remember that heating an aqueous solution of calcium hydroxide will cause it to become *less* soluble, potentially causing solid $Ca(OH)_2$ to precipitate out of a saturated solution. This is the opposite of most standard salts like $NaCl$ or $KNO_3$.
7. Safety and Handling
While commonly used, calcium hydroxide is a strongly alkaline, corrosive substance.
- Corrosivity: Direct contact with the dry powder or concentrated suspensions can cause severe skin irritation, chemical burns, and permanent eye damage.
- Inhalation: Inhaling the fine dust irritates the respiratory tract, causing coughing and shortness of breath.
- First Aid: Spills on the skin or eyes must be flushed immediately with copious amounts of water for at least 15 minutes.
8. Common Misconceptions
- Misconception: Slaking lime is a slow, gentle process.
Scientific Reality: Adding water to fresh quicklime ($CaO$) is violently exothermic. Without proper safety equipment, the sudden release of heat can cause the water to flash-boil, splattering highly corrosive, boiling alkaline slurry into the air. - Misconception: Limewater turns milky because $CO_2$ makes the water cloudy.
Scientific Reality: The milkiness is specifically due to the chemical formation of a solid precipitate—calcium carbonate ($CaCO_3$)—which is insoluble in water. The solid particles reflect light, making the liquid look like milk.
9. Frequently Asked Questions (FAQs)
10. Conclusion and Further Learning
Calcium hydroxide is a magnificent example of how alkaline earth metals interact with their environment. Understanding its exothermic preparation, its unique solubility profile, and the chemistry behind the classic limewater test provides a solid foundation for mastering the s-block elements.
Compare its properties to the alkali metal hydroxides like Sodium Hydroxide, or explore related inorganic salts in our Important Chemical Compounds hub.
