Calcium Carbonate vs Calcium Hydroxide: Key Differences
Compare calcium carbonate and calcium hydroxide by examining their composition, solubility, pH levels, and applications in industry, medicine, and the environment.
Calcium carbonate and calcium hydroxide are two widely used calcium compounds with distinct properties and applications. While both contain calcium, their chemical behavior, solubility, and uses vary significantly, making them suitable for different purposes in biology, industry, and environmental science.
Composition And Crystal Structure
Calcium carbonate (CaCO₃) and calcium hydroxide (Ca(OH)₂) differ in atomic arrangement and bonding, which influence their physical and chemical properties. Calcium carbonate consists of calcium cations (Ca²⁺) bonded to carbonate anions (CO₃²⁻), forming a stable lattice. It naturally occurs in three crystalline polymorphs: calcite, aragonite, and vaterite. Calcite, the most stable form, adopts a trigonal crystal system where calcium ions are surrounded by carbonate groups. Aragonite, denser than calcite, has an orthorhombic structure and forms under higher pressures. Vaterite, the least stable, has a hexagonal arrangement and gradually transforms into calcite or aragonite. These structural variations make calcium carbonate a key component of limestone, marble, and biological structures like shells and coral skeletons.
Calcium hydroxide crystallizes in a hexagonal system, with calcium ions coordinated to hydroxide (OH⁻) groups. This structure results in a layered arrangement with weak interlayer forces, allowing for easy cleavage. Unlike calcium carbonate, which is highly stable under normal conditions, calcium hydroxide is more reactive, absorbing carbon dioxide from the air and gradually converting into calcium carbonate through carbonation. Its softer and more soluble nature contrasts with the rigidity and relative insolubility of calcium carbonate.
pH And Solubility
The pH and solubility of calcium carbonate and calcium hydroxide differ due to their chemical compositions. Calcium carbonate, a salt of carbonic acid and calcium hydroxide, has low water solubility. Its dissolution follows this equilibrium:
\[
\text{CaCO}_3 (s) \rightleftharpoons \text{Ca}^{2+} (aq) + \text{CO}_3^{2-} (aq)
\]
In neutral or basic conditions, calcium carbonate remains largely insoluble, but in acidic environments, carbonate ions react with hydrogen ions to form bicarbonate (\(\text{HCO}_3^-\)), increasing solubility:
\[
\text{CO}_3^{2-} + \text{H}^+ \rightleftharpoons \text{HCO}_3^-
\]
This explains why calcium carbonate dissolves in acidic solutions like gastric acid or carbonated water. The dissolution process plays a role in geological and biological systems, affecting limestone weathering and calcium homeostasis in marine organisms.
Calcium hydroxide, or slaked lime, is more soluble in water. When dissolved, it dissociates into calcium and hydroxide ions:
\[
\text{Ca(OH)}_2 (s) \rightleftharpoons \text{Ca}^{2+} (aq) + 2\text{OH}^- (aq)
\]
This increases hydroxide ion concentration, producing a highly alkaline solution with a pH around 12.4 at saturation (0.16 g/100 mL at 20°C). Its strong basicity makes it highly reactive with acids, forming salts and water. Over time, exposure to carbon dioxide converts it into calcium carbonate, reducing solubility and neutralizing alkalinity. This transformation is used in construction materials, where lime-based mortars harden through carbonation.
Role In Biology And Medicine
Calcium carbonate and calcium hydroxide have distinct biological and medical applications. Calcium carbonate is a major dietary calcium source, essential for bone mineral density and skeletal integrity. The body relies on calcium for cellular signaling, muscle contraction, and enzymatic activity. As a supplement, it helps prevent and treat osteoporosis, particularly in postmenopausal women and the elderly. It is best absorbed with food, as gastric acid enhances solubility. Excessive intake can cause hypercalcemia, leading to kidney stones, vascular calcification, and impaired renal function.
Beyond supplementation, calcium carbonate is a common antacid, neutralizing stomach acid by reacting with hydrochloric acid to form calcium chloride, water, and carbon dioxide. This provides relief from acid reflux and gastroesophageal reflux disease (GERD), though prolonged use may cause acid rebound or mineral imbalances.
Calcium hydroxide is not used for dietary supplementation due to its strong alkalinity, which can irritate mucosal tissues. Instead, it has applications in dentistry, particularly in endodontics. Its antimicrobial properties and ability to promote dentin formation make it useful for pulp capping, root canal disinfection, and apexification. By releasing hydroxide ions, it creates a high-pH environment that inhibits bacterial growth and supports tissue regeneration.
Industrial And Environmental Uses
Calcium carbonate and calcium hydroxide serve distinct industrial roles. Calcium carbonate is a key material in construction, forming the basis of cement, concrete, and lime-based mortars. It is also used in paper manufacturing to enhance brightness and opacity. The plastic and paint industries use it as a pigment extender, improving durability and reducing costs. In agriculture, finely ground calcium carbonate, known as agricultural lime, neutralizes acidic soils, optimizing pH for better crop yields.
Calcium hydroxide is widely used in water treatment and flue gas desulfurization due to its strong alkalinity. Municipal water systems use it to adjust pH and soften water by precipitating dissolved magnesium and calcium ions. In wastewater treatment, it helps remove heavy metals and organic contaminants. Industrially, it plays a role in acetylene gas production, sugar refining, and leather processing, where it removes impurities and regulates acidity. The construction sector benefits from its use in plaster and mortar, where it hardens through carbonation.