Calcium hydroxide, represented by the chemical formula \(\text{Ca}(\text{OH})_2\), is an inorganic compound commonly known as slaked lime or hydrated lime. This white, powdery substance is produced when water is added to calcium oxide, a reaction referred to as slaking. \(\text{Ca}(\text{OH})_2\) is classified as a base. Its identity as a base is the reason for its widespread use across many industrial, agricultural, and construction applications.
How Chemists Classify Acids and Bases
Chemists classify substances as acids or bases using the Arrhenius definition, which focuses on what happens when a compound is dissolved in water. An acid is a substance that increases the concentration of hydrogen ions (\(\text{H}^+\)) in an aqueous solution. Conversely, a base increases the concentration of hydroxide ions (\(\text{OH}^-\)) when dissolved in water.
This classification is linked to the \(\text{pH}\) scale, a logarithmic measure used to specify the acidity or basicity of a solution. The scale typically ranges from 0 to 14, where a value of 7 is neutral. Solutions with a \(\text{pH}\) value less than 7 are acidic, indicating a higher concentration of hydrogen ions.
A solution with a \(\text{pH}\) greater than 7 is considered basic, or alkaline, indicating a higher concentration of hydroxide ions. Understanding this ion-based definition provides the framework for classifying compounds like calcium hydroxide.
Why Calcium Hydroxide is a Base
Calcium hydroxide is categorized as a base because its structure contains the hydroxide ion (\(\text{OH}^-\)) bonded to the metal ion, calcium (\(\text{Ca}^{2+}\)). When the solid compound is introduced into water, it undergoes dissociation, separating into its constituent ions. This process directly releases hydroxide ions into the solution, which is the defining characteristic of an Arrhenius base.
The dissociation reaction is represented by the equation: \(\text{Ca}(\text{OH})_2 (\text{s}) \rightarrow \text{Ca}^{2+} (\text{aq}) + 2\text{OH}^- (\text{aq})\). For every molecule of calcium hydroxide that dissolves, two hydroxide ions are released, significantly increasing the solution’s \(\text{OH}^-\) concentration and raising its \(\text{pH}\) above 7. This mechanism confirms its identity as a base.
Calcium hydroxide is classified as a strong base, meaning that any portion of the compound that dissolves in water dissociates completely into its ions. However, calcium hydroxide is only sparingly soluble in water, meaning that only a small amount can dissolve in a given volume. This low solubility prevents the formation of a highly concentrated solution, even though the dissolved portion is fully ionized.
Common Uses of Calcium Hydroxide
The basic nature of calcium hydroxide is harnessed for practical applications that require \(\text{pH}\) adjustment or chemical reaction.
In agriculture, it is used as a soil amendment to treat acidic soils, a process known as liming. Adding this base neutralizes the soil’s acidity, creating a more favorable environment for many crops to absorb nutrients.
The compound is also a staple in the construction industry, where it is a component in the manufacture of cement, plaster, and mortar. When mixed with sand and water, the \(\text{Ca}(\text{OH})_2\) reacts slowly with carbon dioxide from the air to form solid calcium carbonate. This reaction hardens the mixture and provides the material’s structural strength.
Its alkalinity is employed in environmental applications, particularly in water and wastewater treatment. Calcium hydroxide acts as a flocculant, causing suspended particles to clump together so they can be easily filtered out. It is also used to neutralize acidic wastewater streams before they are safely discharged.