When calcium oxide (CaO), commonly known as quicklime, reacts with water (H2O), it undergoes a vigorous chemical reaction to form calcium hydroxide (Ca(OH)2), also known as slaked lime, accompanied by the liberation of significant heat. This process is known as the slaking of lime.
The interaction between solid calcium oxide and water is an exothermic reaction, meaning it releases heat into the surroundings. This makes the reaction visibly energetic, often producing steam and a noticeable temperature increase.
Chemical Equation
The balanced chemical equation for the reaction is:
CaO(s) + H2O(l) → Ca(OH)2(aq/s) + Heat
- CaO(s): Solid calcium oxide (quicklime)
- H2O(l): Liquid water
- Ca(OH)2(aq/s): Calcium hydroxide (slaked lime), which can be a solid suspension or dissolve to form an aqueous solution.
Formation of Calcium Hydroxide
The primary product of this reaction is calcium hydroxide. While calcium hydroxide is sparingly soluble, it does dissolve in water to form its solution, which is commonly referred to as lime water. When a sufficient amount of water is added, a milky suspension of calcium hydroxide is formed, often called milk of lime.
Characteristics of the Slaking Process
The reaction of calcium oxide with water exhibits several key characteristics that are important for its industrial and practical applications.
Exothermic Nature
The liberation of heat is a defining feature of this reaction. This exothermic property is due to the strong affinity of calcium oxide for water, leading to the formation of stable calcium hydroxide and the release of energy. The heat generated can be substantial, making it necessary to handle the reaction with care.
Physical Changes
As the solid calcium oxide reacts, it gradually transforms into a fine, white powder (calcium hydroxide). If excess water is present, this powder can form a suspension or dissolve to create a clear solution (lime water).
Practical Applications and Significance
The products of the slaking of lime, particularly calcium hydroxide, have a wide range of applications across various industries due to their alkaline nature and reactivity.
- Construction Industry:
- Mortar and Plaster: Slaked lime is a key ingredient in traditional lime mortar and plaster, providing binding properties.
- Soil Stabilization: Used to improve the properties of clayey soils in construction, enhancing their strength and reducing plasticity.
- Water Treatment:
- Water Softening: Calcium hydroxide is used in the lime-soda process to remove hardness (calcium and magnesium ions) from water.
- pH Adjustment: Used to neutralize acidic wastewater and control pH levels.
- Coagulant: Helps in flocculation of suspended solids during water purification.
- Agriculture:
- Soil Amendment: Applied to acidic soils to raise their pH, making them more suitable for certain crops.
- Pesticide and Fungicide: Used in some traditional agricultural sprays.
- Industrial Applications:
- Sugar Refining: Employed to purify sugar cane juice.
- Paper Industry: Used in the pulp and paper manufacturing process.
- Flue Gas Desulfurization: Helps remove sulfur dioxide from industrial emissions.
Summary Table: Calcium Oxide Reaction with Water
| Property | Description |
|---|---|
| Reactants | Calcium Oxide (CaO - Quicklime), Water (H2O) |
| Product | Calcium Hydroxide (Ca(OH)2 - Slaked Lime) |
| Reaction Type | Exothermic (Heat is liberated), Hydration Reaction |
| Process Name | Slaking of Lime |
| Solution Form | Calcium hydroxide dissolves in water to form its solution called lime water, or a milky suspension (milk of lime). |
The reaction of calcium oxide with water is a fundamental chemical process with significant industrial and environmental relevance, transforming a highly reactive oxide into a versatile alkaline compound.
