Calcium hydroxide, known more commonly as slaked lime or hydrated lime, is an inorganic compound with the chemical formula Ca(OH)₂. This alkaline substance is produced through a straightforward chemical reaction called slaking. The process involves combining calcium oxide, or quicklime, with water, which results in a significant release of heat. Understanding the chemistry and practical steps of this conversion is the first step before attempting to create this material.
Required Materials and Equipment
The production of calcium hydroxide begins with the primary reactant, calcium oxide, often referred to as quicklime. Quicklime is a highly reactive, caustic material derived from heating limestone, and it provides the calcium atom for the final product. The only other necessary ingredient for the chemical conversion is clean water.
Since the reaction is highly exothermic, the choice of equipment is paramount for safety. A large, heat-resistant container, such as a metal drum or a sturdy, non-plastic bucket, is necessary to contain the reaction. A long-handled stirring implement, also made of a heat-resistant material, will be needed to mix the materials while maintaining a safe distance. If the goal is to produce a refined powder, filtration tools and a drying area are required to separate the solid calcium hydroxide from any excess water.
The Slaking Process: Step-by-Step Guide
The transformation of quicklime into slaked lime is a hydration process represented by the chemical equation CaO + H₂O → Ca(OH)₂ + Heat. This exothermic reaction converts calcium oxide and water into calcium hydroxide, releasing enough energy to boil the water involved.
Preparation for the slaking process involves placing the quicklime into the heat-resistant container in an open, well-ventilated space. The careful addition of water to the quicklime must be done slowly to control the intense heat release and prevent splattering. Adding large amounts of water all at once can generate steam rapidly, potentially causing a dangerous, explosive expulsion of the caustic mixture.
A typical ratio involves using about three parts quicklime to one part water by weight, though this can be adjusted based on the desired consistency. As water is added, the mixture should be stirred gently with the long-handled tool to ensure all the quicklime is exposed. The quicklime will begin to crumble and swell as the reaction proceeds, accompanied by the visible release of steam and bubbling due to the heat.
The reaction is generally complete when the mixture stops steaming and the bubbling ceases, which indicates that the heat production has subsided. The resulting material will be a thick, milky slurry or paste, often referred to as lime putty. To achieve a fine, dry hydrated lime powder, this slurry must be allowed to settle and “age,” after which the excess water can be drained off and the solid material dried completely.
Essential Safety Protocols
Strict safety protocols must be followed due to the caustic nature of quicklime and the intense heat generated during slaking. Both calcium oxide and calcium hydroxide are alkaline and corrosive, posing a risk of chemical burns. The highly exothermic reaction can heat the mixture to temperatures between 180°F and 220°F, which is hot enough to cause thermal burns and potentially ignite nearby combustible materials.
Mandatory Personal Protective Equipment (PPE) is necessary for handling both the starting material and the reaction. A dust mask or respirator is also recommended to prevent the inhalation of fine quicklime or calcium hydroxide powder, which can irritate the respiratory system. This PPE includes:
- Chemical-resistant gloves.
- A face shield or safety goggles to protect the eyes.
- Protective clothing like long sleeves and pants.
The process must always be conducted in an area with good ventilation to disperse any airborne dust or steam. When adding water, pour it onto the quicklime slowly. Never reverse the process by pouring quicklime into a large body of water, as this dramatically increases the risk of a violent steam eruption. Should any material contact the skin or eyes, flush the affected area immediately with copious amounts of water for at least fifteen minutes, and seek medical attention.
Common Uses of Calcium Hydroxide
Calcium hydroxide, once produced, has a wide range of applications across several industries due to its strong alkaline properties and low cost. In construction, it is a traditional ingredient in lime mortar, plaster, and stucco, improving the workability and durability of these materials. The compound is also used in soil stabilization for road construction.
In the agricultural sector, calcium hydroxide is frequently used as a soil conditioner to neutralize acidic soil, raising the pH level to create a more favorable environment for crop growth. Its use in water treatment is extensive, where it acts as a flocculant to remove smaller particles and helps to adjust the pH of acidic water sources.
The compound also plays a part in the food industry, where it is known as a food additive (E526) used in the process of nixtamalization for corn and in the refining of sugar. Its mild basicity and low toxicity make it suitable for these applications, including its use in the pickling process to maintain the firmness of vegetables.