Slaked Lime

Table of Contents

• What is Slaked Lime?
• How can Slaked Lime be Prepared?
• Properties of Slaked Lime
• Applications of Slaked Lime
• Frequently Asked Questions on Slaked Lime

What is Slaked Lime?

Slaked lime, which is the common name for calcium hydroxide, is a chemical compound whose formula can be represented as Ca(OH)₂. Under standard conditions for temperature and pressure, slaked lime is known to exist as a white, powdery substance. Alternately, this compound can also exist in the form of colourless crystals at STP. Some other common names of slaked lime include caustic lime, hydrated lime, slack lime, and pickling lime. Many important applications of this compound lie in the food industry, where it is used as a food additive. A saturated solution of slaked lime is commonly referred to as limewater.

How can Slaked Lime be prepared?

The most common method for the preparation of slaked lime is via the reaction between calcium oxide (also known as quicklime) and water. When combined with water, a relatively small proportion of the quicklime dissolves, resulting in the formation of a solution known as limewater. The remainder of the quicklime remains in a suspension, which is generally referred to as lime milk.

Properties of Slaked Lime

With a K_sp (solubility product) of 5.5*10⁻⁶, slaked lime can be considered to be relatively insoluble in water. Calcium hydroxide (portlandite) is known to dissolve at room temperature in pure water to create an alkaline solution with a pH of approximately 12.4. It is important to note that chemical burns can be caused by slaked lime solutions. Its solubility dramatically decreases at relatively high pH values (as a consequence of the common ion effect). For cement pastes, this behaviour of slaked lime is considered to be very important.

Aqueous solutions of slaked lime are commonly referred to as limewater and are known to be medium-strength bases that react with acids and can attack certain metals such as aluminium. Some other important properties of slaked lime are listed below.

• The IUPAC name of slaked lime is calcium hydroxide. The chemical formula of this compound is denoted by Ca(OH)₂.
• The molar mass (molecular weight) of slaked lime is roughly equal to 74.1 grams per mole.
• Under standard conditions for temperature and pressure, the density of this compound corresponds to 2.211 grams per cubic centimetre.
• The melting point of slaked lime corresponds to roughly 853 Kelvin.
• This chemical compound is known to crystallise in a hexagonally shaped crystal lattice.
• Slaked lime is not very soluble in water. At a temperature of 20 degrees Celsius, the solubility of slaked lime in water is roughly equal to 1.73 grams per litre.
• At temperatures close to 853 Kelvin (which lies in the range of its melting point), slaked lime is known to lose water and begin undergoing a decomposition reaction.

At 70° C, calcium hydroxide solubility is around half of its value at 25° C. The explanation for this is that the dissolution of slaked lime in water is exothermic in nature and also adheres to Le Chatelier’s principle. Thus, a drop in temperature favours the removal of heat emitted through the dissolution process and raises the equilibrium constant of the calcium hydroxide dissolution, thereby increasing its solubility at low temperatures. This dependence of solubility on temperature is commonly referred to as solubility “retrograde“. As their dissolution reactions are exothermic, the variable hydrated phases of calcium sulphate (gypsum, bassanite and anhydrite) also exhibit retrograde solubility for the same cause.

Applications of Slaked Lime

In sewage treatment, one major application of slaked lime is as a flocculant. It is known to form a fluffy charged solid that leads to the removal of relatively small particles from the water sample, facilitating the formation of a relatively clear product. The low cost and the low toxicity of slaked lime makes it ideal for this specific application. It is often used to increase the pH of the water in freshwater treatment so that pipes do not corrode where the base water is acidic because it is self-regulating and does not increase the pH too much.

During the Kraft method of converting wood to wood pulp, Ca(OH)2 is used in the paper industry. In the preparation of ammonia, slaked lime is known to play a vital role. This compound, because of its basicity, is often used as a pH modifier. Generally, the pickling of cucumbers is performed with the aid of Ca(OH)₂.

The use of calcium hydroxide is not uncommon in the manufacturing of several plastics. It is also used in pesticides, products for hair care, and ebonite processing. This compound is used in root canal operations to fill the cavities of human teeth.

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