What is slaked lime?
Slaked lime, the common name for calcium hydroxide, is a chemical compound whose formula can be expressed as Ca(OH)2. Under standard conditions of temperature and pressure, slaked lime is considered to exist as a white, powdery substance. Alternatively, the compound may also exist as colorless crystals at STP. Some other common names for slaked lime include caustic lime, hydrated lime, slaked lime, and pickled lime. Many important uses of this compound are found in the food industry, where it is used as a food additive. A saturated solution of slaked lime is commonly called lime water.
How can Slaked Lime be prepared?
The most common method of producing slaked lime is the reaction between calcium oxide (also known as quicklime) and water. When mixed with water, a relatively small amount of quicklime dissolves, resulting in a solution known as limewater. The rest of the quicklime remains in suspension, commonly known as milk of lime.
Properties of Slaked Lime
With a Ksp (solubility product) of 5.5*10−6, slaked lime can be considered relatively insoluble in water. Calcium hydroxide (portlandite) is known to dissolve in pure water at room temperature to form an alkaline solution with a pH of about 12.4. It is important to note that chemical burns can be caused by lime solutions. Its solubility decreases dramatically at relatively high pH values (as a result of the common ion effect). For cement paste, this behavior of slaked lime is considered to be very important.
Aqueous solutions of slaked lime are commonly referred to as lime water and are known to be medium strength bases that react with acids and can attack some metals such as aluminum. Some other important properties of slaked lime are listed below.
The IUPAC name for slaked lime is calcium hydroxide. The chemical formula of this compound is Ca(OH)2.
The molar mass (molecular weight) of slaked lime is about 74.1 grams per mole.
Under standard conditions of temperature and pressure, the density of this compound corresponds to 2.211 grams per cubic centimeter.
The melting point of slaked lime corresponds to about 853 K.
This chemical compound is known to crystallize in a hexagonally shaped crystal lattice.
Slaked lime is not very soluble in water. At 20 degrees Celsius, the solubility of slaked lime in water is approximately 1.73 g/L.
At temperatures around 853 K (which is in the range of its melting point), slaked lime is known to lose water and begin to undergo decomposition.
At 70 °C, the solubility of calcium hydroxide is about half of its value at 25 °C. The explanation is that the dissolution of slaked lime in water is exothermic in nature and also obeys Le Chatelier’s principle. Thus, a decrease in temperature favors the removal of heat released by the dissolution process and increases the dissolution equilibrium of calcium hydroxide, thereby increasing its solubility at lower temperatures. This dependence of solubility on temperature is commonly referred to as “retrograde” solubility. Because their dissolution reactions are exothermic, the variable hydrated phases of calcium sulfate (gypsum, basanite, and anhydrate) also exhibit backward solubility.
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