Slaked Lime - Formula, Preparation, Properties, Reactions and Applications

Slaked lime is the common name for the chemical compound calcium hydroxide(Ca(OH)₂). Calcium hydroxide is also known by other common names: caustic lime, hydrated lime, slack lime, and pickling lime. The saturated solution for slaked lime is also referred to as limewater.
This compound exists in nature as a white substance in the form of a powder. However, the compound can also exist as colorless crystals at Standard known Temperature and Pressure.

Preparation of Slaked lime

As we know, Slaked lime has the chemical formula Ca(OH)₂, i.e., calcium hydroxide. The preparation methods involve chemical reactions between calcium oxide and water. Calcium oxide, also otherwise known as quicklime, reacts with water. During this reaction, a small amount of quicklime is dissolved into water, thus forming slaked lime. The chemical reaction for the following can be represented as:-
Cao + H2O 🡪 Ca(OH)2

While some portion of the quicklime is involved in the formation of slaked lime, the remaining portion is present as suspension. This suspension is also popularly known as lime milk.

Another method of preparation of slaked lime is through the chemical reaction between calcium chloride and sodium hydroxide. Calcium chloride and sodium hydroxide can be represented with the help of the chemical formula CaCl2 and NaOH, respectively. When aqueous solutions of calcium chloride and sodium hydroxide are mixed, the chemical reaction results in calcium hydroxide, i.e., slaked lime and salt, i.e., NaCl. The chemical reaction is represented as follows:-
CaCl2 + 2NaOH 🡪 Ca(OH)2 + 2NaCl

Properties of Slaked Lime

Some of the properties of Slaked lime are listed as follows:-

• Its molar mass is 74.09 g/mol.
• It appears as colorless crystals or white powder.
• It is odorless.
• Its melting point is 580℃.
• It is soluble in water, glycerol, and acids. An aqueous solution of calcium hydroxide is called limewater. Its solubility decreases with an increase in temperature. Its suspension in water is called milk of lime.
• It is insoluble in alcohol.
• It is basic or alkaline.
• Due to its alkaline nature, it is said to dissolve at room temperature to generate an alkaline solution of ph approximately equal to 12.4.
• It is relatively insoluble in water as its solubility product is 5.5*10⁻⁶

Slaked lime is observed to undergo a decomposition reaction and lose water when the temperature is as close to 853 K. It is to be noted that there can be side effects caused by slaked lime or slaked lime solutions. With higher ph values, the solubility of slaked lime decreases drastically.

It is also observed that at 70 ° C, calcium hydroxide, aka slaked lime has its solubility at around half of its value at that observed in 25 ° C. The explanation for this is that the dissolution reaction of slaked lime in water is exothermic. It also adheres to Le Chatelier’s principle. Thus, a drop in temperature favors 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.”

Reactions of Slaked Lime with other chemicals

Some of the common reactions of slaked lime with other chemicals are listed as follows:-

• Reaction With Concentrated Acid – When calcium hydroxide reacts with concentrated sulfuric acid, it forms water and precipitates calcium sulfate. The reaction is given below –
  Ca(OH)2 + H2SO4(conc.) 🡪 CaSO4↓ + 2H2O
   
• Reaction With Hydrogen Sulfide – When calcium hydroxide reacts with hydrogen sulfide, it gives calcium hydrosulfide and water. The reaction is given below –
  Ca(OH)2 + 2H2S 🡪 Ca(HS)2 + 2H2O
   
• Decomposition – When calcium hydroxide is heated to 512℃, it decomposes into calcium oxide (quicklime) and water. The reaction is given below –
  Ca(OH)2 → CaO + H2O
   
• Reaction With Dilute HCl (Acid) – When calcium hydroxide reacts with dil. Hydrochloric acid gives calcium chloride and water. The reaction is given below-
  Ca(OH)2 + 2HCl(diluted) 🡪 CaCl2 + 2H2O

Applications of Slaked lime

The applications of slaked lime are listed as follows:-
1. For whitewashing – Slaked lime is used in whitewashing. It reacts with carbon dioxide slowly and forms a layer of                calcium carbonate on the wall, giving the wall shine.
2. For the manufacturing of sugar – Slaked lime is used in manufacturing sugar from sugar cane. It makes sugarcane juice        alkaline and precipitates its impurities.
3. It is used in sewage treatment. It is used as a flocculant in water.
4. It is used in the paper industry.

Other uses include processing and making pickles, water for various beverages, and the cellulose hull of maize kernels. It is also used as a digestive aid, making papadum, etc.