Consider the following pairs 1. Quick lime | Manufacture of Cement 2. Slaked lime | Preparation of Soap 3. Plaster of Paris | Making Toys How many of the above pairs are correctly matched?

Official answer key: B. The answer shown above (C) is the academy’s own.

Quick Lime

• Chemical Formula: CaO
• Common Name: Quick lime or burnt lime
• Preparation: Quick lime is produced by heating limestone (calcium carbonate, CaCO₃) in a kiln at high temperatures (about 900°C to 1000°C). This process is known as calcination.
• Chemical Reaction: CaCO₃ (s) → CaO (s) + CO₂ (g)
• Uses:
  • Construction: Used in the manufacture of cement and mortar. Hence, statement 1 is correct.
  • Chemical Industry: Acts as a reagent in various chemical processes.
  • Metallurgy: Used in the extraction of metals from ores and in the refining of metals.
  • Agriculture: Applied to soil to neutralize acidity and improve fertility.

Slaked Lime

• Chemical Formula: Ca(OH)₂
• Common Name: Slaked lime or hydrated lime
• Preparation: Slaked lime is produced by adding water to quick lime (calcium oxide, CaO). This process is known as slaking.
• Chemical Reaction: CaO (s) + H₂O (l) → Ca(OH)₂ (s)
• Process: The reaction is exothermic and results in the formation of a white, dry powder or a wet paste depending on the amount of water added
• Uses: Slaked lime is a common ingredient in traditional soap making. It's used as a lye substitute to create a milder, more skin-friendly soap. When slaked lime is added to fats and oils, it reacts to form a soap base that's gentle on skin. Hence, statement 2 is correct.

Plaster of Paris

• Chemical Formula: CaSO₄·0.5H₂O
• Common Name: Plaster of Paris
• Preparation: Plaster of Paris is produced by heating gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) to about 150°C. This process removes part of the water content, resulting in calcium sulfate hemihydrate.
• Chemical Reaction: CaSO₄·2H₂O (gypsum) → CaSO₄·0.5H₂O (Plaster of Paris) + 1.5H₂O (steam)
• Uses:
  • Construction: Used for making plasterboard, ceiling tiles, and decorative elements in buildings
  • Art: Commonly used for sculpting and making molds for casts and statues.
  • Medical: Applied in orthopaedics for making casts to immobilize broken bones. Hence, statement 3 is correct.

Additional information

• Soap is a salt of a fatty acid. It is produced through the chemical reaction between fats or oils and an alkali (commonly sodium hydroxide or potassium hydroxide).

Types of Soap

• Hard Soap: Made using sodium hydroxide, typically used for solid soaps.
• Soft Soap: Made using potassium hydroxide, usually results in liquid soaps.