The correct option is Dry slaked lime.
Explanation
Bleaching powder is chemically known as Calcium oxychloride ($CaOCl_2$). It is synthesized through the chemical interaction between chlorine gas and dry slaked lime.
- Production Process: Chlorine gas ($Cl_2$), often obtained as a by-product during the electrolysis of aqueous sodium chloride (brine), is passed over dry slaked lime ($Ca(OH)_2$).
- Chemical Equation: The reaction is represented as:
$Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$
Analysis of Options:
- Dry slaked lime Dry slaked lime: Correct. It provides the calcium and hydroxide base necessary to react with chlorine to form the hypochlorite component of bleaching powder.
- Quicklime Quicklime: Incorrect. Quicklime is Calcium oxide ($CaO$). It reacts with water to form slaked lime but is not directly used with chlorine to produce bleaching powder.
- Wet gypsum Wet gypsum: Incorrect. Gypsum is Calcium sulfate dihydrate ($CaSO_4 \cdot 2H_2O$) and is primarily used in the manufacture of Plaster of Paris and cement.
- Calcium carbonate Calcium carbonate: Incorrect. This is limestone ($CaCO_3$), which is a raw material for producing quicklime but does not react with chlorine to form bleaching powder.
Key Takeaway: Bleaching powder ($CaOCl_2$) is produced by the action of chlorine on dry slaked lime ($Ca(OH)_2$). It is widely used for disinfecting drinking water and bleaching cotton and linen in the textile industry.