MCQ Detailed View

Rectified spirit is a mixture containing about 95% ethanol and 5% water. It is produced by repeated fractional distillation of fermented molasses or other sources of ethanol. However, due to the azeotrope formed between ethanol and water at 95% concentration, simple distillation cannot remove the last fraction of water. To obtain ethanol in its purest form, known as absolute alcohol (100% ethanol), a dehydrating agent is required to chemically or physically remove the water content.

Among the various drying agents, quicklime (CaO) is traditionally used. When rectified spirit is treated with CaO and kept for several hours, the CaO absorbs water from the mixture, leaving behind pure ethanol. After this treatment, the liquid is distilled again to obtain absolute alcohol. The reaction is not chemical but absorptive in nature, where CaO strongly combines with water to form calcium hydroxide, thus removing water from the ethanol-water mixture.

Other options mentioned in the question are not suitable for preparing absolute alcohol. Ca(OH)₂ is already hydrated and cannot remove additional water effectively. CaCO₃ is a neutral salt and has no dehydrating property. CaCl₂ is a drying agent but it forms complexes with alcohol, making it unsuitable for producing absolute ethanol. Hence, the correct choice is CaO.

Absolute alcohol is very important in organic chemistry and industrial applications. It is used as a solvent for reactions where even a small amount of water can interfere with the process. It is also used in the manufacture of synthetic chemicals, pharmaceuticals, dyes, perfumes, and laboratory reagents. In fuel research, absolute alcohol is used for blending with gasoline to form gasohol.

Thus, rectified spirit treated with CaO and redistilled gives absolute alcohol, which is ethanol free from water impurities.