ANS:B - ethyl alcohol-water

In the ethyl alcohol-water system, the minimum boiling azeotrope forms at a composition of approximately 95.6% ethanol and 4.4% water by mass. This azeotropic mixture has a boiling point of around 78.2°C at atmospheric pressure (1 atm). Here's how it works:

1. Ethanol-Water Interaction: Ethanol (ethyl alcohol) and water are miscible in all proportions, meaning they can mix in any proportion without phase separation. This is due to the hydrogen bonding capability of both ethanol and water molecules.
2. Boiling Point Elevation: When pure ethanol or pure water is heated, they boil at different temperatures due to their different boiling points. However, when mixed together, the presence of ethanol molecules disrupts the hydrogen bonding network of water molecules, leading to a higher boiling point for the mixture compared to either pure component. This phenomenon is known as boiling point elevation.
3. Azeotrope Formation: At certain compositions, the interactions between ethanol and water reach a point where the mixture boils at a minimum or maximum temperature. In the case of the ethyl alcohol-water system, the azeotrope with the lowest boiling point occurs at a composition of approximately 95.6% ethanol and 4.4% water by mass. At this composition, the boiling point of the mixture is minimized, resulting in a constant boiling temperature of around 78.2°C.
4. Distillation Limitation: Because of the formation of the azeotrope, traditional distillation methods cannot be used to separate pure ethanol from water when the mixture contains a higher concentration of ethanol than the azeotropic composition. At the azeotropic composition, the vapor and liquid phases have the same composition, making further separation via simple distillation impossible.

This azeotropic mixture is commonly encountered in various industrial processes and laboratory applications. It poses challenges in the separation of ethanol from water through simple distillation, requiring alternative techniques such as extractive distillation or molecular sieves to achieve higher purity ethanol.