ANS:C - constant boiling point

An azeotropic mixture is a constant boiling point mixture. Azeotropic mixtures are binary or ternary mixtures of liquids that have constant boiling points under certain conditions. In an azeotropic mixture, the composition of the vapor phase is the same as the composition of the liquid phase, and as a result, the mixture boils at a constant temperature without changes in composition. This phenomenon occurs when the vapor pressure of the mixture is the same as the vapor pressure of its individual components at a specific composition. So, the correct completion for the statement is: "constant boiling point". A constant boiling point, also known as an azeotropic point, refers to the temperature at which a mixture of two or more liquids boils at a constant composition. In other words, during the process of boiling, the vapor phase and the liquid phase have the same composition. Here's how it works:

1. Boiling Process: When a liquid is heated, it reaches a temperature at which its vapor pressure equals the external pressure (usually atmospheric pressure). At this temperature, bubbles of vapor form within the liquid, and the liquid transitions into the gas phase.
2. Normal Boiling Point: For a pure substance, the temperature at which this transition from liquid to gas occurs is called the normal boiling point. At this temperature, the vapor pressure of the liquid is equal to the atmospheric pressure.
3. Azeotropic Mixture: In the case of a mixture of two or more liquids, the boiling point of the mixture may not remain constant as the composition changes. However, in some cases, there exists a specific composition of the mixture at which the vapor phase and the liquid phase have the same composition. This composition is called the azeotropic composition.
4. Constant Boiling Point: At the azeotropic composition, the mixture boils at a constant temperature. Even if the mixture is heated further, the temperature does not rise above this constant boiling point until one of the components is completely vaporized. This is because any change in composition due to vaporization is offset by the simultaneous condensation of vapor back into the liquid phase.
5. Practical Importance: Constant boiling points have practical significance in various chemical processes, such as distillation and solvent recovery, where precise control over boiling temperatures and compositions is crucial for separation and purification.