Question

Absolute alcohol cannot be obtained by simple fractionation of a solution of water and alcohol because: (a) Ethanol molecules are solvated (b) Boiling points are very near (c) Ethanol remains dissolved in water (d) They form a constant boiling mixture

Short answer

(d) They form a constant boiling mixture.

Step-by-step solution

Understanding the Problem

We need to understand why absolute alcohol cannot be separated and obtained from a solution of water and alcohol using simple fractionation. Simple fractionation separates components based on differences in boiling points.

Exploring Boiling Points

In simple fractionation, components are separated if there are distinct differences in boiling points. If the boiling points are very close, fractionation becomes ineffective because both components evaporate together.

Identifying Constant Boiling Mixtures

Water and ethanol form an azeotrope, a constant boiling mixture, where the mixture boils at a temperature different from either of its components. Because of this, they boil together at a constant ratio, making separation by fractionation ineffective.

Checking Each Option

Option (a) refers to solvation, which isn't the main reason fractionation fails. Option (b) is partly correct since boiling points are close but doesn't fully explain the difficulty. Option (c) implies ethanol is too soluble, but solubility isn't the issue. Option (d) about constant boiling mixtures (azeotropes) is accurate.

Key concepts

Simple Fractionation

Simple fractionation is a technique used to separate liquid mixtures into their individual components based on their boiling points. This is achieved by heating the mixture until it reaches the boiling point of the more volatile component, causing it to vaporize. The vapor is then cooled and condensed back into a liquid in a separate container.

• Simple fractionation relies heavily on differences in boiling points. If the boiling points of the components are significantly different, fractionation can effectively separate them.
• However, if the boiling points are close, like those of water and ethanol, the method becomes problematic. Both components may start to evaporate at nearly the same time, leading to imperfect separation.

Because simple fractionation struggles with closely boiling point mixtures, it is often substituted with more complex methods for such mixtures. This technique sets the stage for understanding why obtaining absolute alcohol through simple fractionation is not feasible.

Boiling Points

The concept of boiling points is central to understanding why simple fractionation may fail in certain scenarios, such as in the separation of alcohol and water.

• The boiling point is the temperature at which a substance transitions from a liquid to a gas. For pure water, this is 100°C (212°F), and for ethanol, it's around 78.37°C (173.07°F).
• When two liquids have closely aligned boiling points, like ethanol and water, separating them becomes a challenge. If their boiling points are very close, both may enter the vapor phase simultaneously, creating a mixture of gases.
• This mixture then condenses back into a liquid that still contains both components, rather than separating them.

In systems that form azeotropes, boiling points do not determine separation effectiveness. Instead, the constant boiling nature of the azeotrope dictates behavior.

Absolute Alcohol

Absolute alcohol, also known as anhydrous ethanol, refers to ethanol that contains no more than 1% water. Obtaining it requires overcoming the limitations posed by azeotropes.

• An azeotrope is a mixture that behaves like a single substance during boiling; it has a constant boiling point and vapor composition.
• In the case of water and ethanol, the azeotrope forms at around 95.6% ethanol and 4.4% water, with a boiling point of approximately 78.1°C, which is close to ethanol's boiling point.
• This means that boiling the mixture does not change its composition, posing a significant barrier to achieving pure ethanol through simple fractionation.

Instead, other methods, such as using chemical drying agents or more sophisticated distillation techniques, are required to obtain absolute alcohol. Understanding this highlights the complexities involved in purifying ethanol beyond the capabilities of simple fractionation.