Question

A binary liquid solution is prepared by mixing n-heptane and ethanol. Which one of the following statements is correct regarding the behaviour of the solution?
(A) The solution formed is an ideal solution.
(B) The solution is non-ideal, showing positive deviation from Raoult’s law.
(C) The solution is non-ideal, showing negative deviation from Raoult’s law.
(D) n-heptane shows positive deviation while ethanol shows negative deviation from Raoult’s law.

Answer 1

Raoult’s law is stated for the ideal mixtures. But as all mixtures are real in nature, there would be positive and negative deviation from the law depending upon the polarities of the solutions comprising the mixture.

Complete step by step solution:
Let us know about Raoult's law before going towards the deviations.
Raoult’s law- The partial pressure of the components of an ideal mixture of liquids is equal to the product of the vapour pressure of the pure component and its mole fraction in the mixture.
Generally,
$P={{P}^{*}}x$
where,
P = partial pressure of the component
${{P}^{*}}$= vapour pressure of the pure component
x = mole fraction of the component in the mixture.
Real solutions- Most of the pairs of liquids forming the mixture do not have uniformity of attractive forces (adhesive and cohesive). Thus, deviation from Raoult's law is observed in these mixtures.
-The two types of deviations observed are;
Negative deviation- If the vapour pressure of a mixture is lower than as expected from Raoult’s law then the mixture shows negative deviation. This arises when the forces between the particles in the mixture are stronger than the mean forces of the particles in pure liquid. For example, a mixture of acetone and chloroform.
Positive deviation- If the vapour pressure of a mixture is higher than as expected from Raoult’s law then the mixture shows positive deviation. This arises when the cohesive forces between similar molecules are greater than the adhesive forces between dissimilar molecules. These dissimilarities in the polarities of the solution results in positive deviation from Raoult’s law. For example, a mixture of chloroform and ethanol.
Illustration- Here, n-heptane is non-polar and the ethanol is polar. Thus, the non-ideal mixture formed will deviate from Raoult's law. The cohesive forces of similar molecules are greater than that of the adhesive forces of dissimilar molecules. Thus, there will be positive deviation from Raoult's law.

Therefore, option (B) is correct.

Note: The polarities and the forces between the molecules of the mixture are responsible to decide the deviation pattern from Raoult's law. Also, when mixture is formed, it would be impossible to show different deviations within the solutions. So, option (D) can never be the option.