Ideal and Non-Ideal Solution: Raoult’s Law & Deviation

In an ideal solution, there is no change in volume upon mixing two components, i.e. ΔV_mixing. While, in Non-Ideal Solutions, the condition requires= ΔV_mixing​ = 0, ΔH_mixing​ = 0. Thus, each component in an Ideal and non-ideal solution either follows Raoult's law or it does not.

All the components in an ideal solution follow Raoult's law under every condition of temperature and concentration. However, a Non-ideal solution cannot be expressed by Raoult's law or by Henry's law over an entire range of concentrations.

When two or more two substances are mixed homogeneously then they form a solution. A solution is a homogeneous mixture, meaning that its properties and composition is uniform throughout the mixture. The two necessary substances present in solutions are solute and solvent.  The substance with low quantity is the solute, while the substance with excess quantity is called the solvent. Solutions can be further divided into many types:

• Ideal Solutions
• Non-ideal Solutions
• Binary Solutions
• Ternary Solutions
• Saturated Solutions
• Unsaturated Solutions
• Supersaturated Solutions

Key Terms: Solutions, Solubility, Solvent, Mole Fraction, Colligative Properties, Ideal Solutions, Non-ideal Solutions, Solute

Raoult's law

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According to Raoult's law, “the vapour pressure of any solution containing a non-volatile solute in a volatile solvent is proportional to the mole fraction of the solvent in the solution.”

Raoult's law was first given by French chemist François-Marie Raoult in the year 1887. It is applicable for a solution of two volatile liquids. It claims that the vapour pressure of the solution is proportional to the mole fraction of the solvent.

Thus, Raoult’s Law formula is:

Thus,

P_solution = Χ_solvent.P_solvent

Where,

• P_solution = Vapor pressure of the solution
• Χ_solvent = Mole fraction of the solvent
• P_solvent = Vapor pressure of the pure solvent

Solutions Video Lecture

Example of Raoult’s Law

Assume a solution of volatile liquids A and B in a given beaker. Since both A and B are volatile, both particles A and B are in the vapour phase. Consequently, the vapour particles of A and B thus exert partial pressure, contributing to the total pressure above the solution.

Raoult’s Law

Raoult’s Law Also states that in equilibrium,

⇒ P_A = P°_Ax_A, P_B = P°_Bx_B

Here,

• P_A = Partial pressure of A
• P°_A = Vapour pressure of pure A at that temperature
• x_A = Mole fraction of A in the liquid phase

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Ideal and Non-Ideal Solution 

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A homogeneous mixture composed of two or more components is called a Solution.

• The component which has the highest quantity in a solution is the solvent.
• The solvent helps determine the physical state of the solution.
• A solute is a substance, in a solution, found in a smaller amount.
• The solution that has two components is called a Binary Solution.
• The properties of a solution are further defined on basis of the Vapour Pressure, Boiling Point, Freezing Point and other Colligative Properties.

The binary liquid in liquid solutions can be further divided into two terms:

• Ideal Solution
• Non-ideal Solution

Ideal Solution

The solutions which follow Raoult's law at all temperatures and concentrations are called Ideal Solution. During its formation no change in enthalpy or volume takes place. In ideal solutions, the interaction between solute-solute and solvent-solvent molecules same as the interaction between solute and solvent molecules.

An ideal solution should possess the following characteristics:

• It should follow Raoult's law
• No heat or energy released during the formation of the solution means the enthalpy of mixing should be zero. e, ΔH_(mix) = 0
• The volume of the solution should be constant means the volume of the solution will be equal to the volume of the components. ΔV_(mix) = 0

Practically, no solution behaves as an ideal solution. The substances of similar structure and polarity mix together to form an ideal solution.

Examples of Ideal Solution

The following examples of nearly ideal solutions :

• Benzene and Toluene
• n-hexane and n-heptane
• Chlorobenzene and Bromobenzene
• Ethyl bromide and ethyl iodide 

Non-Ideal Solutions

The solutions which do not follow Raoult's law at all concentrations and temperatures are termed as non-ideal solutions. It is also termed real solutions.

The enthalpy of a non-ideal solution does not equal zero and it also changes volume. For example, when sulphuric acid is mixed in water then heat is released and a volume change is also observed. Thus, it is a non-ideal solution.

Non-Ideal solutions possess the following characteristics:

• It does not follow Raoult's law
• The enthalpy during the formation of the solution is not zero. ΔH_(mix), not 0
• The volume after mixing may not be constant. ΔV_(mix) not 0.

The non-ideal solutions show deviation from Raoult's law. Based on deviation the non-ideal solutions are of the following two types:

• Positive Deviation
• Negative Deviation

Positive and Negative Deviation from Raoult’s Law

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Depending on the deviation of Non-ideal solutions,it can be further classified as:

• Positive Deviation
• Negative Deviation

Positive Deviation from Raoult’s Law

Most of the liquid mixtures form a non-ideal solution.

• In this solution, the interaction between solute and solvent is weaker than the interaction between solute-solute and solvent-solvent. Thus, A – B < A – A or B – B.
• P_A > P_A⁰ x_A and P_B > P⁰_B x_B (as total vapour pressure, i.e.P_A⁰ x_A + P⁰_B x_B) is greater than predicted with respect to Raoult’s Law
• Sometimes the partial vapour pressure of a miscible liquid is greater than the expected vapor pressure as per Raoult's law.
• The total vapour pressure will be greater than the corresponding vapor pressure in the case of an ideal solution of the same composition.
• The boiling point of such solutions is lowered. This behaviour of a non-ideal solution shows a positive deviation from Raoult's law.

Graphically, it can be represented as:

Positive Deviation from Raoult’s Law

The solutions showing positive deviations have an intermediate composition at which the total vapor pressure is maximum, which is greater than the vapor pressure of the pure liquid. The boiling point of such solutions is minimum. This type of solution boils at a constant temperature and gets distilled without changing its composition. Such a mixture is called an Azeotropic mixture.

The non-ideal solutions showing positive deviation are as follows:

• Acetone + Carbon disulphide
• Acetone + Ethyl Alcohol
• Acetone + Benzene
• Carbon tetrachloride + Chloroform

Negative Deviation from Raoult’s Law

In case of Negative deviation,

• In this solution, the total vapour pressure P_A < P_A⁰ x_A and P_B < P⁰_B x_B, of the solution, becomes lower than the expected vapour pressure as in Raoult's law.
• Here, the interaction between the solute-solvent molecules is stronger than the solute-solute and solvent-solvent molecules, i.e. A – B > A – A or B – B. This solution is said to exhibit a negative deviation from Raoult's law.
• The boiling point of these solutions can be increased with the addition of solute.

Graphically, it can be represented as:

Negative Deviation from Raoult’s Law

​For a certain intermediate composition, the total vapour pressure of the solution is minimum and the boiling point is maximum. This solution can be distilled without any change in its composition. Hence, it is also called an azeotropic mixture or maximum boiling azeotrope.

Some examples of non-ideal solutions showing negative deviation from Raoult's law are as follows:

• Chloroform + Benzene
• Acetone + Aniline
• HCL + water
• Nitric acid + water
• Chloroform + Diethyl ether

Difference between Ideal and Non-Ideal Solutions

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The main differences between ideal and non-ideal solutions based on certain properties are mentioned in the below table :

Things to Remember

• In a solution, a solute is a substance with low quantity, while that with excess quantity is called a solvent.
• Solutions are of many types namely Binary Solution, Ternary Solution, Dilute Solution, Concentrated solution, Saturated solution, Unsaturated solution, Supersaturated solution, Ideal solution, and Non-Ideal solution.
• According to Raoult's law, the vapour pressure of any solution containing a non-volatile solute in a volatile solvent is proportional to the mole fraction of the solvent in the solution.
• The Vapour pressure of a solution is proportional to the mole fraction of solvent, thus, P_solution = Χ_solvent.P_solvent
• The solutions which follow Raoult's law at all temperatures and concentrations are called Ideal solutions.
• The solutions which do not follow Raoult's law at all concentrations and temperatures are termed as non-ideal solutions.

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Previous Year Questions

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