What is ideal solution?

The binary liquid-liquid solution can be divided into two types:

1. Ideal solutions
2. Non-ideal solutions

What is an Ideal Solution?

An ideal solution is one which obeys Raoult’s law over the whole collection of concentration.

• This type of solutions is formed by combining two components which have the same molecular size, structure and same intermolecular forces.
• The intermolecular interactions which take place between the components (A-B attractions) are of the same size as the intermolecular interactions taking place in pure components (A-A and B-B attractions).
• The result of mixing of pure components to form the solution is zero
• This means that heat is absorbed or emitted when the components are mixed in the solution.
• The resultant volume for the formation of the solution is also zero.
• The volume of the entire solution is equal to the sum of volumes of the components.
• The intermolecular attractive interactions of pure components are of A-A and B-B type.
• The intermolecular attractive interactions of binary solutions are of A-A, B-B and A-B type.
• When the intermolecular attractive forces between the A-A and B-B type are equal to the intermolecular attractive forces between A-B, then it results in the formation of ideal solution.
• There is no such perfect ideal solution. Only that they are almost ideal in behavior.

As per Raoult’s law, the partial vapor pressure of the two components of the solution can be shown as below:

The ideal solutions have the following traits:

1. The change in heat in mixing is zero: As no change takes place in the magnitude of the attractive forces in the two components which are present, the change in heat on mixing in such solutions is zero.
2. The change in volume on mixing is zero: In ideal solution, the volume of the solution is the total of the volume of the components before mixing which means that there occurs no change in volume on mixing oris zero.

The solutions become ideal once they are diluted.

In short, following are the characteristics of an ideal solution:

• It obeys Raoult’s law.
• must be zero.
• must be zero.

Examples of Ideal Solutions

• Benzene and toluene
• n-hexane and n-heptane
• Bromoethane and iodoethane
• Chlorobenzene and bromobenene

The solutions which obey Raoult’s law are known as ideal liquid solution.

Composition in Vapor Phase

The composition of vapor phase which is equal with the solution is made by the partial pressure of the components. Suppose y1 and y2 are the mole fraction of the two components, 1 and 2 respectively in the vapor phase, then as per Dalton’s law of partial pressure:

Partial pressure of a component= Mole fraction of the component * total pressure

            P1=y1p

            P2=y2p

Or, p1=y1p total

Mole fraction of component 1 in vapor phase y1=p1/p

Mole fraction of component 2 in vapor phase y2=p2/p

Mole fraction of a component in vapor phase=Partial vapor pressure of component/Total vapor pressure

What are non-ideal solutions?

The solutions which do not obey Raoult’s law over the whole collection are called non-ideal solutions. For such solutions,

The vapor pressure of non-ideal solutions will be either higher or lower than said by Raoult’s law.

In non-ideal solutions, there occurs change in volume and heat energy when the two components are combined together.

Most of the solutions are non-ideal because they donot behave in the same manner as ideal solution.

So, for non-ideal solutions, no components obey Raoult’s law over the whole collection i.e.

Types of non-ideal solutions

Non-ideal solutions either show positive or negative traits from the ideal behavior depending upon their nature.

Non-ideal solutions showing positive deviations from Raoult’s law

Let us consider a binary solution of two components A and B.

If the A-B interactions in the solutions are weak than the A-A and B-B interactions, then A and B types of molecules becomes more. Thus, the vapor pressure of the component is greater than that predicted by Raoult’s law. This is defined as positive deviations from Raoult’s law.

Some examples of solutions showing positive deviations are:

• Ethyl alcohol and cyclohexane
• Acetone and carbon disulphide
• Benzene and acetone
• Carbon tetrachloride and chloroform
• Acetone and ethyl alcohol
• Ethyl alcohol and water

Non-ideal solutions showing negative deviations from Raoult’s law.

In this type of solution, the A-B interactions are stronger than the A-A and B-B interactions which are present in the two liquids. As A-B interactions are strong, the tendency to escape of A and B molecules are less. Thus, it has a partial vapor pressure than that predicted by Raoult’s law. Hence, the total vapor pressure is less than that of an ideal solution.

Examples of Negative Deviation

• Acetone and chloroform
• Chloroform and diethyl ether
• Chloroform and nitric acid
• Acetone and aniline
• Water and nitric acid
• Diethyl ether and chloroform