Explain what an ideal liquid solution is, and discuss why it is physically reasonable that a mixture that is well modeled as an ideal solution would have no miscibility gap.

Question

What will be an ideal response?

Answer 1

An ideal solution is one in which the components of the mixture interact in a way so that the intermolecular interactions that occur within the mixture are nearly equivalent to the intermolecular interactions that occur within the pure components.
It is reasonable to assume that an ideal solution would have no miscibility gap. Liquid-liquid equilibrium occurs when a mixture can lower its overall Gibbs free energy by separating into two phases. Remember that Gibbs free energy is expressed as:
?G=?U-T?S+P?V
The internal energy term is minimized when more intermolecular interactions occur (such as hydrogen bonding). The introduction of a second component into a liquid mixture can interrupt the intermolecular interactions of the pure component. Therefore, the mixture can split into two equilibrium phases that are more pure in one component. Although this lowers the entropy of the system (which increases the Gibbs free energy), the internal energy is minimized enough to overcome this.
For an ideal solution, the phase-splitting will not occur because the intermolecular interactions will not be substantially affected by the addition of a new component. Therefore, if the mixture were to split into two phases, it would result in the same internal energy term and a lower entropy term, therefore increasing the Gibbs free energy of the system. Therefore, it is favorable for an ideal solution to not exhibit a miscibility gap.