[kartikwat]

Why there is no change in enthalpy or volume when components mixed form ideal solution and why there is change in volume and enthalpy when components mixed form non ideal solution.

[Corribus]

This is pretty much just the definition of an ideal solution. It is what it is. The reason the enthalpy change is zero is because it is assumed in an ideal solution that there are no significant interactions between the solute and solvent. Thus the only thermodynamic driving force for dissolution is entropy. An ideal solution is often also approximated as a dilute limit, although this isn't absolutely necessary, in which case any change of volume is small compared to the overall volume of the solution.

In a real (or non-ideal) solution, there are intermolecular interactions between solute and solvent molecules, so there is an enthalpy change as well as entropy change during mixing. Likewise, when the concentration is sufficiently large, the volume change can no longer be neglected.

Strictly speaking, "ideal" and "non-ideal" I am fairly sure refer only to the thermodynamic component, not the volume change. Even in a hypothetical ideal solution, if you add enough solute, you can't neglect the volume change.

[kartikwat]

This is pretty much just the definition of an ideal solution. It is what it is. The reason the enthalpy change is zero is because it is assumed in an ideal solution that there are no significant interactions between the solute and solvent. Thus the only thermodynamic driving force for dissolution is entropy. An ideal solution is often also approximated as a dilute limit, although this isn't absolutely necessary, in which case any change of volume is small compared to the overall volume of the solution.

In a real (or non-ideal) solution, there are intermolecular interactions between solute and solvent molecules, so there is an enthalpy change as well as entropy change during mixing. Likewise, when the concentration is sufficiently large, the volume change can no longer be neglected.

Strictly speaking, "ideal" and "non-ideal" I am fairly sure refer only to the thermodynamic component, not the volume change. Even in a hypothetical ideal solution, if you add enough solute, you can't neglect the volume change.

Hey but my question is why do volume or enthalpy increase or decrease in +ve deviation and -ve deviation respectively

[Corribus]

Well, what do you think?

[kartikwat]

I dont know

[Corribus]

If you mix two substances, A and B, and the attractive forces between A and B are stronger than those from either A to A or B to B, would you expect the enthalpy change of mixing to be exothermic or endothermic?

[kartikwat]

Exothermic

[Corribus]

It looks like you have your answer, then. Any other questions?

[kartikwat]

But why does volume decrease

[Corribus]

In a real solution the total volume of the mixture can be slightly more or slightly less than the total volume of the two substances when separate. Water and ethanol is a classic case, where if you mix X mL of water with Y mL of ethanol, the total volume of the mixture is slightly less than X + Y. As you may be aware, most of an atom or molecule is actually empty space. Molecules exist at an average distance from one another, and that distance is dependent on, among other things, their mutual attraction or repulsion from each other. In an ideal solution, which neglects these forces, this average intermolecular distance won't depend on what the nature of the two substances are, and so will be the same in the mixture as in the separate substances, so the total volume doesn't change. In a real solution, if the average degree of attractiveness between molecules changes between pure substances and the mixture, then the total volume element per molecule can change, which will impact the total volume of the mixture. This effect is certainly more noticeable and easy to understand in gasses, but the principles are virtually the same.