In chemistry there are ideal models/concepts of ideal crystal, ideal gas and even ideal solution, but there is no model/concept of ideal liquid.

In physics (fluid dynamics, to be precise) there is a model of perfect fluid, a fluid without internal friction and, as a consequence, no viscosity, and thermal conductivity. The model covers macroscale physical properties and doesn't really touch what's happening on the "chemistry level".

Why chemistry lacks a proper model of ideal liquid when a seemingly more complex model of an ideal solution has been developed? Isn't there a need for one, and if so, is there a concept of ideal fluid that comes the closest to be an alternative of, let's say, an ideal gas law, but for liquids?


The nearest to an ideal fluid is a hard sphere fluid, but this is removed from the ideal gas or even solution concept of ideality in a critical way.

Ideal (also called perfect) gases are ideal because they lack intermolecular interactions. A statistical description that ignores the intermolecular potential suffices to describe an ideal gas. A first extension toward nonideality, following the vdW equation, describes particles in a gas as hard spheres (having an excluded volume).

The property of reduced compressibility encoded by a hard sphere potential also provides the simplest model for a liquid. The hard sphere potential suffices to describe a simple liquid, and it is about as ideal an abstraction of a liquid as you can get. However, while it shows a liquid-solid phase transition, it does not display a liquid-gas transition. In that sense it is not a universal model, it does not capture some key properties of liquids, in the same sense that the ideal gas law is not universal because it too does not address some simpler key properties of real gases (such as excluded volume). However, introduction of an additional attractive term as in the full vdW model, bringing us further away from ideality, allows us to observe a critical point below which a phase transition clearly occurs between liquid and gas. I should add that the hard sphere potential can be expressed in reduced units, making it universal within the scope of that limited model. It is a suitable first approximation for understanding some properties of nonpolar liquids, for instance condensed noble gases.

I would add that Raoult's law is another ideal model for a liquid, and consistent with the ideal gas concept (or rather Dalton's law). It is also a limiting law (or model), like the ideal gas, and applies to dilute solutions.

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    $\begingroup$ The vdW equation exhibits a critical point, and a liquid-gas transition below the critical point. It does not show a liquid-solid transition. See, for example, physics.stackexchange.com/questions/453490/… $\endgroup$ – Jon Custer Jan 21 at 17:35
  • $\begingroup$ @JonCuster I meant the hard sphere fluid shows a liquid-solid transition, not the vdW gas. The hard sphere fluid can be considered a van der Waals substance without an attractive potential term. $\endgroup$ – Night Writer Jan 21 at 17:38
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    $\begingroup$ Fair enough. But, I might claim the vdW model as an 'ideal' fluid. But we all know that ideality is never the case for our real problems (sigh). $\endgroup$ – Jon Custer Jan 21 at 17:41
  • $\begingroup$ @JonCuster Yes, the vdW fluid is an ideal fluid, no doubt about it. You can regard hard sphere fluids as a subset of vdW fluids, without an attractive potential term. $\endgroup$ – Night Writer Jan 21 at 18:30

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