Advanced #37

No aqueous solution is ideal

Aqueous solutions always deviate from ideal behavior due to the unique properties of water.

Scientific Explanation

An ideal solution obeys Raoult’s law: the vapor pressure of each component is proportional to its mole fraction, and mixing produces no heat change or volume change. In practice, many organic solvent mixtures approximate this ideal reasonably well. Aqueous solutions, however, never do — they invariably deviate from ideal behavior.

The reason lies in the unique structure of liquid water. The extensive hydrogen bond network responds sensitively to any dissolved substance. Even small amounts of a foreign substance disrupt this network: some solutes strengthen the local order (kosmotropes), others break it down (chaotropes). In no case does the network remain unaffected.

Aqueous solutions therefore always exhibit either positive or negative deviations from Raoult’s law. Positive deviations occur when water-solute interactions are weaker than water-water bonds (for example, ethanol-water). Negative deviations arise when the solute forms particularly strong bonds with water (for example, hydrochloric acid-water).

Non-Ideal Aqueous Solutions Diagram comparing ideal mixing (Raoult's law) with real aqueous solution behavior. The ideal line is straight, but the real curve deviates significantly, showing either positive or negative deviation from ideality. Vapor Pressure Mole Fraction Solute 0 0.5 1.0 Ideal (Raoult) Positive deviation (e.g., ethanol-water) Negative deviation (e.g., HCl-water) No aqueous solution follows the ideal straight line Aqueous Solutions — Always Non-Ideal
Aqueous solutions always deviate from ideal behavior (dashed line) — either positively or negatively.

Everyday Relevance

The non-ideality of aqueous solutions has far-reaching consequences. In the food industry, it affects the freezing and boiling points of solutions (which is why salt water freezes below 0 degrees Celsius). In pharmacy, it determines drug solubility and bioavailability. Oceanography also relies on understanding non-ideal salt solutions, since the salinity of the oceans influences every physical property of seawater — from density and freezing point to osmotic pressure.