Advanced #75

The molar ionic volumes of salts show maxima as a function of temperature

The apparent molar volumes of ions in water show unusual temperature maxima.

Scientific Explanation

The apparent molar volume of an ion describes how much space one mole of that ion occupies in solution — including its effect on the surrounding water. In most solvents, this volume changes monotonically with temperature. In aqueous solutions, however, many ions exhibit a maximum in their apparent molar volume at specific temperatures.

This unusual behavior reflects the complex interaction between ions and the hydrogen bond network. At low temperatures, water molecules arrange themselves in a highly ordered fashion around an ion — the electrostriction effect compresses the water in the hydration shell. Upon warming, the hydration structure loosens, and the apparent volume increases. Above a certain temperature, however, the general thermal expansion of the solvent dominates, and the volume increase of the hydration shell relative to the solvent reverses, producing the maximum.

The temperature of the maximum depends on the type of ion. Small, strongly charged ions (like Na+) show their maximum at different temperatures than larger, weakly charged ions (like Cl-). This provides valuable insights into the structure of the hydration shell around different ions.

Molar Ionic Volumes Show Temperature Maxima Line chart showing apparent molar volumes of ions in water versus temperature. The curves show maxima at different temperatures depending on the ion, an unusual behavior not seen in most solvents. Two example ion curves are shown with peaks. Apparent Molar Volume (cm³/mol) Temperature (°C) 0 25 50 75 100 Na⁺ max Cl⁻ max Maxima occur at different temperatures Molar Ionic Volumes — Temperature Maxima
Apparent molar volumes of ions in water show temperature maxima that depend on the type of ion.

Everyday Relevance

The temperature dependence of ionic volumes has practical importance for oceanography, where precise density data of seawater is needed for current models. Seawater density depends not only on salinity and temperature but also on which specific ions are present at which concentrations. The unusual volume maxima must be accounted for in precise equations of state for seawater.

In chemical process engineering, where salt solutions are processed at various temperatures, these volume effects can influence process design and equipment sizing.