The animation compares pairs of compounds chosen according to the following criteria. Paired together are:

• Familiar salts of very different solubility
• Salts of similar packing structures
• Salts whose solubility is either not significantly affected by ion reaction with water (such as acid–base and complex formation reactions), or affected in a way that opposes the observed solubility differences
• Salts that essentially require discussion of only configurational disorder or only thermal disorder

The pairs chosen for these animations are NaCl/CaCO₃ and MgCO₃/MgSO₄. In the former case, both dissolution phenomena are almost athermic, hence the major differences are in configurational disorder and solvation effects. In the latter pair, the major difference lies in thermal disorder, as the dissolution of magnesium sulfate is quite exothermic.

The program simulates the “before” and the “after” states for the dissolution of equal amounts of the two salts of each pair in identical amounts of water. Changes in total entropy and in its components are represented qualitatively in bar graphs.

In the case of NaCl versus CaCO₃, the animation illustrates that the small solubility of CaCO₃ in water is due to the decreased mobility of the water molecules associated with the small, dipositive calcium ion. This decrease results in a decrease in total entropy upon dissolution of calcium carbonate, and thus the amount dissolved is very small. On the other hand, the dissolution of NaCl is substantial because it yields a gain in total entropy since there is not a similar decrease in motility.

Figure 1. A screenshot from Computer Simulations of Salt Solubility comparing the dissolution of NaCl and CaCO₃.

For the pair MgCO₃/MgSO₄, where the solvation effects are similar, the increased thermal disorder of dissolved MgSO₄ is illustrated by greater particle agitation. The dissolution of MgSO₄ yields greater thermal entropy and particle movement, and thus it is much more soluble than MgCO₃.

Figure 2. A still image from Computer Simulations of Salt Solubility of the animation comparing the dissolution of MgCO₃ and MgSO₄.

A word of reservation is due. First, the simulations are qualitative in nature, and no rigorous calculations of the contributions to entropy changes are illustrated. Also, when using these pictorial simulations, students should be alerted specially to the unrealistically small number of particles represented and the unrealistically large spaces between them, to the symbolic representation of ions and molecules, and to qualitative illustrations of only some molecular motions. In addition, exchange processes involving water molecules of the hydration sphere are not shown.