For a substance to dissolve, what must be true about delta S?

For a substance to dissolve, it is important for the entropy change (delta S) of the system to be positive. This increase in entropy reflects the greater disorder in the solution when the solute molecules separate and disperse among the solvent molecules.

When a solid solute dissolves in a solvent, the ordered structure of the solid breaks down as solute molecules are surrounded by solvent molecules and spread throughout the solution, increasing the randomness of the system. This higher level of disorder promotes solubility, as systems tend to favor configurations with greater entropy at a given temperature.

A positive delta S is particularly significant at constant temperature, as it relates to the second law of thermodynamics, which states that processes that increase total entropy tend to be favored. Therefore, a higher entropy associated with the dissolution of a substance favors the process of solvation and results in dissolution.

In contrast, a negative delta S would indicate a decrease in disorder, making dissolution less favorable, while a delta S of zero suggests no change in disorder, which does not support the impurification necessary for dissolution to occur. Additionally, the concept of delta S being equal to the heat of solution does not appropriately characterize the thermodynamic principles governing solubility, as these two quantities can be