Which of the following processes are spontaneous, and which are nonspontaneous? (a) Freezing of water at 2 °C (b) Corrosion of iron metal (c) Expansion of a gas to fill the available volume (d) Separation of an unsaturated aqueous solution of potassium chloride into solid KCl and liquid water

A spontaneous process is one that occurs naturally under specific conditions without the need for external energy input. It is often associated with an increase in entropy, or disorder, in the system and its surroundings. The spontaneity of a process can be predicted using thermodynamic principles, particularly the Gibbs free energy change (ΔG), where a negative ΔG indicates a spontaneous reaction.

Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it is a key factor in determining the spontaneity of a process. Generally, processes that lead to an increase in the total entropy of the universe (system plus surroundings) are spontaneous. For example, the expansion of a gas increases entropy as the gas molecules spread out into a larger volume.

Phase changes, such as freezing or melting, are influenced by temperature and pressure conditions. For instance, water freezes at 0 °C under standard atmospheric pressure, but at 2 °C, the process may not be spontaneous unless the conditions favor it. Understanding the phase diagram of a substance helps predict whether a phase change will occur spontaneously under given conditions.