What is the standard enthalpy change of hydration associated with?

The standard enthalpy change of hydration specifically refers to the energy change that occurs when gaseous ions dissolve in water to form an aqueous solution. When a gaseous ion is surrounded by water molecules, an interaction takes place where the polar water molecules stabilize the ion. This process is exothermic—releasing energy—as the hydration shell forms. The enthalpy change is a measure of the energy released when these gaseous ions become hydrated.

In contrast, the other options do not accurately describe this process. The formation of a gas from a solution pertains to changes in the state of matter rather than hydration. The dissolving of solids in a solvent often involves different types of energy changes, such as lattice energy in the case of ionic solids, rather than specifically discussing the hydration of ions. Lastly, the mixing of two different liquids is more related to interactions between liquid molecules rather than the hydration of ions. Thus, the correct response focuses on the specific interaction of gaseous ions with water, which encapsulates the principle of standard enthalpy change of hydration.