CIE Chemistry Advanced Subsidiary (AS) Level Practice Test

The standard enthalpy change of solution is the energy change when one mole of solute dissolves in a solvent to form a solution that is infinitely dilute, measured under standard conditions. This means the amount of solute is so small that solute–solute interactions are negligible and only solute–solvent interactions contribute to the enthalpy change. That exact description—one mole dissolving to give an infinitely dilute solution under standard conditions—is why this option is correct. The other scenarios refer to dissolving to form a concentrated or saturated solution, or to processes like forming a compound from solvent, which do not define the standard enthalpy of solution.