Final answer:
The solution was a homogeneous mixture because physical changes allowed the separation of components with different boiling points, indicating the presence of more than one substance.
Step-by-step explanation:
The evaporation of liquid with subsequent formation of salt crystals after the boiling of an unknown solution suggests that the solution was a homogeneous mixture and not a pure substance. In this scenario, the physical changes involved demonstrate that multiple components with different boiling points were present. The fact that all the liquid evaporated, indicating a lower boiling point, and left behind salt crystals suggests that water and salt were present and were separated physically through the process of evaporation and crystallization. Pure substances have a unique boiling point, and upon reaching it, all of the substance would transition uniformly. Since the components of the mixture were separated by their different boiling points, the original solution was not chemically pure but a mixture.