The transition of a substance from solid to liquid and then to gas represents a physical change. A physical change is one that does not alter the chemical composition of the substance. During these transitions, the molecules of the substance move from a structured arrangement in the solid state to a more fluid arrangement in the liquid state, and finally to a dispersed arrangement in the gas state. At no point in this process does the chemical identity of the substance change, which is the key factor that distinguishes physical changes from chemical changes.
When an impurity is present in a solid, two primary things happen to its melting point. First, the melting point typically decreases due to the disruption of the orderly arrangement of atoms or molecules in the solid structure. This phenomenon, known as melting point depression, occurs because the impurity interferes with the forces holding the solid together. Second, the melting range becomes broader, meaning that the solid may not melt at a specific temperature but rather over a range of temperatures. This is also a consequence of the impurity altering the structural integrity of the solid.
As for the effect of decreasing the external pressure, it generally lowers the melting point of a solid. When the pressure is reduced, the molecules of the solid require less force to break free from their arranged state, enabling them to transition into a liquid at a lower temperature. This relationship is particularly evident in substances that are sensitive to pressure changes, like ice, which can melt at lower temperatures under reduced pressure.