Final answer:
Impure samples have lower melting points because impurities disrupt the orderly structure of a solid, resulting in a system that requires less heat to transition from solid to liquid. Examples include lower melting points in the production of aluminum and in the manufacturing of glass.
Step-by-step explanation:
Impure samples melt at lower temperatures due to a few fundamental reasons associated with thermal dynamics and the structure of the mixtures. The presence of impurities within a solid disrupts its ordered structure, causing a less structured system where the melting point is reduced compared to that of a pure substance. This phenomenon is observed in various materials, including the production of aluminum and the manufacturing of types of glass.
For instance, when Hall and Heroult were trying to electrolyze natural aluminum compounds, they discovered that by using a mixture of Al2O3 and cryolite (Na3AlF6), the melting point could be reduced, enabling more efficient production. In glass-making, the use of various oxides such as SiO₂, Al₂O₃, CaO, and Na₂O can result in glasses with lower melting points, making them cheaper to produce.
When we discuss the process of melting or the enthalpy of fusion, we look at how much heat is absorbed during the transition from solid to liquid. Impurities disrupt the orderly crystal lattice of a solid, allowing for a broader distribution of molecular speeds and resulting in a lower temperature required for the solid to transition into the liquid phase.