Question

Why do impurities reduce the melting point measurement value and increase the measurement range?

Answer 1

The presence of impurities in a crystalline substance causes lowering in the melting point of the substance. This is a colligative property, called "depression in freezing point".

The freezing point of a solid is the temperature at which the vapor pressures of the substance in solid and liquid phases become equal to each other. On addition of an impurity to a substance, the vapor pressure of its liquid phase decreases because of the relative decrease in the concentration of the liquid at the surface. Since pressure is directly proportional to temperature, reduction in vapor pressure leads to reduction in melting point of the crystalline solid.

Introduction of an impurity to a crystalline solid compromises its crystallinity. What this means is the crystalline nature of a solid decreases due to the impurities present in a substance. Since crystalline solids have sharp melting points and non-crystalline solids do not, the impure solids have a range of melting instead of a sharp melting point.