Why do impurities decrease the melting point melting point and increase the range of the melting point of a substance?
Impurities in a solid disrupt the bonds that hold the solid together. Atoms or molecules in a solid lattice are held together by their attraction for each other. These intermolecular attractions can be ion-ion, dipole-dipole or dispersion forces. When other atoms or molecules are present their interactions with the pure substance reduce the attractions between the like atoms or molecules, reducing the amount of energy needed to separate them. Pure solids tend to melt at a specific temperature, remaining at that temperature until the entire sample is melted. Mixtures, on the other hand, melt over a range. This is because the particles being held together in a pure substance have a specific lattice energy while those in a mixture have more variation in intermolecular forces.