The Melting Point is a primary criterion for the purity of solid organic compounds.

• Pure Solid: Characterized by a sharp melting point, meaning it transitions from solid to liquid over a very narrow temperature range (typically $<1^{\circ }C$).
• Impure Solid: The presence of even small amounts of impurity lowers the melting point (Melting point depression) and causes the substance to melt over a much wider temperature range.
• Identical compounds can be confirmed by 'Mixed Melting Point' - if a mixture of two samples shows no depression or widening of m.p., they are the same substance.