By Claire Gillespie – Updated Mar 24, 2022
Melting point is the temperature at which a solid turns into a liquid. In a reversible phase change, the melting point of a pure substance is equal to its freezing point – the temperature at which it solidifies. For example, ice melts at 0 °C (32 °F) and freezes at the same temperature, turning back into solid water. Determining a substance’s melting point is a reliable way to confirm its identity, because it is difficult to heat solids far beyond their intrinsic melting thresholds.
Three key factors shape a melting point: molecular composition, the strength of intermolecular forces, and the presence of impurities.
When molecules pack tightly and symmetrically, the material resists melting. Symmetrical neopentane, for instance, has a higher melting point than its branched isopentane counterpart because its molecules fit together more snugly. Size matters too: smaller molecules require less thermal energy to disrupt their arrangement. Ethanol (C₂H₆O) melts at –114.1 °C (–173.4 °F), whereas the bulkier polymer ethyl cellulose melts at 151 °C (303.8 °F). Giant covalent networks such as diamond, graphite, and silica contain hundreds of strong covalent bonds that must break before melting, giving them exceptionally high melting points.
Strong attractions between molecules raise the melting point. Ionic compounds exhibit high melting points because of powerful electrostatic ion–ion interactions. In organic chemistry, polarity and hydrogen bonding further elevate melting points. For example, the polar molecule iodine monochloride melts at 27 °C (80.6 °F), while the non‑polar bromine melts at –7.2 °C (19.0 °F). The greater the polarity or hydrogen‑bonding ability, the more energy is needed to separate the molecules.
Pure solids have a narrow, sharp melting range—typically 1–2 °C—because their molecules are uniformly packed. Any impurity introduces structural defects that weaken intermolecular cohesion, leading to a lower melting temperature and a broader melting range. This phenomenon, known as melting‑point depression, is a classic indicator of sample purity. For instance, a crystalline organic compound that is a single, well‑ordered molecule will melt at a distinct temperature, whereas a mixture of two different organic molecules will melt over a wider range because they cannot fit together perfectly.
