Every solid has a breaking point — a temperature at which the organized structure of its atoms or molecules can no longer hold together against thermal vibration, and the substance begins to flow as a liquid. This is the melting point, one of the most fundamental physical properties of any substance. At this exact temperature, solid and liquid phases coexist in equilibrium: add heat and more solid melts, remove heat and more liquid freezes. For pure water, this happens at 0°C — a temperature so familiar it became one of the anchor points for the Celsius scale itself.
The range of melting points across the periodic table is staggering. Mercury melts at -39°C, which is why it is the only metal that is liquid at room temperature — and why it was used in thermometers for centuries. Gallium melts at just 29.8°C, meaning it will literally melt in your hand on a warm day. At the other extreme, tungsten holds the record for the highest melting point of any element: 3422°C, which is why it was chosen for incandescent light bulb filaments — it glows white-hot without melting. Carbon does not truly melt at normal pressures; at about 3550°C it sublimes directly from solid to gas, though under extreme pressure it melts around 4800°C.
Melting points reveal the strength of intermolecular forces. Ionic compounds like sodium chloride (801°C) have high melting points because enormous energy is needed to overcome electrostatic attraction in the crystal lattice. Metals vary based on bonding strength — lead melts at 327°C but iron requires 1538°C. Molecular compounds with weak intermolecular forces melt at low temperatures: oxygen melts at -218°C, and helium is unique in never forming a solid at normal pressure — you would need about 25 atmospheres of pressure to freeze it even at absolute zero. Engineers consult melting points constantly: choosing solder for electronics, selecting alloys for jet engines, and designing furnace linings that will not soften during use.