Here's why:
* Phase diagrams show the conditions of temperature and pressure at which different phases (solid, liquid, gas) of a substance exist.
* Melting point is the temperature at which a solid changes into a liquid.
* The solid-liquid equilibrium line on the phase diagram represents the conditions where solid and liquid phases can coexist in equilibrium. This means that at any point along this line, the solid and liquid phases are at the same temperature and pressure, and they can transition between each other.
Important Note: The melting point is specifically the temperature at which the solid and liquid phases are in equilibrium at a given pressure. This point on the solid-liquid equilibrium line will be different for different pressures.
