What is meant by melting point?

The term "melting point" refers specifically to the temperature at which a substance transitions from a solid state to a liquid state. This phase change occurs because the thermal energy supplied to the solid causes its particles to vibrate more vigorously, ultimately breaking the bonds that hold them in a fixed position.

At the melting point, the energies of the solid and liquid phases become equal, allowing the material to exist in both states simultaneously until the phase transition is complete. Knowing the melting point is crucial in identifying and characterizing materials, as it is a unique property for many substances.

The other options refer to different phase changes or properties of matter: the first choice addresses sublimation (solid to gas), the second describes freezing (liquid to solid), and the last one pertains to density, which relates to mass and volume rather than phase changes. Therefore, the chosen answer accurately captures the definition of melting point.