The change of state in which a solid becomes liquid by adding heat or changing pressure is called melting.

Here's a breakdown:

* Adding heat: When you add heat to a solid, the molecules within the solid gain kinetic energy (energy of motion). This increased energy causes the molecules to vibrate more rapidly and break free from their fixed positions in the solid structure, leading to the transition to a liquid state.

* Changing pressure: Increasing pressure can also cause melting. This is because increasing pressure forces the molecules closer together, which can overcome the forces holding them in a solid structure.

Let me know if you'd like to know more about the process of melting!