Final answer:
A solid changes to a liquid when heat is added because energy is used to increase the spacing between particles, overcoming attractive forces, which turns the solid into a liquid without raising the temperature until the phase change is complete.
Step-by-step explanation:
When heat is added to a solid, the spacing between particles increases because the thermal energy overcomes the forces holding them together. During the melting process, the energy is used to partially overcome the attractive forces between particles in a solid to form a liquid, which explains why no temperature rise occurs despite the addition of energy. As a consequence, the structure of the solid becomes less ordered and gains the ability to flow, transitioning into the liquid state.
The heat supplied during melting goes into breaking the intermolecular bonds in the solid without increasing the particles’ kinetic energy, which is why we don't observe a temperature change until the phase change is complete. Once the solid has turned into a liquid, if more heat is added, the temperature will then continue to rise until the liquid's boiling point is reached, where a similar stabilization of temperature will occur as the liquid becomes a gas.