What happens to a solid when thermal energy is added?

This happens at the boiling temperature of every substance that can vaporize. At the boiling temperature, adding heat energy converts the liquid into a gas WITHOUT RAISING THE TEMPERATURE.The same thing happens when a solid changes into liquid. For instance, ice and water can exist together at the melting temperature.

First, it depends on the solid, solids melt at different temperatures, but to answer your question, we would say this:

When a solid is heated with thermal energy, the particles in the solid start to vibrate rapidly until the particles cannot hold each other together anymore, then they break apart into a liquid.

Hope I helped.