What happens when you add thermal energy to a solid?

The thermal energy of a solid is a function of its temperature, and transferring energy to the solid by heat will result in the increase of the solid's temperature.
On a microscopic level, the thermal energy of a solid is determined by the average speed of the random motion of the molecules inside the solid. Typically, the molecules in a solid are constrained, so they cannot freely move or rotate, but they can vibrate. The faster the molecules vibrate, the higher the thermal energy of the solid, and thus, the higher the temperature. When energy is transferred to the solid, the change in the solid's temperature will depend on the characteristic of the solid called "specific heat," which depends on the solid's molecular structure.
If the temperature of a solid reaches a value called the "melting point," and the energy is still being added to the solid, then the solid will undergo a phase change, becoming a liquid—that is, it will melt. (The melting point depends on the type of material of the solid.) During the phase change, until all of the solid is melted, the temperature of the solid remains constant. Additional incoming energy is used to break the bonds between the molecules, which then can move more freely, rather than to increase the temperature of the material. The distance between the molecules will then increase, resulting in the change in the state of matter.
Once the solid is completely liquefied, if the energy is still being added, the temperature of the resultant liquid will increase until it reaches the point of vaporization. Then, the liquid will start turning into a gas.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/inteng.html