Question

Explain why energy must be added to a solid to cause it to melt.

Answer 1

This has to do with what it means for something to be a solid. A solid state means that the atoms are pretty much locked into one position. They are all kind of stuck together in this giant lattice (relative to their size at least hah) and they don't have enough energy to break away from each other. But when you add energy through heat... well then it's possible for the atoms to gain enough energy that they begin to break away from the attractive forces that keep them in a lattice structure. This is the process known to us humans as melting :)

Answer 2

@115.cowgirl.22 Furthermore, please you have to keep in mind that, when atoms are organized in a lattice, namely solid state, they aren't at rest, but they are oscillating continuosly. Finally when substance is melted, between those atoms an attractive force will exist, even if it is less stronger than the one which is acting on the same atoms, when they are arranged in a lattice structure. In other words, the melting process means that atoms increase their oscillation motion around their equilibrium positions until the strength force acting on them decreases so that they can also slide each other.