Introduction
This program was one of the first I made on Khan Academy. It was designed for the chemistry playlist. It's based on a simple physics (particle) simulation, which I've used for many things. But this one has charged particles, so has more interesting interactions.
The simulation is pretty effective despite being so simple. It illustrates quite nicely how a liquid can freeze or a solid can melt. It also show the phase transition: if you lower the temperature (of the container) until it's all frozen, then raise the temperature to 6, the temperature of the substance raises, then plateaus as the bonds melt and it becomes liquid, then the temperature rises again.