This model represents the changes of state at the micro-particle level. By heating up and cooling down a set of particles, students can explore the fundamental features of three states.
They are offering Mechanisms (software for modeling organic chemistry reaction mechanisms) and ModelAR (a virtual molecular modeling program) for free. (9-12)
Lots of resources listed here.
NGSS Aligned Chemistry Units (9-12).
Great site to teach Claim, Evidence, and Reasoning.
Great resources connected to NGSS.