Use the kinetic molecular theory to explain the properties of solids, liquids, and gases, and describe the energy changes that occur during phase changes.
The kinetic molecular theory explains the properties of matter using one central idea: all particles are constantly in motion, and the amount of that motion (kinetic energy) depends on temperature. In a solid, particles vibrate in place but are held tightly by strong attractions, giving solids a fixed shape and volume. In a liquid, particles have enough energy to slide past one another while still staying close together, giving liquids a fixed volume but a shape that flows to match its container. In a gas, particles have enough energy to move independently and spread far apart, so gases have neither a fixed shape nor a fixed volume, expanding to fill whatever container holds them.
A phase change is a physical change between these states, and it always involves energy moving in or out even while temperature can temporarily hold still. Melting (solid to liquid), vaporization (liquid to gas), and sublimation (solid directly to gas, like dry ice) all require adding energy to overcome attractions between particles. Freezing (liquid to solid), condensation (gas to liquid), and deposition (gas directly to solid, like frost) all release energy as attractions between particles are reestablished.
A heating curve shows this clearly: as a solid is heated, its temperature rises steadily until it reaches its melting point, where the temperature FLATTENS even though heat is still being added -- all of that added energy is going into breaking apart the solid's rigid structure, not into raising temperature, until melting is complete. Once fully melted, temperature rises again as a liquid until it reaches its boiling point, where it flattens again for the same reason during vaporization.
Mastery looks like: They explain the properties of each state of matter using particle motion and attraction, and correctly describe the energy flow during any phase change, including why temperature holds flat during the change itself.
Common stumbles: Assuming temperature always rises whenever heat is added (missing the flat region during a phase change); confusing which phase changes release energy and which require it.