Lesson 3: Particles in Motion – We learned about heat and changes of state. When thermal energy is added to solid ice its temperature increases until it starts to melt changing to liquid water. When thermal energy is removed from liquid water its temperature decreases until it starts to freeze changing to solid ice. Then we learned that when thermal energy is removed from a gas it changes to a liquid through a process called condensation. Next we learned that sublimation occurs when a solid absorbs energy and changes directly to a gas without first becoming a liquid. Deposition occurs when a gas changes directly to a solid without first becoming a liquid. Then we learned that thermal energy moves quickly in a thermal conductor and slowly in a thermal insulator.