Final answer:
Heating a substance increases molecular motion by adding kinetic energy to its particles, which affects the state of the substance by potentially overcoming intermolecular forces that hold it together in a particular phase.
Step-by-step explanation:
Heating a substance increases molecular motion. As we heat a substance, its particles, which may be molecules or atoms, gain kinetic energy and move faster. This is because the temperature of a substance is directly related to the average kinetic energy of its particles. Moreover, the state of a substance is determined by the balance between this kinetic energy and the intermolecular forces that draw the particles together. For example, gases expand when heated due to increased molecular motion, liquids may start to boil, and solids can melt if the temperature increases enough to overcome the intermolecular forces holding their molecules in a fixed structure. Conversely, cooling a substance decreases its thermal energy, leading to slower molecular motion and potentially a change in state, such as condensation or freezing.