Final answer:
Flat lines on a Thermal Energy Graph indicate phase changes, such as melting or boiling, where temperature stays constant due to the absorption or release of latent heat.
Step-by-step explanation:
On a Thermal Energy Graph, the flat lines represent periods where the temperature of the substance remains constant despite the addition of heat. This occurs during phase transitions such as melting or boiling. During these phases, the energy added to the substance is used not for increasing the kinetic energy of the particles, which would raise the temperature, but for overcoming the intermolecular forces that hold particles in a structured solid or closely packed liquid phase.
When ice melts or water boils, the temperature remains constant at the melting point (0°C) and boiling point (100°C) respectively, indicating the absorption of latent heat. This latent heat is required to change the state of the substance without changing its temperature. For condensation, the process is reversed, and latent heat is released, which actually increases the temperature of the surroundings, although we often associate condensation with feeling cold due to the removal of heat from our skin or a cold surface.