Energy which is stored by particles (atoms and molecules) within a system is called Internal Energy.

The energy takes the forms of:

• Kinetic Energy (vibration of atoms etc.)
• Potential Energy (between the particles)

Internal energy is the energy stored in a system by the particles. It is the total kinetic energy and potential energy of all the particles that make up a system.

Processes:

• Melting: Solid to Liquid
• Freezing: Liquid to Solid
• Boiling: Liquid to Gas
• Condensation: Gas to Liquid
• Sublimation: Solid to Gas

Remember in Changes of State:

• Mass is conserved
• Physical changes
• Reversible