Internal energy is the energy stored by the particles that make up a system

• The particles in a system vibrate or move around-they have energy in their kinetic energy store

• They also have energy in their potential energy stores due to their positions

• The energy stored in a system is stored by its particles. The internal energy of a system is the total energy that its particles have in their kinetic and potential energy stored

• Heating the system transfers energy to its particles(they gain energy in their kinetic energy stores and move faster), increasing the internal energy

• This leads to a change in temperature or a change in state. If the temperature changes, the size of the change depends on the mass of the substance, what it’s made of and the energy input.

• A change in state occurs if the substance is heated enough-the particles will have enough energy in their kinetic energy stores to break the bonds holding them together

A change of state conserves mass

• When you heat a liquid, it boils and becomes a gas. When you heat a solid, it melts and becomes a liquid. These are both changes of state.

• The state can also change due to cooling. The particles lose energy and form bonds. The changes of states are

• A change of state is a physical change. This means you don’t end up with a new substance-it’s the same substance as you started with, just in a different form

• If you reverse a change of state, the substance will return to its original form and get back its original properties

• The number of particles doesn’t change-they’re just arranged differently. This means mass is conserved-none of it is lost when the substances change state.