6.3 - Internal Energy & Changes of State

Heating or cooling changes temperature or state

• Heating transfers energy to the particles in a substance; cooling transfers energy away.
• Heating or cooling leads to a temperature change or a change of state.

Heating and cooling

• Temperature change - Heating makes particles move or vibrate faster, raising the temperature. Cooling slows particles down, lowering the temperature.
• Change of state - If heated enough, the energy breaks the bonds between particles rather than raising their temperature. When cooled enough, bonds form between particles.

Temperature stays constant during a change of state

• While a substance is changing state, its temperature does not change.
• During melting or boiling, energy is absorbed to break bonds rather than raise the temperature.
• During condensing or freezing, bonds form and release energy, so the temperature does not fall until the change of state is complete.
• Melting point is the temperature at which a substance melts; boiling point is the temperature at which it boils.
• For pure water at normal atmospheric pressure: melting point 0 °C, boiling point 100 °C.

Internal energy

Particles in a system store energy in two ways:

1. Kinetic energy stores - Energy from the movement or vibration of the particles.
2. Potential energy stores - Energy from the positions of the particles relative to each other, set by the forces between them.

Defining internal energy

• The internal energy of a system is the total energy stored by all its particles in their kinetic and potential energy stores combined.
• Heating increases internal energy: the energy either raises the temperature (increasing the kinetic stores) or changes the state (increasing the potential stores).
• Cooling decreases internal energy.

Temperature and kinetic stores

• The temperature of a substance depends on the average energy in the kinetic energy stores of its particles.
• The higher the temperature, the faster the particles move or vibrate on average.

A change of state conserves mass

A change of state is when a substance changes from one state of matter to another:

• Melting and freezing (between solid and liquid)
• Boiling (or evaporating) and condensing (between liquid and gas)
• Subliming (between solid and gas)

A change of state is a physical change

• It's a physical change, not a chemical one - no new substance is formed.
• The change can be reversed to recover the original material with its original properties.

Why mass is conserved

• During a change of state, the number of particles stays the same - they're only rearranged.
• Mass is the total mass of the particles, so mass is conserved.