Changes of State and Internal Energy

This lesson covers the changes of state between solids, liquids and gases, and the concept of internal energy, as required by the AQA GCSE Combined Science Trilogy specification (8464, section 6.3.1). Understanding internal energy is key to explaining why temperature stays constant during a change of state.

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Changes of State

A change of state is a physical change — no new substances are formed, and the process is reversible. The mass of the substance is conserved (stays the same).

graph LR
    S["Solid"] -->|"Melting"| L["Liquid"]
    L -->|"Freezing"| S
    L -->|"Boiling /<br/>Evaporating"| G["Gas"]
    G -->|"Condensing"| L
    S -->|"Sublimation"| G
    G -->|"Deposition"| S

Change of State	Direction	Energy
Melting	Solid → Liquid	Energy absorbed
Freezing	Liquid → Solid	Energy released
Boiling / Evaporating	Liquid → Gas	Energy absorbed
Condensing	Gas → Liquid	Energy released
Sublimation	Solid → Gas (directly)	Energy absorbed