How Is Energy Related to the Change of State Represented by the Model?

The answer

A change of state (phase change) is all about energy moving in or out of the substance to change how strongly its particles are held together. In the model, if the substance is moving toward a higher-energy state—solid → liquid (melting), liquid → gas (evaporation/boiling), or solid → gas (sublimation)—then the particles must gain energy. That energy is absorbed from the surroundings to break the attractive forces holding the particles close. These are endothermic changes.

If the model instead shows movement toward a lower-energy state—gas → liquid (condensation), liquid → solid (freezing), or gas → solid (deposition)—then the particles lose energy, releasing it to the surroundings. These are exothermic changes.

So the relationship is simple to state: as particles gain energy they move faster and farther apart (toward gas); as they lose energy they slow and pack closer (toward solid). Which way the arrow points in the model tells you whether energy is absorbed or released.

What the particles are doing

Energy added during a phase change does not speed the particles up in the way it does within a single state—instead it does the work of overcoming the attractions between particles. That is why, during a phase change, the temperature stays constant even though energy is still being added. On a heating curve, this shows up as flat plateaus at the melting point and boiling point: the absorbed energy (the latent heat of fusion or vaporization) goes into pulling particles apart, not into raising the temperature. Only after the phase change is complete does added energy raise the temperature again.

For example, when a solid changes directly to a gas (sublimation), particles absorb a large amount of energy to break free of nearly all their attractions and spread out to fill their container. When a gas condenses, those attractions re-form and the stored energy is released as heat.

Why the common wrong answers fail

• "Energy is released as the solid changes to a gas." Wrong—melting, evaporating, and subliming all absorb energy; energy is only released in the opposite (condensing, freezing) directions.
• "The temperature rises during the phase change." Wrong—temperature is constant during a phase change because the energy goes into breaking bonds, not increasing kinetic energy.
• "No energy is involved because mass is conserved." Wrong—mass is conserved, but a phase change always involves an energy transfer.

The bigger picture

Every phase change comes in an endothermic/exothermic pair that mirror each other. Melting absorbs the same energy that freezing releases; boiling absorbs what condensation releases. Reading the model comes down to two questions: which direction is the substance changing, and therefore is energy being absorbed or released? Higher-energy state = energy in (endothermic); lower-energy state = energy out (exothermic).