Phase changes that move matter from a more ordered state to a less ordered one result in an increase in entropy. Specifically, melting (solid to liquid), vaporization (liquid to gas), and sublimation (solid to gas) all increase entropy. The reverse processes, freezing, condensation, and deposition, decrease it.
Why These Three Phase Changes Increase Entropy
Entropy is a measure of how many different ways the particles in a system can be arranged. A solid has its molecules locked in a rigid structure with very limited movement. When that solid melts into a liquid, the molecules gain freedom to slide past one another, rotate, and move around. That jump in possible arrangements means more entropy. When a liquid vaporizes into a gas, the effect is even more dramatic: molecules spread out to fill whatever space is available, moving in every direction with far more possible configurations.
Sublimation, where a solid transforms directly into a gas (think dry ice or iodine crystals producing visible vapor), combines both jumps at once. The molecules go from a fixed crystal lattice straight to the high-freedom state of a gas, producing the largest entropy increase of any single phase change.
How Big Is the Entropy Increase?
Water makes a useful example because NIST provides precise entropy values for each state. At standard conditions, solid ice would have an entropy around 45 J/mol·K, liquid water has an entropy of about 69.95 J/mol·K, and water vapor reaches 188.84 J/mol·K. Notice the pattern: going from liquid to gas nearly triples the entropy, while going from solid to liquid produces a more modest increase. This is because gas molecules have vastly more ways to arrange themselves in space compared to molecules in a liquid.
You can calculate the exact entropy change during any phase transition using a simple relationship: divide the heat absorbed during the transition by the temperature (in Kelvin) at which it occurs. For melting, that looks like the heat of fusion divided by the melting point. For boiling, it’s the heat of vaporization divided by the boiling point. Because vaporization requires much more energy than melting, the entropy change during boiling is always larger than during melting for the same substance.
The Connection to Heat Absorption
Every phase change that increases entropy is endothermic, meaning the substance absorbs heat from its surroundings. That absorbed energy doesn’t raise the temperature. Instead, it goes into breaking the forces holding molecules together, giving them greater freedom of motion. This is why ice sitting at 0°C absorbs heat steadily without getting warmer until it has fully melted. All that energy is being used to disrupt the crystal structure, and the result is higher entropy.
The reverse is also true. Freezing, condensation, and deposition all release heat and decrease the entropy of the substance. The molecules are losing freedom, settling into more organized arrangements, and shedding energy in the process.
Which Phase Changes Decrease Entropy
For completeness, here are the six common phase changes grouped by their effect on entropy:
- Increase entropy: melting (solid → liquid), vaporization (liquid → gas), sublimation (solid → gas)
- Decrease entropy: freezing (liquid → solid), condensation (gas → liquid), deposition (gas → solid)
The simple rule: if particles end up with more freedom to move and more possible arrangements, entropy goes up. If they end up more constrained, entropy goes down. Any transition that moves to the right on the solid → liquid → gas spectrum increases entropy, and any transition moving left decreases it.
What Molecular Motion Has to Do With It
At the molecular level, particles can store energy in three main ways: translation (moving from place to place), rotation (spinning), and vibration (atoms within a molecule stretching and compressing). In a solid, molecules are mostly limited to vibrating in place. Liquids add translation and rotation. Gases have all three types of motion at their maximum, with molecules traveling freely across large distances.
Each additional type of motion creates more possible “microstates,” which is the technical term for distinct arrangements the system can occupy. More microstates means higher entropy. This is why increasing the temperature of any substance also raises its entropy, even without a phase change. The molecules move faster, accessing more microstates. But phase transitions produce the biggest jumps because they fundamentally change how molecules are allowed to move, not just how fast they’re moving.