Two phases can have no net flow between them {equilibrium between phases}. At boiling-point or melting-point temperature, both phases, liquid/gas or solid/liquid, respectively, have same free energy and are in equilibrium. If temperature decreases, substance becomes denser, because entropy change is small but potential-energy change is large. If temperature increases, substance becomes less dense, because entropy change is large but potential-energy change is small.
Physical Sciences>Chemistry>Inorganic>Phase
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Date Modified: 2022.0224