Matter typically exists in the solid, liquid, or gaseous state. In gases, the high-energy molecules move quickly and are far apart, so attractive intermolecular forces are weak. Changes in temperature, pressure, or both can trigger phase transitions, or phase changes, between states of matter. During these transitions, energy flows to or from the system without a change in temperature. The energy change required for the complete transition of one mole of a substance is the molar enthalpy of that transition. Compressing or cooling a gas makes it harder for the molecules to move away from each other, transforming them into a denser, less compressible liquid. Cooling the liquid reduces the kinetic energy, transforming it into a densely packed arrangement of an incompressible solid.