Quantum mechanics theorists showed physicists how the behavior of particles at the atomic level and smaller was not consistent with Newtonian ideas about matter and energy. Newtonian physics categorized matter and energy separately, operating under the assumption that transitions from one energy level to the next were continuous and uninterrupted. Quantum physics showed that matter and energy are not always distinguishable from one another and that particles such as electrons did not traverse every point along an energy continuum, but they “leap” from one level to another. This “quantum leap” is called "discontinuity."
Matter behaves differently at different temperatures and pressures. At sea level, water forms into crystals that interlock with one another at 32 degrees Fahrenheit, for example, and the water's volume expands as it becomes solid. At 212 degrees Fahrenheit at sea level, water becomes gaseous. These phase transitions correspond to critical points along a thermodynamic continuum. The change in matter is often abrupt and dramatic at those points, however, because this phase transition is “discontinuous.” The matter hits this point and “leaps” into a new phase. Not all phase transitions are discontinuous.
Four states characterize matter at differing thermodynamic phases: solid, liquid, gas and plasma. Phase transitions take matter from one of these states to another according to a set of rules. Solids can become differing solids, liquids or gases but cannot become plasmas. Liquids can only become solids or gases. Gases can become solids, liquids and plasmas, but one gas will never become a different gas (though one liquid can become more than one gas). Plasmas can only transition into gases.
Depending on the substances used, a variety of phase change types exist. One solid might liquefy into two separate liquids, for example, or into one solid and one liquid. Crystal structures may change in a solid to solid transition. A liquid-to-gas transition might separate into two separate condensable liquids, such as when people make liquor by distilling it. Dry clay — a solid — changes into a different kind of solid after it has been fired. This change results from molecular transition catalyzed by a different thermodynamic phase — high heat.