Fasa juga kadangkala dipanggil '''keadaan jirim''', namun istilah ini boleh menyebabkan kekeliruan dengan keadaan [[termodinamik]]. Sebagai contoh, dua gas pada tekanan yang berlainan mempunyai keadaan termodinamik yang berbeza, tetapi keadaan jirim yang sama.
== Takrifan ==
===Example 1: Solid, liquid, and gas phases===
[[Water]] (H<sub>2</sub>O) is composed of water [[molecule]]s, each of which is an [[oxygen]] [[atom]] attached to two [[hydrogen]] atoms. At [[room temperature]], the molecules are packed closely together, and interact weakly. They do not stick together, and are able to slide past one another like the sand grains in an [[hourglass]]. This microscopic behavior of water molecules gives rise to the physical properties of liquid water with which we are all familiar. Because the molecules do not form any rigid structure, water has no fixed shape, and adapts to the shape of any container in which it is placed. Because the molecules are very close to one another, water resists [[Physical compression|compression]]; try squeezing a [[water balloon]], and you will find that it is practically impossible to reduce its volume, unlike an ordinary air balloon.
If we make slight changes in the physical conditions, such as lowering the [[temperature]], we will observe no abrupt changes in the properties of water. Cold water behaves little differently from lukewarm water. For instance, its compressibility changes slightly with temperature, but remains very low.
However, if we reduce the temperature below a certain point, an abrupt and dramatic change occurs. At the microscopic level, the molecules suddenly align with one another to form a rigid [[hexagonal (crystal system)|hexagonal lattice]], losing the ability to slide past one another. The system as a whole acquires rigidity, and can hold a definite shape (though it may deform or fracture into pieces if sufficient force is applied.) This is the solid phase of water, commonly known as [[ice]]. The transition from a liquid phase to a solid phase is called freezing (or "[[melt]]ing" if we go in the opposite direction), and it is a type of phenomenon known as a '''[[phase transition]]'''.
Another phase transition, known as [[boiling]], occurs if we raise the temperature of liquid water past a certain point. The water abruptly enters a gaseous phase, where it is called [[water vapor]]. In the gaseous phase, the molecules are spread far apart, and interact extremely weakly. Like a liquid, a gas has no fixed shape, but unlike a liquid, it has little resistance to compression because there is enough space for the molecules to move closer to one another. Whereas a liquid placed in a container will form a puddle at the bottom of the container, a gas will expand to fill the container.
We can use other physical parameters, not just temperature, to produce these phase transitions. For example, we can change a liquid into a gas by decreasing the pressure, or, equivalently, increasing the volume. As before, small changes do not have much effect; the phase transition occurs abruptly when the change exceeds a certain amount.
===General definition of phases===
In general, we say that two different states of a system are in different phases if there is an abrupt change in their physical properties while transforming from one state to the other. Conversely, two states are in the same phase if they can be transformed into one another without any abrupt changes.
An important point is that different types of phases are associated with different physical quantities. When discussing the solid, liquid, and gaseous phases, we talked about rigidity and compressibility, and the effects of varying the pressure and volume, because those are the relevant properties that distinguish a solid, a liquid, and a gas. On the other hand, when discussing [[paramagnetism]] and [[ferromagnetism]], we looked at the magnetization, because that is what distinguishes the ferromagnetic phase from the paramagnetic phase. Several more examples of phases will be given in the following section.
Not all physical quantities are relevant when we are looking at a certain system. For example, it is generally not useful for us to compare the magnetization of liquid water to the magnetization of ice. In this sense, what constitutes a "phase" depends on what parameters you are looking at, and vice versa. It is this idea that allows us to generalize the concept of phases to encompass a wide variety of phenomena.
In more technical language, a phase is a region in the [[parameter space]] of [[thermodynamics|thermodynamic variables]] in which the [[free energy]] is [[analytic function|analytic]]. As long as the free energy is analytic, all thermodynamic properties (such as [[entropy]], [[heat capacity]], [[magnetization]], and [[compressibility]]) will be [[well-behaved]], because they can be expressed in terms of the free energy and its [[derivative]]s. For example, the [[entropy]] is the first derivative of the free energy with [[temperature]].
When a system goes from one phase to another, there will generally be a stage where the free energy is non-analytic. This is a [[phase transition]]. Due to this non-analyticity, the free energies on either side of the transition are two different functions, so one or more thermodynamic properties will behave very differently after the transition. The property most commonly examined in this context is the [[heat capacity]]. During a transition, the heat capacity may become infinite, jump abruptly to a different value, or exhibit a "kink" or discontinuity in its [[derivative]]. See also [[differential scanning calorimetry]].
''Possible graphs of heat capacity (C) against temperature (T) at a phase transition.''