•   Solid
•   Liquid
•   Gas
•   Plasma

Each state of matter in this sequence possesses increasing amounts of energy, as reflected by the temperature ranges that they exist in.

The fundamental units of a substance consist of atoms and molecules. The 4 phases of matter represent the degree of compactness in which these units are packed and how much energy they possess. The diagram above shows how matter in one physical state can change into another phase. The relationship between these phases as a function of pressure and temperature is illustrated on a phase diagram.

How are the phases of matter affected by temperature and pressure?

The figure above is a phase diagram for a pure substance, as a function of temperature and pressure. Besides the main phases of solid, liquid and gas, two other areas exist - vapor and supercritical fluid. If plasma was shown, it would be on the extreme right beyond the temperatures for gases. 

A vapor is a gas existing below the substance's critical temperature (temperature at the critical point, c.p.). Below the critical temperature, a vapor can be transformed to a liquid or solid if a large enough pressure is applied to it. If the pressure and temperature both exceed those of the critical point, the distinction between gas and liquid vanishes and the substance behaves like a supercritical fluid. The vapor and supercritical fluid behavior are intermediate between those of liquids and gases.

For some substances, there is also a "gray area" in physical properties between those of a solid and a liquid. Substances in the liquid crystalline state illustrates this.

Liquid Crystals defy the convention definition of liquids and solids

Humans looking for a simplistic understanding of Nature attempts to categorize its behavior in distinct categories. The phase diagram in the previous page shows a distinct boundary between the solid and liquid phase of a substance. We often think in terms of black and white.

Nature is more subtle in that its behavior, are often "varying degrees of gray". This idea is illustrated below.

Substances that behaves as liquid crystals often transform between a crystalline solid and an isotropic liquid, as a function of temeprature and constant pressure, through the formation of intermediate phases, as illustrated below.

The intermediate region in the phase diagram between a solid and liquid for a liquid crystalline substance is more complicated then the figure above, if pressure is also varied.

What is intermediate between a solid and a liquid?

A liquid crystalline substance can form intermediate phases between that of solid and liquid. Some crystalline solid do not transform directly to form an isotropic liquid.

How do the Characteristics of Liquid Crystal Compare and Contrast with Other Phases?  

The physical properties of a substance can be divided into macroscopic and nanoscopic behaviors.