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Phases - Gas, liquid and solid

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Phases - Gas, liquid and solid

Gas, liquid, and solid are known as the three states of matter or material, but each of solid and liquid states may exist in one or more forms. Thus, another term is required to describe the various forms, and the term phase is used. Each distinct form is called a phase, but the concept of phase defined as a homogeneous portion of a system, extends beyond a single material, because a phase may also involve several materials. For example, a homogeneous solution of any number of substances is a one-phase system. Phase is a concept used to explain many physical and chemical changes (reactions).

A solid has a definite shape and volume. A liquid has a definite volume but it takes the shape of a container whereas a gas fills the entire volume of a container. You already know that diamond and graphite are solids made up of the element carbon. They are two phases of carbon, but both are solids.

Solids are divided into subclasses of amorphous (or glassy) solids and crystalline solids. Arrangements of atoms or molecules in crystalline solids are repeated regularly over a very long range of millions of atoms, but their arrangements in amorphous solids are somewhat random or short range of say some tens or hundreds of atoms.

In general, there is only one liquid phase of a material. However, there are two forms of liquid helium, each have some unique properties. Thus, the two forms are different (liquid) phases of helium. At a definite temperature and pressure, the two phases co-exist.

So far, all gases behave alike as do mixtures of gases. Thus, a gas is usually considered as a phase.

The Concept of Phase

A phase is a distinct and homogeneous state of a system with no visible boundary separating it into parts. Conversion between these states is called a phase transition.

Water, H2O, is the most common substance that its gas (steam), liquid (water), and solid (ice) phases are widely known. An ice water mixture has two phases, so are systems containing ice-and-vapour, and water-and-vapour. To recognize the vapour system in these system may require a keen observation, because the vapour usually blends with air, and is not detected directly.

You probably also know that several solids may exist for a substance, and each of the solid form is also called a phase. Diamond and graphite being the most quoted examples. Graphite and diamond are both solid carbon. They have different crystal shapes, colors, and structures. They represent two different phases of carbon.

Under 1 atm, ice has hexagonal symmetry, while cubic ice is formed under high pressure. In fact, there are at least 8 different types of ice, each being a solid phase.

When you mix water and alcohol, regardless of the relative amounts that you use, they are fully miscible. The resulting mixture has only one phase (a solution). However, water and oil are normally immiscible, and their boundary of separation is visible. They form a two-phase system.

Sometimes you cannot "see" the boundary, and you will need scientific reasoning to realize the number of phases present in system.

Well, there is so much concept packed into one term that we can not make the definition any simpler for you. However, the term is useful because it can be used to explain many phenomena. There is no substitute for it. Learn it and use it to explain physical changes.

Phase Transitions

A state change of any material due to temperature or pressure change is a phase transition. A phase transition is a physical change (or reaction). The following diagram illustrates the key phase transitions: You should know the names of the process for these phase transitions.
           sublimation            deposition
   SOLID  ============>  GAS    ==============> SOLID

            melting               freezing
   SOLID  ============>  LIQUID  =============>  SOLID

         condensation            vaporization
   GAS  =============>   LIQUID  =============> GAS

The concepts of phase and phase transition introduce you into fields of materials studies. For example, if you search the internet with the phrase "phase transition", you get thousands of websites, some are related to the concept we have discussed here, but some may be using "phase transition" as a catchy phrase.

For example, a Netscape Search gives a site for Studies of the phase transitions in calcite includes several papers published by a professor of Earth Sciences in UK whose career is based on the study of calcite, CaCO3, a very common mineral. The concept of phase transition is also applied to the study of nuclear matter such as protons and neutrons.

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