Thinking on Different Phases of Matter and their Transformations

by B.H.S.Thimmappa

Exploring the Lack of Overall Perspective

A phase is a homogeneous and physically distinct form of matter separated from other parts of the system by interfaces. Solid, liquid, and gases are the common classical phases of matter or physical states. Solid ice, liquid water, water vapor, solid iron, liquid molten iron, iron vapor, and solid oxygen (-218oC), liquid oxygen (-118 oC & compress), and oxygen gas are typical examples of these three common phases. One can have one-phase, two-phase, or three-phase systems of water/iron/oxygen. Solids have a definite shape and volume and particles are closely packed with regular repeating patterns. They have a high density and are difficult to compress. Liquids assume the shape of their containers, have a fixed volume, and particles have some degree of freedom of movement in a disordered way. They have moderate density and minor compressibility possible. Gases take shape of the container, expand in volume, and particles are in constant motion, sliding past each other. They have low density and are easy to compress. An increase in pressure or a decrease in the temperature of the gas makes it liquefy to form a more ordered liquid phase. Further increase in pressure or drop in temperature results in a more ordered solid phase. Solids and liquids have particles that are closer together than in the gaseous state and are thus called condensed phases.

Solid states of matter can be classified into the following three categories; i) crystalline solids where atoms, molecules, and ions are arranged in an orderly array (single crystal, polycrystalline, microcrystalline) e.g. sugar, salt, diamond, ice ii) amorphous solids where atoms arranged randomly, e.g. flour, silica, glasses, polymers, iii) polycrystalline solids (an aggregate of many small single crystals (grains)) e.g. metals, ceramics, and iv) quasicrystalline solids (possess long-range order but lacks translational symmetry) e.g. Al-Mn alloy. The researchers recently found a ‘hexatic phase’ of water where it acts as something in between solid and liquid and a ‘superionic phase’ which occurs at high pressures, the water becomes highly conductive. A new form of amorphous ice with a density very similar to that of water has been discovered. There are 20 known crystalline phases of ice. An oil-water mixture is a liquid that will separate into two phases as they are immiscible. Miscible alcohol-water forms one phase and salt and saltwater combination form different solid/solution phases. Carbon tetrachloride (CCl4)-water forms two phases and gas mixtures exist in only one phase at normal pressures as they mix in all proportions to give a homogeneous mixture. The discontinuity in structure between the two states is called the phase boundary.

There are a few special phases like plasma state, colloidal state, a Bose-Einstein condensate (BEC), and Fermionic condensate (FC). A plasma is a hot ionized gas of positively charged ions and negatively charged electrons (free charged particles) at very high temperatures (10K) in magnetic/inertial confinement. Examples of terrestrial plasmas include the interior of the stars/sun, switching devices (d.c. discharge), lightning, fluorescent light, electric welding arc, nuclear fireball, neon sign, and aurora. This higher energy ionized state of matter has applications in plasma fusion energy, semiconductor fabrication, plasma pyrolysis, defense aircraft, plasma television for wide-angle view, and surface modification.  At ultra-low temperatures, bosons and fermions can each condense together, creating Bose-Einstein or Fermionic condensates. A BEC is a phase of matter formed by the bosons cooled to temperatures very near absolute zero. It is formed by cooling a gas of extremely low density to super low temperatures. FC is a superfluid phase formed by fermionic particles at low temperatures.

A colloidal state of matter involves the presence of a dispersed phase of tiny particles in a dispersion medium. The particle size is greater than that in a true solution and smaller than that in a coarse suspension (1-100 nm). Examples of colloidal systems from daily life include milk, blood, smoke, fog, inks, paints, clouds, aerogel, foams, cosmetics, and detergents. The different types of colloidal systems involving various dispersed phases and dispersion mediums (continuous phase) include aerosol (liquid dispersed in gas/fog, mist), foam (gas dispersed in liquid/froth, soap lather), emulsion (liquid in liquid/milk, hair cream), sol (solid in liquid/paints, cell fluids), solid sol (gas in solid/pumice stone, foam rubber), aerosol (solid in gas/smoke, dust), and gel (liquid in solid/cheese butter, jellies). Many food items like milk, butter, ice cream, fruit jellies, and whipped cream are colloidal. The use of colloids in paints, inks, water treatment processes, and treatment and diagnosis of diseases is well known. Silver colloid as a germicidal agent, copper colloid as an anticancer drug, and mercury colloid as anti-syphilis preparation are some examples of therapeutic agents. Interestingly, proteins in muscle, bone, and skin, plasma proteins in drug-target site binding, plant macromolecules in drug coating, and dextran injection (colloidal dispersion) as plasma substitutes are examples of natural colloids in pharmaceutical applications.

Superconductivity is a state of exactly zero electrical resistance. The recent discovery of room-temperature superconducting material could lead to magnetic levitating trains and future fusion power plants. Superfluid is a state in which cryogenic liquids can flow without any kind of friction (no viscosity). Superfluidity is a state of matter in which it behaves like a fluid with zero viscosity, where it appears to exhibit the ability to self-propel and travel in a way that defies the forces of gravity and surface tension. Examples of superfluids are helium-3 and helium-4 when they are liquefied by cooling to cryogenic temperatures. Ionic liquids are composed entirely of organic cations and inorganic or organic anions that are liquids below 100 oC and have the potential to replace the volatile organic solvents used in several chemical processes. Ionic liquids are used as solvents or electrolytes and used in separation, heat storage, electroelastic materials, lubricant and additives, and protein crystallization. Of particular interest is the subatomic color glass condensate (CGC), a theoretically predicted type of matter in atomic nuclei when they collided near the speed of light.

There are some mesophases in between these phases of matter. The liquid crystal phase (LCP) is a mesophase in between solid (crystal) and isotropic (liquid) states exhibiting the properties of both (wristwatch, calculator displays, screens on laptops, instrument panels). The supercritical fluid (SCF) phase is the mesophase in between liquids and gases and it has properties of both gas and liquid states (e.g. sc-carbon dioxide, sc-xenon, sc-ethane). SCF is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. This phase is used in the extraction of caffeine from coffee beans, removing fats from potato chips/oil seeds/vegetables, and extracting flavor/fragrances from citrus oils. In various chromatographic separation techniques, different stationary and mobile phases (S/L/G/SCF) are used.  Unstable and metastable mesophases can assist in the nucleation of porous crystals. The progress in materials science has led to the discovery of many liquid crystalline anisotropic mesophases such as columnar/discotic, smectic, nematic, and cholesteric. Lipid bilayers, cell membranes, and gelatin are examples of mesophases with a partially ordered structure. Soft matter comprises a variety of physical systems that are deformed or structurally altered by thermal or mechanical stress. Classes of soft matter include polymers, foams, gels, colloids, liquid crystals, and biological membranes. Soft matter and functional interfaces for formulation and industrial innovation are a topic of research in several top-level universities. Soft matter science includes soft matter interfaces from membranes to tribology, from design to multifunctional materials and devices, and from physical concepts to material properties. The development of single-phase and multiphase (gas & liquid) fluid flow processes and liquid-liquid phase separations are currently being pursued by researchers to have novel products, processes, and systems.

The poem of ‘blind men and the elephant’ illustrates the concept of identifying the different parts of the elephant by touching it (tail as rope, leg as a tree, trunk as a spear, ear as fan, body as a wall). Their perceptions are very limited and may be only partially right. Based on the partial information we have on different phases in science, individual solutions to some global scientific problems may have limited use. But if we follow an interdisciplinary research approach to solve global environmental issues a better solution will come out. An interdisciplinary collaboration needs deep commitments and personal relationships. The compartmental attitude and lack of overall perspective are detrimental to having comprehensive outcomes and the benefits of merging areas of expertise in research activities are enormous. The phase changes play an important role in several biogeochemical cycles, particularly the water cycle in the planetary ecosystem. An interdisciplinary and multidisciplinary approach to materials research will lead to the discovery of many new phases of matter with novel applications. It is necessary to transfer updated knowledge comprehensively on each topic for science students in higher education institutions to provide an overall picture of the current scenario.

Thinking Caps about Phase Changes in Matter  

The state of matter changes as we add more energy (S -> L -> G -> P). The phase transformations include solid to liquid (melting), liquid to solid (freezing), solid to gas (sublimation), gas to solid (deposition), liquid to gas (vaporization), gas to liquid (condensation), gas to plasma (ionization), and plasma to gas (recombination) where the enthalpy of the system increases from solid to liquid to gas to plasma. These phase conversions have applications in transformations in steel, precipitation, solidification, crystallization, glass transition, recovery, recrystallization, and grain growth. Depending on the temperature’s influence on the rate at which phase transformations occur, heat treatments for metal alloys are carried out. Phase diagrams show the relationships between the different phases that appear within the system under equilibrium as a function of composition, temperature, and pressure, and reflect exactly what phases are present at any given temperature and pressure. Understanding the relationship between the structure and properties of a material is essential to tailor its microstructure to obtain the desired properties.

Phase transformations of elements under high pressure or temperature often lead to several new materials. A phase change material (PCM) absorbs or releases enough energy during phase transition to produce a heating or cooling effect. They can absorb, store, and release energy when the material transforms from one of the classical states to another (e.g. solid & liquid, crystallization (heat released), melting (heat absorbed). The phase transition can also take place between non-classical states of matter like crystal conformance. Here the material changes from one crystalline structure to another with a different energy level. This new-age energy conversion technique is used in solar energy heating/storage, building temperature regulation, electronic cooling, and industrial waste heat recovery. They are used in the smart textile industry, to produce clothes for protection from extreme environmental conditions because of the thermo-regulating effect (e.g. clothes for astronauts). Tunable PCM-based thermal energy storage has many applications in thermal textiles, building/power plant cooling, battery thermal management, and transportation of pharmaceutical/medical supplies. Synthesis of organic and inorganic PCMs, thermal property enhancement of low-temperature nano-enhanced PCM, and nanomaterials based on PCM for antibacterial application are hot areas of current research activity.

Think Big and Think Ahead Without Limits

The origin of all the naturally occurring elements falls into two phases. i) primordial nucleosynthesis (big bang) produces light elements and ii) stellar nucleosynthesis forms the heavy elements.  A phase is a form of material having a characteristic structure and properties. The three forms of iron at atmospheric pressure but at different temperatures are alpha-iron (ferrite), gamma-iron (austenite), and delta-iron. Another form called epsilon iron (hexaferrum) exists at very high pressure. A bar of solid iron may contain multiple phases. The many phases of carbon include amorphous carbon, diamond, carbon nanotube, graphene, fullerene, and graphite. Silicon has crystalline and amorphous forms and tin has grey, white, and rhombic tin phases. Sulfur exists in four different phases – two solid (rhombic, monoclinic), one liquid, and one vapor phase. Similarly, many elements in the periodic table have different numbers of known phases. There is a scope for the discovery of new phases for different elements or alloys. Several synthetic applications of the SCF phase in a wide range of areas from biology to advanced manufacturing are known. The discovery of a new phase of matter that acts like it has two time dimensions by shining a laser pulse sequence inspired by the Fibonacci sequence of atoms inside a quantum computer can dramatically boost their performance. The power of thinking big allows us to move out of our comfort research zone, create multiple phases of innovation, and make new phases or develop an amazing mixture possible. Thinking beyond the limits by considering interdisciplinary or multidisciplinary research projects and following new approaches or strategies is essential to create a new trend to change our world for the betterment of humankind. Proper operational planning matter even more that provides opportunities to enhance our research efficiency and move forward in the right direction in phases.


  1. Kotz, John C., and Paul Jr. Treichel. Chemistry & Chemical Reactivity. N.p.: Saunders College Publishing, 1999.
  2. Oxtoby, David W., H. P. Gillis, and Alan Campion. Principles of Modern Chemistry. Belmont, CA: Thomson Brooks/Cole, 2008.

About the Author:

B.H.S. Thimmappa is a writer from Udupi, India

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