Chemistry: States of Matter: Solids, Liquids, Gases, and Plasma


There are three common states of matter on Earth: solid, liquid, and gas. State defines a physical property of matter. The defining characteristics that determine state include the number and chemical makeup of the molecules and how a particular collection of them is represented by such physical properties as their collective volume and shape, their reaction to temperature, and their reaction to pressure.

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The solid state of matter is characterized by a fixed shape that is dependent on the number and characteristics of the specific atoms and molecules that make up the solid. The solid state resists change in shape due to external pressure. To a very small degree relative to the other states, the density (mass/volume) of solids is temperature dependent. Solids can be crystalline, which means they have very orderly structure to the particles that make up the solid, or the solid can be amorphous and have no symmetry in the arrangement of the particles that make up the solid.

In the liquid state, matter will flow to take the shape of any container. The density of liquids is significantly dependent on temperature, but confined liquids resist change due to pressure. The gaseous state also will take the shape of a container but differs from liquids in that it will expand to fill any container so that if the container size is increased, the gas will expand to fill it. Temperature, pressure, and the number of molecules are all important parameters in defining the volume of a gas. Different gases are always completely miscible (mixable), but different liquids are not always miscible.

The term phase is used in certain contexts rather than state to describe the physical properties of matter. Phase is a more restricting term that is commonly used in scientific studies, but it is not often used in general conversation. Where phase differs from state can best be understood in describing liquid mixtures such as oil and water. Each physically and chemically different and separable liquid represents a separate phase. The individual entities, like the mixture, are in the liquid state.

Plasma is called the fourth state of matter. It is not common on Earth"s surface, but 90% of the matter in space is in the plasma state. Naturally occurring plasma does make up the ionosphere, a layer of low-density charged particles in the upper atmosphere of Earth. The plasma state is similar to the gaseous state, except it does not contain atoms and molecules. Instead, plasmas consist of subatomic particles such as negatively charged electrons and positive ions.

Historical Background and Scientific Foundations

The terms solid and liquid are English versions of Latin words for the terms. The term gas has a history. It can be traced back to the Renaissance period (1400–1650) when Jan Baptist van Helmont (1579–1644) gave the name gas to what is now known to be carbon dioxide. Until that time gases were called airs or spirits or vapors. Studies of gases became a significant part of the evolution of modern chemistry. The Ideal Gas Laws that are used by chemists today emerged from a series of studies that followed van Helmont"s famous work.

British scientist Robert Boyle (1627–1691) experimented with gases and published his observations in 1660 in a treatise he titled The Spring of the Air. Boyle observed that the product of the pressure on a gas times the volume of the gas remains constant at a constant temperature for any trapped gas. This fact is now called Boyle"s Law.

A fascination with gases in general and with the recently-discovered, lighter-than-air gas hydrogen in particular led French chemist Jacques Charles (1746–1823) to recognize in 1787 that the volume of a fixed weight of gas is directly proportional to its temperature at constant pressure. Known as Charles" Law today, this is the second of the gas laws.

Another French scientist, Joseph Louis Gay-Lussac (1778–1850), experimented with the reactions of particular gases and found that they always reacted in volume ratios of small whole numbers. He announced his finding in 1808. Gay-Lussac"s discovery is now known as the law of combining volumes.

Gay-Lussac"s discovery is not one of the Ideal Gas Laws, but it was part of the background that led Italian physicist Amadeo Avagadro (1776–1856) to suggest in 1811 that the volumes of gases are determined by the number of molecules of gas present. Avogadro"s hypothesis was also inspired by the ideas promoting the theory of atoms and molecules by British scientist John Dalton (1766–1844) that were published in 1808. Avogadro"s major contribution was to recognize that it is the number of molecules, not atoms, that determines the volume of a gas at a given temperature and pressure.

Molecules are made of atoms. The number of atoms in a molecule varies with the nature of the particular molecule. Oxygen has two atoms in a molecule. Water vapor has three, and methane, commonly found in decaying material, has four.

Combining all the individual contributions, the Ideal Gas Laws state that the pressure times the volume of a gas will equal the number of molecules times the temperature with a constant multiplied to relate the various units used in the measurements. This is stated mathematically as: PV = nRT, where n is the number of molecules and R is the constant.

Solids and liquids are the condensed states of matter. Physicists conduct studies of condensed matter, particularly at the atomic and molecular level. Condensed Matter Physics is also known as Solid State Physics. Liquids are included in the study of Solid State Physics under the heading of Soft Condensed Matter. Interest in crystalline solids has been documented to prehistoric times in collections of native crystals that have holes and engravings made in them. Ancient Greeks also studied crystals.

The discovery of the structure of crystals was made accidentally by a French monk, René-Just Haüy (1743–1822), when he dropped a fine calcite crystal from a crystal collection of the Treasurer of France. Upon examination of the pieces, Haüy noted the consistent regular and repeating shapes of all the pieces. In 1817, he published the story of this discovery and his later work on crystals in a number of books that included Traité de Cristallographie (Treatise on Crystallography). Haüy"s work is considered to be the beginning of the important science of crystallography, which has led to major advances in understanding the chemical structure of crystalline solids.

When x rays were discovered, the study of crystals expanded. German physicist Max von Laue (1879–1960) received the Nobel Prize in 1914, and the fatherson team of British physicists Sir William Henry Bragg (1862–1942) and William Lawrence Bragg (1890–1971) got the Nobel Prize in 1915 for their contributions to x-ray crystallography. Through advances in these early studies, the study of crystalline molecules has provided structural information on a great many compounds that include proteins and polymers.

There is no similar “Eureka!” moment related to the history of the liquid state of matter. However, the fluidity and solvent properties of liquids have had considerable importance in the advancement of all sciences, as in life science, physics, and chemistry.

The Science

At about the same time Avogadro was presenting his hypothesis and John Dalton was promoting the theory of atoms and molecules, British botanist Robert Brown (1773–1858) was observing through his microscope the random and ceaseless motions of pollen grains suspended in water. This observation in 1827 became known as Brownian motion. Brown did not attempt to explain the motion, but in 1905 German-American physicist Albert Einstein (1879–1955) suggested the motion was the result of random thermal motions of the liquid molecules. Einstein"s work and the studies that produced the Ideal Gas Laws led to the Kinetic Molecular Theory, which explains states of matters and changes of state.

Gases are described as composed of randomly moving molecules with perfectly elastic collisions as they bound off the walls of their confining container. In liquids the molecules have equally random motions, but the molecules are tumbling and interacting in close proximity to each other so they flow to fill a container. The viscosity of the liquid depends on the type of molecules and how they interact and also on the temperature of the fluid. Solids have random thermal vibrations, the degree of which depends on the nature of the molecules and the temperature of the solid. The molecules of solids vibrate in place. They do not move about in space like liquid molecules; that is why a solid has a fixed shape.

Changing state involves the addition (or subtraction) of heat to change the degree of motion of the molecules, that is to change the temperature. Starting with a solid that is a pure substance, adding heat makes the molecular vibrations increase. At the melting temperature adding heat no longer makes the solid warmer but instead, the heat causes the bonds between molecules to break. When enough heat has been added to break all the intermolecular bonds, the solid melts and becomes a liquid. The amount of heat needed to melt a pure solid is called the heat of fusion for that solid.

A similar increase in motion and increase in temperature occurs as a liquid is heated until the liquid molecules reach the temperature where the energy absorbed by the molecules has increased their velocity to the point the molecules can escape the weak bonding that kept them close as a liquid. At the point of vaporization, the temperature again does not raise because all the energy added is used to cause molecules to escape from the surface of the liquid until the liquid is completely converted to a gas. Once a gas, adding heat again increases the temperature. The heat needed to convert a pure liquid to a gas is called the heat of vaporization.

At very high temperatures, such as those in lightening or on the sun, the molecules of the gas are converted to atoms, and the atoms are converted to subatomic particles that are positively and negatively charged. At that point the gas has become plasma.

Influences on Science and Society

The gaseous, liquid, and solid states of matter all have had significant roles in the development of modern chemistry and physics. Starting with Robert Boyle"s experiments with gases that led to his publishing The Spring of the Air treatise in 1660, research with gases significantly influenced the development of modern theories on chemical elements, atoms, and molecules.

British scientist Joseph Priestley (1733–1804) made a key discovery when he isolated oxygen, a gas he called dephlogisticated air, in keeping with the theory of that time that a mysterious substance called phlogiston was in all substances. The confusion of exactly what Priestley had discovered was solved by French chemist Antoine Laurent Lavoisier (1743–1794) who named the gas oxygen and called it an element, an element being by his definition a unique chemical substance. Priestley did not agree with Lavoisier. The existing concept of elements was very limited, but Lavoisier gained popularity among scientists and the discovery of more elements accelerated.

Gases also figured in the advancement of the theory that matter is composed of atoms and molecules. Although this theory was not widely accepted at the time Dalton and Avogadro were promoting their ideas, it gained momentum with the growing popularity of Lavoisier"s ideas. Unfortunately, his career as a leader in the advancement of modern chemistry was abruptly ended. Lavoisier was not only a chemist but he was also wealthy. His family"s aristocratic status caused his death at the guillotine during the French Revolution (1789–1799).

Liquids played a part in the advancement of the concept that matter is made of atoms with Robert Brown"s observation in 1827 of pollen grains moving in a liquid, albeit the influence was slow in coming. Even though evidence was gathering in the support of the existence of atoms and molecules, the concepts were not universally accepted until after Einstein suggested that the motion of the liquid molecules caused Brownian motion. When Einstein presented his ideas in 1905, theoretical physics was not a well-organized science. Einstein"s studies attracted other key scientists to advance the theory of atoms and molecules, as well as the science of physics.

The solid state has been particularly influential as a result of Einstein"s contributions and the advances in physics that have led to the development of atomic energy. The development of the atomic bomb and the applications of nuclear energy are all encompassed in a scientific discipline that is now called Solid State Physics.

Modern Cultural Connections

Austrian botanist Friedrich Reinitzer (1857–1927) found a solid, crystalline, cholesterol-based substance that did not melt all at once, as a pure solid is expected to melt. The substance appeared to have two melting points. At 229°F (145°C) the crystals melted to a cloudy liquid. With further heating, to 288°F (178°C), the substance changed to a clear liquid. Reinitzer asked German physicist Otto Lehmann (1855–1922), an expert in crystal optics, if he could help explain the unusual melting of the cholesterol-based crystals. Lehmann determined the cloudy phase represented a new state of matter, which he called liquid crystals. Researchers following the discovery of the cholesterol-based liquid crystals found that other substances, often rod-shaped molecules, also formed liquid crystals.

The curious new state of matter was investigated in the 1960s by French physicist Pierre-Gilles de Gennes (1932–2007). He received the Nobel Prize in 1991 for his contributions to the science of liquid crystals. Today, liquid crystals are used in electronic information displays for everything from cell phones to flat panel displays for computers and TVs. These liquid crystal displays are commonly called LCDs. The development of these applications for liquid crystals was made possible by de Gennes" work.

Competing with liquid crystal displays for TVs are plasma displays that use xenon gas and neon gas trapped in tiny cells in which the gases are converted to plasma through high voltage applied to electrodes that are attached to the cells. Plasma displays have the advantage of a wider angle view than is available with a liquid crystal display.

Man-made plasma is also used in fluorescent lighting and in neon signs. Scientists are working on plasma reactors that may some day produce energy on a very large scale by using the same fusion process that is used in the sun.

See Also Chemistry: Chemical Bonds; Chemistry: Chemical Reactions and the Conservation of Mass and Energy; Physics: Fundamental Forces and the Synthesis of Theory; Physics: Maxwell"s Equations, Light and the Electromagnetic Spectrum; Physics: Newtonian Physics.



Moore, F.J. History of Chemistry. New York: McGraw-Hill Book Company, Inc., 1939.

Moore, Walter J. Physical Chemistry. Englewood Cliffs, NJ: Prentice Hall, 1979.

Morris, Richard. The Last Sorcerers: The Path from Alchemy to the Periodic Table. Washington, D.C.:Joseph Henry Press, 2003.

Tabor, D. Gases, Liquids, and Solids. Baltimore, MD: Penguin Books, Inc. 1969.

Web Sites

Purdue Department of Chemistry. “General Chemistry Help: States of Matter.” (accessed August 18, 2007).

Chemical Heritage Foundation. (accessed August 18, 2007).

Department of Physics, Brown University. http://www. (accessed August 18, 2007).

Nobel Prize Organization, Physics Prize Information. (accessed August 18, 2007).

Princeton Plasma Physics Laboratory: Fusion Energy Educational Web Site. (accessed August 18, 2007).

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Science Week: History of Physics: Einstein and Brownian Motion. (accessed August 18, 2007).