Plasma

Plasma (physics), in physics, usually gaseous state of matter in which a part or all of the atoms or molecules are dissociated to form ions (see Ionization). Plasmas consist of a mixture of neutral particles, positive ions (atoms or molecules that have lost one or more electrons), and negative electrons. A plasma is a conductor of electricity, but a volume with dimensions greater than the so-called Debye length exhibits electrically neutral behavior. At a microscopic level, corresponding to distances shorter than the Debye length, the particles of a plasma do not exhibit collective behavior but instead react individually to a disturbance, for example, an electric field.

On the earth, plasmas usually do not occur naturally except in the form of lightning bolts, which consist of narrow paths of air molecules of which approximately 20 percent are ionized, and in parts of flames. The free electrons in a metal can also be considered as a plasma. Most of the universe, however, consists of matter in the plasma state. The ionization is caused either by high temperatures, such as inside the sun and stars, or by radiation, such as the ionization of interstellar gases or, closer to the earth, the upper layers of the atmosphere (see Ionosphere), producing the aurora.

Plasmas can be created by applying an electric field to a low-pressure gas, as in neon or fluorescent tubes (see Neon Lamp). A plasma can also be created by heating a neutral gas to very high temperatures. Usually the required temperatures are too high to be applied externally, and the gas is heated internally by the injection of high-speed ions or electrons that collide with the gas particles, increasing their thermal energy. The electrons in the gas can also be accelerated by external electric fields. Ions from such plasmas are used in the semiconductor industry for etching surfaces and otherwise altering the properties of materials.

In very hot plasmas the particles acquire enough energy to engage in nuclear reactions with each other during collisions. Such fusion reactions are the heat source in the sun's core, and scientists are trying to create artificial plasmas in the laboratory in which fusion reactions would produce energy for the production of electricity.

See also Fusion; Nuclear Energy; Physics: Plasma Physics.

States of Matter

States of Matter, in classical physics, three forms in which matter occurs—solid, liquid, and gas. Plasma, the collection of charged gaseous particles containing nearly equal numbers of negative and positive ions, is sometimes called the fourth state of matter (see Ion; Ionization). Solid matter is characterized by resistance to any change in shape, caused by a strong attraction between the molecules of which it is composed. In liquid form, matter does not resist forces that act to change its shape, because the molecules are free to move with respect to each other (see Molecule). Liquids, however, have sufficient molecular attraction to resist forces tending to change their volume. Gaseous matter, in which molecules are widely dispersed and move freely, offers no resistance to change of shape and little resistance to change of volume. As a result, a gas that is not confined tends to diffuse infinitely, increasing in volume and diminishing in density.

Most substances are solid at low temperatures, liquid at medium temperatures, and gaseous at high temperatures, but the states are not always distinct (see Temperature). The temperature at which any given substance changes from solid to liquid is its melting point, and the temperature at which it changes from liquid to gas is its boiling point; (see Freezing Point). The range of melting and boiling points varies widely. Helium remains a gas down to -268.9°C (-452°F), and tungsten remains a solid up to about 3420°C (about 6190°F).

For further discussion of the properties of matter in its different states, see Atom; Crystal; Fluid; Glass; Liquid Crystal; Thermodynamics; Vapor. See also Critical Point; Cryogenics.

Changes in Matter

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Changes in Matter is an alteration in the form or composition of matter. In science, matter is defined as anything that occupies space and possesses the attributes of gravity and inertia. Matter occurs in three forms: solid, liquid, or gas. Changes in matter may be of two types: physical or chemical. See also States of Matter

PHYSICAL CHANGES

A physical change is a change in matter that involves no chemical reaction. When a substance undergoes a physical change, the composition of its molecules remains unchanged, and the substance does not lose its chemical identity. Melting, evaporating, and freezing are three types of physical change. For example, water is a liquid that freezes to form the solid ice, which may again be melted into water. Because molecules of water and ice are composed of the same chemical elements in the same proportions, the change from water to ice is a physical change. Physical changes include any alteration in the shape and size of a substance. For example cutting, grinding, crushing, annealing, dissolving, or emulsifying produce physical changes. Still another physical change is sublimation, the change from a solid to a gas.

CHEMICAL CHANGES

When a substance undergoes a chemical change, the composition of its molecules changes. The properties of the original substance are lost, and new substances with new properties are produced. An example of a chemical change is the production of rust (iron oxide) when oxygen in the air reacts with iron. Chemical changes may also result in physical changes. For example, when wood (a solid) is burned, it is combined with oxygen gas to produce gaseous carbon dioxide , liquid water, and solid carbon.

Some of the various chemical changes that matter may undergo are classified below. For a more detailed discussion of chemical reactions, See Chemistry and Chemical Reaction.

A. Combination Reactions -> Combination reactions occur when two substances unite to form a third substance. For example, combining magnesium (Mg) and oxygen results in the production of magnesium oxide (MgO): . This reaction can be accomplished by burning magnesium in air, which supplies the oxygen.

B. Decomposition Reactions -> Decomposition reactions occur when a single compound breaks down into two or more simpler substances. In the decomposition of mercuric oxide (HgO), the elements mercury (Hg) and oxygen are produced: .

C. Displacement Reactions -> When one element replaces another in a compound, it is known as a displacement reaction. For example, iron (Fe) may displace copper (Cu) in a solution of cupric sulphate : .

D. Double Decomposition Reactions -> When two compounds interact to form two other compounds, it is known as a double decomposition reaction. For example, sodium iodide (NaI) and lead nitrate react to form lead iodide and sodium nitrate : .

E. Hydrolysis -> Hydrolysis is a double decomposition reaction in which water reacts with a second substance. When ammonium chloride is combined with water , it undergoes hydrolysis, yielding ammonium hydroxide and hydrochloric acid (HCl): .

F. Neutralization Reactions -> Neutralization is the interaction of an acid with the equivalent quantity of a base (see Acids and Bases). If the process is carried out in an aqueous solution (dissolved in water), the products are water and a salt. For example, hydrochloric acid (HCl) and sodium hydroxide (NaOH) neutralize each other when dissolved in water, forming sodium chloride (NaCl), a salt, and water : HCl + NaOH → NaCl + .

G. Substitution Reactions -> Substitution reactions occur when an element, such as chlorine (Cl), replaces one or more hydrogen atoms in a hydrocarbon, such as methane : .