The Properties of Matter

The general properties of matter result from its relationship with mass and space. Because of its mass, all matter has inertia (the mass being the measure of its inertia) and weight, if it is in a gravitational field (see gravitation). Because it occupies space, all matter has volume and impenetrability, since two objects cannot occupy the same space simultaneously.

The special properties of matter, on the other hand, depend on internal structure and thus differ from one form of matter, i.e., one substance, to another. Such properties include ductility, elasticity, hardness, malleability, porosity (ability to permit another substance to flow through it), and tenacity (resistance to being pulled apart).

The States of Matter

Matter is ordinarily observed in three different states, or phases (see states of matter), although scientists distinguish three additional states. Matter in the solid state has both a definite volume and a definite shape; matter in the liquid state has a definite volume but no definite shape, assuming the shape of whatever container it is placed in; matter in the gaseous state has neither a definite volume nor a definite shape and expands to fill any container. The properties of a plasma, or extremely hot, ionized gas, are sufficiently different from those of a gas at ordinary temperatures for scientists to consider them to be the fourth state of matter. So too are the properties of the Bose-Einstein and fermionic condensates, which exist only at temperatures approximating absolute zero (-273.15°C;), and they are considered the fifth and sixth states of matter respectively.

Early Theories of Matter

In ancient times various theories were suggested about the nature of matter. Empedocles held that all matter is made up of four "elements"—earth, air, fire, and water. Leucippus and his pupil Democritus proposed an atomic basis of matter, believing that all matter is built up from tiny particles differing in size and shape. Anaxagoras, however, rejected any theory in which matter is viewed as composed of smaller constituents, whether atoms or elements, and held instead that matter is continuous throughout, being entirely of a single substance.

Modern Theory of Matter

The modern theory of matter dates from the work of John Dalton at the beginning of the 19th cent. The atom is considered the basic unit of any element, and atoms may combine chemically to form molecules, the molecule being the smallest unit of any substance that possesses the properties of that substance. An element in modern theory is any substance all of whose atoms are the same (i.e., have the same atomic number), while a compound is composed of different types of atoms together in molecules.

The difference between a mixture and a compound helps to illustrate the difference between a physical change and a chemical change. Different atoms may also be present together in a mixture, but in a mixture they are not bound together chemically as they are in a compound. In a physical change, such as a change of state (e.g., from solid to liquid), the substance as a whole changes, but its underlying structure remains the same; water is still composed of molecules containing two hydrogen atoms and one oxygen atom whether it is in the form of ice, liquid water, or steam. In a chemical change, however, the substance participates in a chemical reaction, with a consequent reordering of its atoms. As a result, it becomes a different substance with a different set of properties.

Many of the physical properties and much of the behavior of matter can be understood without detailed assumptions about the structure of atoms and molecules. For example, the kinetic-molecular theory of gases provides a good explanation of the nature of temperature and the basis of the various gas laws and also gives insight into the different states of matter. Substances in different states vary in the strength of the forces between their molecules, with intermolecular forces being strongest in solids and weakest in gases. The force holding like molecules together is called cohesion, while that between unlike molecules is called adhesion (see adhesion and cohesion). Among the phenomena resulting from intermolecular forces are surface tension and capillarity. An even larger number of aspects of matter can be understood when the nature and structure of the atom are taken into account. The quantum theory has provided the key to understanding the atom, and most basic problems relating to the atom have been solved.

The atomic theory of matter does not answer the question of the basic nature of matter. It is now known that matter and energy are intimately related. According to the law of mass-energy equivalence, developed by Albert Einstein as part of his theory of relativity, a quantity of matter of mass m possesses an intrinsic rest mass energy E given by E = mc², where c is the speed of light. This equivalence is dramatically demonstrated in the phenomena of nuclear fission and fusion (see nuclear energy; nucleus), in which a small amount of matter is converted to a rather large amount of energy. The converse reaction, the conversion of energy to matter, has been observed frequently in the creation of many new elementary particles. The study of elementary particles has not solved the question of the nature of matter but only shifted it to a smaller scale.

Distribution of Interstellar Matter

Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the interstellar matter constitutes nearly all the physical volume of a galaxy. Although the density of interstellar matter is far lower than in the best laboratory vacuum, the total mass contained between stars is about 5% of the mass of the universe. Interstellar matter is mostly gaseous, but about 1% is interstellar grains or dust. The grains are not distributed uniformly in space but are found in clumpy clouds.

Some of the interstellar material is visible, sometimes through small telescopes, as nebulae. In the vicinity of bright stars the grains appear as glowing regions because of the intensity of the light they scatter; these regions are called reflection nebulae. Regions where the clouds are so thick that they obscure all starlight are called dark nebulae. Highly ionized matter, densely clustered around a hot star, is visible by the light emitted by the ions and electrons when they recombine; this is called an emission nebula.

Composition and Properties of Interstellar Matter

The interstellar gas, which constitutes about 99% of the interstellar matter, consists mostly of hydrogen and helium. In addition to the spectra (see spectrum) of those elements, some spectral lines not formed under ordinary laboratory conditions ("forbidden lines") are seen. The prominent green color of certain emission nebulae is due to a forbidden line of doubly ionized oxygen. In H I regions (regions of unionized hydrogen), neutral hydrogen atoms absorb and emit radio waves with a wavelength of 8 in. (21 cm), due to a reorientation of the proton spin in the magnetic field produced by the electron spin (see magnetic resonance). Besides atomic hydrogen and helium, many molecules, including formaldehyde and water vapor, have been detected in the interstellar medium by the techniques of radio astronomy.

The interstellar gas is electrically neutral at points far removed from any star (H I regions) but is highly ionized (the electrons are detached from their atoms) in the immediate vicinity of the most massive and hottest stars (H II regions). The gas is virtually transparent to visible light; there is weak optical absorption by certain trace atoms (sodium, calcium) and molecules (cyanogen, carbon hydride). However, within a short distance from a hot star nearly all its ultraviolet light is absorbed; the energy from this light maintains the state of ionization in the circumstellar H II region, which is called the Strömgren sphere (for Bengt Strömgren, the Danish astrophysicist who postulated its existence in the 1930s) and is the source of emission nebulae.

The nongaseous interstellar matter exists in the form of tiny solid particles called interstellar grains or dust. The grains are believed to be elongated in shape, and aligned with the magnetic field; they are believed to contain graphite or silicate material as well as polycyclic aromatic hydrocarbons. The clouds obscure the view of the galaxy in certain directions, particularly in the direction of the galactic center. They polarize and selectively scatter the starlight passing through them; blue light is scattered more than red light so that stars partially obscured by interstellar matter appear redder than their true color. Since the distances and intrinsic luminosities of many stars are estimated from analysis of their spectra, this effect, called interstellar reddening, has been responsible for errors in calculating the distances and luminosities of these stars.

Dark matter may consist of dust, planets, intergalactic gas formed of ordinary matter, or of MACHOs [Massive Astrophysical Compact Halo Objects], nonluminous bodies such as burned-out stars, black holes, and brown dwarfs; these are the so-called hot dark matter and would be dispersed uniformly throughout the universe. The discovery in 2001 of a large concentration of white dwarf stars in the halo surrounding the Milky Way indicates that these burned-out stars could represent as much as a third of the dark matter in the universe.

Other theories hold that it is made of elementary particles that played a key role in the formation of the universe, possibly the low-mass neutrino or theoretical particles called axions and WIMPs [Weakly Interacting Massive Particles]; these are the so-called cold dark matter and would be found in clumps throughout the universe. In 1996 a Japanese team at the Univ. of Tokyo led by Yasushi Ikebe reported on dark-matter clumping in the galactic cluster Fornax. Clumps were found in two distinct regions: around a massive galaxy in the center of the cluster and, in larger amounts, around the entire cluster. This suggests that the slower, cold dark matter might form the smaller clumps associated with the galaxy while the faster, hot dark matter might form the larger clumps associated with the galactic cluster.

Computer simulations of the formation of the universe favored the cold dark matter but tended to predict the formation of too many dwarf galaxies when compared to the observed universe. This led to the postulation of warm dark matter, which resolved the simulation problems. Unlike cold dark matter, which has mass but virtually no velocity or temperature, or hot dark matter, which has mass and is highly energetic, warm dark matter has mass and a low temperature corresponding to an extremely low velocity.

See also interstellar matter.

Material substance that constitutes the observable universe and, together with energy, forms the basis of all objective phenomena. Atoms are the basic building blocks of matter. Every physical entity can be described, physically and mathematically, in terms of interrelated quantities of mass, inertia, and gravitation. Matter in bulk occurs in several states; the most familiar are the gaseous (see gas), liquid, and solid states (plasmas, glasses, and various others are less clearly defined), each with characteristic properties. According to Albert Einstein's special theory of relativity, matter and energy are equivalent and interconvertible (see conservation law).

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