The matter is any substance (composed of various types of particles) that has mass, inertia, and occupies physical space by having volume. The atom is the simplest example of matter particles, which represent the smallest unit of matter composed of electrons, the protons, and the neutrons. They retain all of the chemical properties of an element. Massless particles such as photons, energy phenomena, or waves like light or sound, are not included in this definition.

Matter and mass should not be confused with each other because they are not the same thing in modern physics. The matter is a general term describing any physical substance, and the mass is a quantitative property of matter.

This definition of matter, sufficient for macroscopic physics, the subject of study of mechanics and thermodynamics, does not fit well with modern theories in the microscopic field, typical of atomic and subatomic physics. For example, the space occupied by an object is mainly empty, given the large ratio between the average radius of the electronic orbits and the typical dimensions of an atomic nucleus; moreover, the mass conservation law is violated on subatomic scales.

The term matter can be traced directly to the Latin term mater, which means mother. The etymology of the term, therefore, suggests how matter can be considered the constituent foundation of all bodies and all things: the first substance of which all other substances are formed. The term matter derives from philosophical jargon.

Classification of matter

Matter can be classified into different categories, but the main ones are mixtures and pure substances. A pure substance (usually referred to simply as a substance) is matter that has distinct properties and a composition that does not vary from sample to sample. Water and table salt (sodium chloride) are examples of pure substances. All substances are either elements or compounds.

The matter can be classified according to the states of aggregation, or divided into organic or inorganic and can belong to one of the three kingdoms of nature (mineral, vegetable, animal). All these classifications, however, cease to be rigorous when the matter is studied in its elementary constituents (molecules, atoms, etc.).

Physical and chemical properties of matter

Physical properties of the matter are characteristics that describe matter not associated with a change in its chemical composition. They include characteristics such as density, color, hardness, melting and boiling points, electrical conductivity, size, shape, color, and mass. Other examples of physical changes include magnetizing and demagnetizing metals and grinding solids into powders. In each of these examples, there is a change in the physical state, form, or properties of the substance, but no change in its chemical composition.

Chemical properties of the matter are characteristics that describe how matter changes its chemical structure or composition. In other words, the change of one type of matter into another type (or the inability to change) is a chemical property. Examples of chemical properties include flammability, toxicity, acidity, reactivity (many types), and heat of combustion.

Extensive and intensive properties

If you think about the various observable properties of matter, it will become apparent that these fall into two classes. Some properties, such as mass and volume, depending on the quantity of matter in the sample we are studying. Clearly, these properties, as important as they may be, cannot by themselves be used to characterize a kind of matter; to say that “water has a mass of 2 kg” is nonsense, although it may be quite true in a particular instance. Properties of this kind are called extensive properties of matter.

This definition of the density illustrates an important general rule: the ratio of two extensive properties is always an intensive property.

Suppose we make further measurements, and find that the same quantity of water whose mass is 2.0 kg also occupies a volume of 2.0 liters. We have measured two extensive properties (mass and volume) of the same sample of matter. This allows us to define a new quantity, the quotient m/V which defines another property of water which we call the density. Unlike the mass and the volume, which by themselves refer only to individual samples of water, the density (mass per unit volume) is a property of all samples of pure water at the same temperature. Density is an example of an intensive property of matter.

Intensive properties are extremely important because every possible kind of matter possesses a unique set of intensive properties that distinguish it from every other kind of matter. In other words, intensive properties serve to characterize matter. Many of the intensive properties depend on such variables as the temperature and pressure, but the ways in which these properties change with such variables can themselves be regarded as intensive properties.

The more intensive properties we know, the more precisely we can characterize a sample of matter.

Some intensive properties can be determined by simple observations: color (absorption spectrum), melting point, density, solubility, acidic or alkaline nature, and density are common examples. Even more fundamental, but less directly observable, is chemical composition.

States and phase transitions of matter

The states of aggregation of matter depend both on the nature of the matter and on the temperature and pressure of the environment in which it is located; based on the variations of these two environmental parameters, physical transformations also called state transitions (or phase transitions) take place. Matter can exist in several states, also called phases; the four fundamental states are:

  • solid state;
  • liquid state;
  • gaseous state;
  • plasmatic state.

These four descriptions, each implying that the matter has certain physical properties, represent the three phases of matter. A single element or compound of matter might exist in more than one of the three states, depending on the temperature and pressure.

A phase transition is a physical process in which a substance goes from one phase to another. Usually, the transition occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point of the substance.

The nature of the phase change depends on the direction of the heat transfer. Heat going into a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat from a substance changes a gas to a liquid or a liquid to a solid.

  • solid to liquid = melting (or fusion)
  • solid to gas = sublimation
  • liquid to gas = boiling, evaporation, vaporization
  • liquid to solid = solidification, freezing
  • gas to liquid = condensation
  • gas to solid = deposition
  • gas to plasma = ionization
  • plasma to gas = deionization, recombination

A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium, certain properties of the medium change, often discontinuously, as a result of the change of some external condition, such as temperature, pressure, or others. A phase transition is achieved by changing the thermodynamic parameters to reach a particular limit.

The law of conservation of matter

“Nothing comes from nothing” is an important idea in ancient Greek philosophy that argues that what exists now has always existed since no new matter can come into existence where there was none before. Antoine Lavoisier (1743-1794) restated this principle for chemistry with the law of conservation of mass, which “means that the atoms of an object cannot be created or destroyed, but can be moved around and be changed into different particles.”

This law says that when a chemical reaction rearranges atoms into a new product, the mass of the reactants (chemicals before the chemical reaction) is the same as the mass of the products (the new chemicals made). More simply, whatever you do, you will still have the same amount of stuff (however, certain nuclear reactions like fusion and fission can convert a small part of the mass into energy.

The law of conservation of mass states that the total mass present before a chemical reaction is the same as the total mass present after the chemical reaction; in other words, mass is conserved. The law of conservation of mass was formulated by Lavoisier as a result of his combustion experiment, in which he observed that the mass of his original substance—a glass vessel, tin, and air—was equal to the mass of the produced substance—the glass vessel, “tin calx”, and the remaining air.

The law of conservation of matter summarizes many scientific observations about the matter:

It states that there is no detectable change in the total quantity of matter present when matter converts from one type to another (a chemical change) or changes among solid, liquid, or gaseous states (a physical change).

This is really a consequence of “conservation of atoms” which are presumed to be indestructible by chemical means. In chemical reactions, the atoms are simply rearranged but never destroyed.

Mass conservation had special significance in understanding chemical changes involving gases, which were for some time not always regarded as the real matter at all. Owing to their very small densities, carrying out actual weight measurements on gases is quite difficult to do, and was far beyond the capabilities of the early experimenters.

Thus when magnesium metal is burned in air, the weight of the solid product always exceeds that of the original metal, implying that the process is one in which the metal combines with what might have been thought to be a “weightless” component of the air, which we now know to be oxygen.


Dark matter

Quark matter (QCD)

Quark matter or QCD matter refers to any of a number of theorized states of matter whose degrees of freedom include quarks and gluons.

In regular cold matter, quarks, fundamental particles of nuclear matter, are confined by the strong force into hadrons that consist of 2-4 quarks, such as protons and neutrons. Quark matter or quantum chromodynanamical (QCD) matter is a group of phases where the strong force is overcome and quarks are deconfined and free to move. Quark matter phases occur at extremely high densities or temperatures, and there are no known ways to produce them in equilibrium in the laboratory; in ordinary conditions, any quark matter formed immediately undergoes radioactive decay.

Strange matter is a type of quark matter that is suspected to exist inside some neutron stars close to the Tolman–Oppenheimer–Volkoff limit (approximately 2-3 solar masses), although there is no direct evidence of its existence. In strange matter, part of the energy available manifests as strange quarks, a heavier analogue of the common down quark. It may be stable at lower energy states once formed, although this is not known.

Quark–gluon plasma is a very high-temperature phase in which quarks become free and able to move independently, rather than being perpetually bound into particles, in a sea of gluons, subatomic particles that transmit the strong force that binds quarks together. This is analogous to the liberation of electrons from atoms in a plasma. This state is briefly attainable in extremely high-energy heavy ion collisions in particle accelerators, and allows scientists to observe the properties of individual quarks, and not just theorize. Quark–gluon plasma was discovered at CERN in 2000. Unlike plasma, which flows like a gas, interactions within QGP are strong and it flows like a liquid.

At high densities but relatively low temperatures, quarks are theorized to form a quark liquid whose nature is presently unknown. It forms a distinct color-flavor locked (CFL) phase at even higher densities. This phase is superconductive for color charges. These phases may occur in neutron stars but they are presently theoretical.

Degenerate matter

Degenerate matter is a highly dense state of fermionic matter in which particles must occupy high states of kinetic energy in order to satisfy the Pauli exclusion principle.

Under extremely high pressure, as in the cores of dead stars, ordinary matter undergoes a transition to a series of exotic states of matter collectively known as degenerate matter, which are supported mainly by quantum mechanical effects. In physics, “degenerate” refers to two states that have the same energy and are thus interchangeable.

Degenerate matter is supported by the Pauli exclusion principle, which prevents two fermionic particles from occupying the same quantum state. Unlike regular plasma, degenerate plasma expands little when heated, because there are simply no momentum states left. Consequently, degenerate stars collapse into very high densities. More massive degenerate stars are smaller, because the gravitational force increases, but pressure does not increase proportionally.

Electron-degenerate matter is found inside white dwarf stars. Electrons remain bound to atoms but are able to transfer to adjacent atoms. Neutron-degenerate matter is found in neutron stars. Vast gravitational pressure compresses atoms so strongly that the electrons are forced to combine with protons via inverse beta-decay, resulting in a superdense conglomeration of neutrons. Normally free neutrons outside an atomic nucleus will decay with a half life of just under 15 minutes, but in a neutron star, the decay is overtaken by inverse decay.

Cold degenerate matter is also present in planets such as Jupiter and in the even more massive brown dwarfs, which are expected to have a core with metallic hydrogen. Because of the degeneracy, more massive brown dwarfs are not significantly larger. In metals, the electrons can be modeled as a degenerate gas moving in a lattice of non-degenerate positive ions.


  1. Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD. OpenStax. Chemistry 2e.
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