The matter is any substance (composed of various types of particles) that has mass, inertia, and occupies physical space by having volume. This definition, adequate for macroscopic physics, the object of study of mechanics and thermodynamics, does not fit well with modern theories in the microscopic realm, proper to atomic and subatomic physics. In physics, there is no unanimous consensus on the definition of matter, in part because the notion of “occupying space” is inconsistent within quantum mechanics. Instead, many physicists prefer to use the concepts of mass, energy, and particle.

For example, the space occupied by an object is mostly empty, given the large ratio (≈105) between the average radius of the electronic orbits and the typical size of an atomic nucleus; moreover, the law of conservation of mass is strongly violated on subatomic scales. In these fields, it is possible instead to adopt the definition that matter is constituted by a certain class of particles, which are the smallest and fundamental entities physically detectable: these particles are called fermions and follow the Pauli exclusion principle, which states that no more than one fermion can exist in the same quantum state.

Because of this principle, the particles that compose the matter are not all at the minimum energy state and for this reason it is possible to create stable structures of assembled fermions. Particles of the complementary class, called bosons, constitute instead the fields. They can then be considered the agents that operate the assemblies of fermions or their modifications, interactions and energy exchanges. A metaphor not entirely correct from a physical point of view, but effective and intuitive, sees the fermions as the bricks that make up the matter of the universe, and bosons as the glues or cements that hold them together to constitute the physical reality.

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.

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:

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.

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.

The matter in philosophy

In philosophy, the definition of matter corresponds to a constitutive principle of sensible reality in the original philosophical meaning. In ancient thought matter is, in the composition of things, the passive, feminine, chaotic element, which receives a form, in contrast to the active, forming one: in Pythagoreans matter is both father and mother, the matrix of things; taken on its own it is the opaque, shapeless, chaotic element; in reality it is always united to a form, so it is not perfectly identifiable.

In the pre-Socratics matter is the indistinct element from which things originate; sometimes it is conceived as a set of particles.

For Plato matter is the negative element, resistant to the impression of form that comes from the divine world, it is the dark receptacle of forms, the cause of the presence of evil (as corruption, deprivation of form) in the world and the reason for the instability of the sensory world.

For Aristotle, matter is instead the substrate of every change, every movement of form; it is a principle of things, constitutive of corporeality. Thus conceived, matter is pure indeterminacy, knowable only indirectly, arguing its existence as necessary for the composition of reality. In this sense arises a meaning of matter as power, pure capacity, which is nothing but can become something. Matter, thus defined, of itself taken is a non-entity. Aristotle brings to its maximum development the eternity of matter as a principle.

In Stoicism matter has again a physical connotation: it is the passive element of the Universe, but it gives corporeity to the entities. Plotinus and late Platonism affirm that matter is not life, nor soul, nor intellect, nor form, it is devoid of any determination and does not even have being. As indeterminacy it is also found in the intelligible, where, however, for the unity of it, it assumes the properties of the entity; insofar as it is joined with the concept of total otherness it is the chaotic indeterminacy from which the sensible world is drawn.

In Christian thought matter, while retaining the general Platonic meaning of original informality, is not in itself negativity, since it is created by God, and is the potential aspect of every creature, angelic and terrestrial. In the medieval doctrine of ilemorphism, that is universal matter and form, the interweaving of matter and form concerns every creature and means the composition of beings in the two principles.

In Renaissance thought matter is studied in its positivity: Bernardino Telesio thinks of it as visibility and corporeity; Giordano Bruno and Tommaso Campanella identify it with nature; for G. Bruno matter has in its bosom all complicated forms, and these are drawn to light as nature.

In Descartes matter has lost all metaphysical meaning and is defined in essentially physical terms: in particular matter is extension, movement, and its objective properties are expressed in mathematical ratios; they are valid for all matter, and consequently any distinction between incorruptible celestial matter and corruptible terrestrial matter falls.

Spinoza considers the extended matter an attribute of the divine substance; Leibniz instead defines matter as force, activity, energy: this concept will be developed in the romantic thought in conjectures about nature with theosophical perspectives. From the consideration of matter as a reality to which are attributable original properties, comes the modern materialistic doctrine that in the nineteenth-century evolutionism has its most typical expression.

In contemporary thought, the increasing mathematization of matter has caused the abandonment of the problem of matter as a principle, substance of reality.

Matter and form are correlative and complementary terms that indicate the constitutive principles of reality. With Aristotle, the structure of entities is explained in terms of matter and form because everything consists of an indeterminate principle and a determining principle: they correspond to power and act in the sense that every entity has a movement, a becoming that is the transition from matter to form, that is, consists in achieving a form. Only God is totally act, therefore only form, and does not presuppose a matter. This conception of the structure of things is called ilemorphism, and after Aristotle remains in Western philosophy, used both by later Platonism and Christian thought.

In the Middle Ages developed the theory of universal ilemorphism, according to which matter and form are not entities existing separately, but every immaterial or material reality is both matter and form: the angels have a matter as well as the body has a form that in the Franciscan tradition is identified in the light. St. Thomas opposed to this doctrine stating that while power and act are concepts that apply to all entities, matter and form apply only to the corporeal ones: matter is connotation of the element that constitutes the sensory reality.

In the Renaissance, with G. Bruno, is resumed the neoplatonic thesis of matter and form as constitutive of the whole universe, both of spiritual beings and nature. With Kant the concepts of matter and form take on a meaning no longer metaphysical, but logical-transcendental; for matter Kant means the material of knowledge, the multiplicity of phenomena, to which the activity of the subject gives the unity of a form determining it as an object.

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