Chemistry
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What is matter?
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What is matter?
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.
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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.).
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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.
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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.
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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.