Matter (chemistry)

See also: Matter

From the perspective of classical mechanics, chemists describe matter as anything that occupies space and has mass. Without matter the universe would not contain things, atoms and all of the things of which atoms serve as building blocks, the stuff of universe, from stars and planets to the inanimate things of our planet and all its living things, including people, among whom include chemists whose interests center on studying and exploiting for knowledge and human benefit the properties of matter and the ways matter of one form with one set of properties transforms into matter of a different form with a different set of properties, as, for example, when hydrogen atoms and oxygen atoms transform into liquid water, or when liquid water transforms into solid water or vapor.

From the classical chemistry perspective of matter viewed as space-occupying mass, matter includes the subatomic particles that scientists can discern as having physical extension and mass (e.g., protons, neutrons, quarks), electrons, the chemical elements, also called elementary substances — "the substances from which everything tangible is made" — and all the 'compounds' and 'mixtures' chemical elements make up.

Chemists do not ignore the fact that matter has structure both at the subatomic level, and at the atomic and supra-atomic level. They understand that facts at the former level help explain facts at the latter, for example, that the electrons of atoms play an essential role in determining chemical reactivity.

From the perspective of classical chemistry, informed by quantum or nuclear chemistry, all matter consists of chemical elements, either uncombined with other elements (e.g., pure gold), or bonded, each element with itself (e.g., dioxygen [O₂), or in various combinations and proportions of chemical elements of differing species organized as 'compounds' (e.g., salts such as sodium chloride [NaCl], molecules of DNA) or as 'mixtures' (e.g., a solution of glucose in water, an alloy of copper and tin).

A minimal account of matter from the chemist´s classical perspective requires discussion of the meanings of the terms 'thing' (or 'anything' or 'something' or 'everything'), 'mass', 'substance', 'chemical elements', 'molecules', 'ions', 'compounds', 'mixtures', 'properties', and 'chemical reactions'. This article continues with that discussion.

This article will use the words 'object' and 'substance' generically to refer to something that occupies space and has mass, from a simplest bit or unit of matter, to a tangible sample of matter — matter from a chemist's perspective.

Overview

Thing

When chemists define matter as anything (any thing) that occupies space and has mass they do not, in conjunction, define thing, presumably because they assume common knowledge of what the word 'thing' means. Indeed, semantic linguists have discovered that the word 'thing' has a primary meaning not definable by the use other words whose definitions do not require knowledge of the definition of 'thing', definable only by the use of words whose definitions themselves ultimately require knowledge of the definition of the word 'thing'.

Though semantically primitive, 'thing' still has meaning, a meaning a child learns from the way its elders use it, the word's origin going back to the deep-time beginnings of human speech, however pronounced then. A child hears his English-speaking parents frequently uttering 'thing' in reference to what we would call material objects: "This drawer has too many things in it", "Give me that thing before you hurt yourself", "Put your things away".

We would understand, then, that any 'thing' that occupies space and has mass represents matter, providing we know the meaning of the words 'occupy', 'space', and 'mass'. A semanticist might readily define the first two, 'occupy' and 'space', in terms of semantic primitives, but not so readily the third, 'mass', the definition of which we consider in the next section.

Mass

See: Mass

Mass gives a measure of the quantity of matter in an object, expressed in kilograms (kg), a basic unit of the International System of Units (SI units). Three related measures of mass exist, referred to as 'inertial mass', 'passive gravitational mass', and 'active gravitational mass'. Physicists have established that the three measures give equivalent values despite their different conceptual bases.

Inertial mass relates to a quantity of matter's resistance to motion in response to an applied force, resistance measured in terms of the degree of acceleration it undergoes in response to the applied force. For a given force, an object with a larger mass accelerates more slowly than an object with a smaller mass. For an iron block to achieve the same acceleration of a wood block requires a larger force than that acting on the wood block. Newton´s Second Law of Motion formulates the mass: force equals mass times acceleration, F=ma, mass expressed in kilograms, force expressed in newtons, and acceleration expressed in meters per second per second. From the chemist´s Newtonian perspective, one cannot create mass or destroy it, consequent to the law of conservation of mass.

Passive gravitational mass gives a measure of the quantity of matter in virtue of its reference to the property of an object to react to a gravitational field, that is, to react by attraction to another mass generating a mass-attracting force, a reaction which Newton called gravitation. The magnitude of the force attracting the object measures its weight, which increases with larger attracting masses, but the object´s mass remains constant, indicating no fixed weight for any given quantity of matter in an object.

Active gravitational mass gives a measure of the quantity of matter in virtue of its reference to the property of an object to create a field of force surrounding it that attracts another object — its property of creating a so-called gravitational field.

The equivalence of inertial mass and passive gravitational mass derives from Newton´s law of universal gravitation and the observation that different masses accelerate equally when let loose from the same height in a given gravitational field. The equivalence of passive and active gravitational mass derives both from Newton´s law of universal gravitation, Newton´s law of action and reaction, and the observation that one cannot shield an object from the force of gravity. The derivations are the provenance of physics.

Three points to note:

1. An object´s mass gives a measure of the quantity of matter comprising the object;
2. Objects have the same mass whether measured as inertial, passive, or gravitational mass;
3. Einstein´s theories of special and general relativity modify the Newtonian concept of mass, which however give a useful measure of mass for most purposes in general chemistry.

Substances

Chemistry conceptualizes matter as consisting of distinguishable types, referred to as 'substances'. Examples of substances include such commonly recognized space-occupying masses as water in a glass container, the glass container itself, copper wire, a gem of pure diamond, air enclosed in a balloon, atoms, and molecules.

Different substances have different properties, either physical or chemical properties, depending on whether or not testing for the property involves the formation of another substance or substances. They may also exist in different 'states', or 'phases', solid, liquid, and gaseous the most familiar.

All substances fall under two generic categories, 'pure substances' and 'mixtures'. Chemists classify as the quintessentially pure substances the chemical elements, types of matter composed solely of a single species of atom, such as the copper atoms fashioned into copper wire, the carbon atoms comprising a diamond gem, or iron atoms in a chunk of purified iron. Ninety-four different species of atoms occur naturally on Earth, each collection, or sample, of which that consists solely of atoms of a single species constitutes a pure substance of the type of matter referred to as a chemical element, or elementary substance.

Compounds

The atoms of two or more different chemical elements potentially can bind to each other, in constant proportions, by any one of a variety of types of chemical bonds, forming in the process new types of pure substances referred to as 'compounds'. Water exemplifies a compound, composed of units of hydrogen and oxygen atoms tightly bonded, in the same proportion per bonded unit particle, in this case, two hydrogen atoms and one oxygen atom per unit particle of compound, expressed in chemical formula as H₂O. Chemists have identified the bonds in a unit particle of the water compound as so-called covalent bonds, a type of bond that involves electron sharing between the two hydrogen atoms and the oxygen atom, and refer to the unit particle as a molecule. Chemists express quantities of H₂O with a variety of measures of mass, such as kilograms, a basic quantitative unit in the International System of Units (SI units), among six other basic quantitative units, and as moles, defined in terms of the number of atoms of a specified isotope of carbon in a specified quantity of isotope expresed in kilograms.