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Matter (chemistry) for CZ

From the perspective of classical mechanics, or more specifically, Newtonian mechanics, chemists describe matter as anything that occupies space and has mass. That includes the subatomic particles that scientists can discern as having physical extension and mass, all the chemical elements, or elementary substances — "the substances from which everything tangible is made," — and all the substances chemical elements make up.

A minimal account of matter from the chemist´s Newtonian perspective requires discussion of the meanings of the terms 'mass', 'substance', 'chemical elements', and 'compounds'.

Overview

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 the resistance to motion exhibited by an unknown quantity of matter in response to an applied force, measured in terms of the 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. To get an iron block to equal the acceleration of a wood block requires a larger force. Newton´s Second Law of Motion formulates the mass: force equals mass times acceleration, F=ma, mass in kilograms, with 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, gravitation. The magnitude of the force attracting the object measures its weight, which increases with larger attracting masses, but the object´s remains constant, in keeping with the unchanged quantity of matter in the 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 gravitational field.

The equivalence of inertial mass and passive gravitational mass derives from Newton´s Law of Gravitation and the observation that different masses accelerate equally when let loose 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, and air enclosed in a balloon.

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.

All substances fall under two generic categories, 'pure substances' and 'mixtures'. Chemists classify as the quintessentially pure substances the [[Chemical elements}chemical elements]], types of matter composed solely of a single species of atoms, 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, in this case, two hydrogen atoms and one oxygen atom per unit of compound, expressed in chemical formula as H₂O. Chemists have identified the bonds in a unit of the water compound as so-called covalent bonds, a type of bond that involves electron sharing between the hydrogen atoms and the oxygen atom, and refer to the unit as a molecule. Chemists express quantities of H₂O with a variety of measures of mass, such as kilograms, a basic unit in the International System of Units (SI units), among six other basic units used to define all the myriad other units of quantity.