Matter
In ancient Greece, some philosophers, most notably Heraclitus, believed that everything in the world was in a state of fluctuation. Others argued that there must be some permanence, otherwise it would not be possible to see anything as being real.
The fifth-century Greeks were apparently the first to attribute structure to matter. They postulated that matter consisted of very small particles that were firmly bound together in the solid state, but that could change position to accommodate compression and deformation. At higher temperatures, these particles were thought to slide past e ach other and to eventually separate, as the material object underwent melting and evaporation. The Greeks called these particles atoms, Greek for indivisible. This philosophical position offered a reconciliation between the views of a world in fluctuation and one that is permanent.
In 1687, Isaac Newton published his Principia, in which he described his laws of motion. As had his Greek predecessors, Newton viewed matter as passive and inert, and as consisting of "solid, massy, impenetrable, movable particles." Thus, for Newton and his cont emporaries, there was little distinction between the properties of matter in the material world and the building blocks of which it was composed.
In 1785, French chemist Antoine Lavoisier proposed his law of conservation of matter, which states that matter can neither be created nor destroyed, only changed into different forms. This law has since been superseded by the law of conservation of mass and energy, which takes into account Albert Einstein's observation that mass and energy are interchangeable under certain conditions.
Today, scientists believe that matter is made up of molecules; that molecules consist of atoms bound together; that each atom contains a nucleus surrounded by electrons; that the nucleus contains protons and neutrons, bound together; and that protons and neutrons consist of quarks bound together by gluons.
The modern viewpoint is consistent with the definition of matter as anything that occupies space and has mass. Mass is the material property that shows up as weight when the object is acted upon by gravity. Mass can be measured from an object's tendency to resis t moving, i.e., its inertia. The ratio of an object's mass to the volume it takes up is known as its density. The basic unit of mass in chemistry is the gram (1 kilogram = 1,000 grams).
Matter may exist in the solid, liquid, or gaseous state. Solids have definite sizes and shapes; liquids have definite volumes, but no fixed shapes; and gases possess neither shapes nor fixed volumes.
Matter that is made up of only one type of atom is called an element. Loosely, if a substance contains two or more kinds of atoms joined together and grouped in a definite way, that substance is called a compound; mixtures have a composition that is not uniform, but heterogeneous.
The physical properties of matter are those that can be observed when no change is taking place in the atomic structure. Examples include color, density, melting point, and hardness. Chemical properties have to do with a substance's tendency to react with other substances. An example is iron's tendency to rust in moist air.
In the case of ordinary chemical transformations, the mass of the products always equals the mass of the reactants.
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