Matter

Matter is the substance of which all things are made. All objects consist of matter. The objects may differ widely from one another. But they have one thing in common--they all occupy space. Therefore, scientists usually define matter as anything that occupies space. All matter has inertia. This means that it resists any change in its condition of rest or of motion. The quantity of matter in an object is called its mass, but scientists usually prefer to define mass as a measure of inertia. The earth's gravitational attraction for a given mass gives matter its weight. Gravity's pull on an object decreases as it moves away from the earth. For this reason, objects that move from the earth into outer space "lose weight" even though their masses remain the same.

Matter can be changed into energy and energy into matter. For example, matter changes into energy when radium and other radioactive elements disintegrate and when atomic bombs explode. Energy changes into matter when subatomic particles collide at high speeds and create new, heavier particles.

The Properties of Matter

All of us easily recognize many varieties of matter. Each variety possesses certain characteristics common to all samples of its kind. We base our recognition of each variety of matter on knowledge of its special characteristics, or properties. These properties distinguish one kind of matter from others. Matter has two main types of properties--physical and chemical.

Physical properties. People recognize certain kinds of matter by sight, smell, touch, taste, or hearing. We can recognize gold by color, sugar by taste, and gasoline by odor. These are examples of physical properties of matter. Another such property is density, the amount of mass for each unit of volume. Because of the difference in density, a block of cork weighs less than a block of all common woods the same size. Solubility (the ability of one kind of matter to dissolve in another) and conductivity (the ability of matter to conduct heat or electricity) are also physical properties.

Chemical properties of matter describe how a substance acts when it undergoes chemical change. For example, a chemical property of iron is its ability to combine with oxygen in moist air to form iron oxide, or rust. Scientists call such changes in the composition of matter chemical changes. Some changes alter the value of physical properties, such as weight or density, but produce no change in the composition of the matter. Scientists call these physical changes. When water changes to steam it undergoes physical, but not chemical, change.

Compounds and elements. By using chemical processes, scientists may be able to separate a substance into two or more simpler kinds of matter with new properties. If so, they call the substance a compound substance, or a chemical compound. Substances that do not break down into simpler varieties of matter by chemical means are called elementary substances, or chemical elements.

Structure of Matter

All ordinary matter is made up of atoms. An atom is the smallest quantity of an element that can enter into chemical reaction to form a compound. Atoms are composed of particles called protons, neutrons, and electrons. Protons and neutrons, in turn, are made up of particles called quarks. Quarks are held together by particles called gluons.

All the atoms of an elementary substance have identical chemical properties. When two or more elements combine to form a compound, the atoms of one substance combine with the atoms of the other substances. The atoms form larger particles called molecules. Water consists of molecules, each of which contains two atoms of hydrogen and one of oxygen. Atoms and molecules are extremely small. If the molecules in one drop of water were counted at the rate of 10 million each second, a person would need about 5 million years to count them all.

Compounds may be organic or inorganic. Organic compounds contain the element carbon. They are called organic because most of the compounds found in living organisms contain carbon. All other compounds are classed as inorganic. These classifications are not completely rigid. Organic molecules are among the largest molecules and may contain thousands of atoms.

Molecules are bound together by electrical force. This force comes from the electrons in the atoms. Electrons in a molecule may be exchanged between atoms in what chemists call ionic bonding. In addition, electrons may be shared between atoms in what chemists call covalent bonding.

Conservation of Matter

Before the famous German-born scientist Albert Einstein developed his theory of relativity, scientists believed that matter was never created or destroyed. This idea was called the conservation of matter. But Einstein proved that mass and energy are interchangeable. For example, if a chemical change gives off energy as heat and light, then the substances that changed must have lost some mass. In all ordinary chemical reactions that take place in factories, homes, and laboratories, the amount of mass lost is far too small to be measured. Measurable quantities of mass are changed into energy only in nuclear reactions such as those that occur in nuclear reactors, atomic bombs, or particle accelerators. Because of Einstein's work, scientists now state the conservation law this way: Mass-energy may not be created or destroyed, but each may be converted into the other.

States of Matter

Matter can ordinarily exist in three physical states--solid, liquid, and gas. For example, ice is solid water. When heated, it melts at a definite temperature to form liquid water. When heat causes the temperature of the water to rise to a certain point, the water boils, producing steam, a gas. Removal of heat reverses these processes. In spite of these changes, the chemical composition of water remains the same. A fourth state of matter, called plasma, exists under special conditions.

Solids. All solids have form. They also have hardness and rigidity, or the ability to oppose a change of shape. For example, stone does not change shape easily. Some solids, like salt or sulfur, are brittle and will shatter when struck. Others have great tensile strength and resist being pulled apart. Still others, particularly metals, have malleability (the ability to be beaten into thin sheets) and ductility (the ability to be drawn into wires). These properties depend on the particles that make up the substance and the forces acting among them. The atoms in almost all solids are arranged in regular patterns, called crystals.>

Liquids have no shape of their own. But they have the ability to flow. They take the shape of any container in which they are placed. They fill it only when their volume equals that of the container. Iron and steel are rigid in their solid state. But manufacturers often melt them and pour them into molds.

Gases. All gases, regardless of the composition of their molecules, have almost identical physical behavior. Compared with liquids or solids, they have low densities. They exert pressure equally in all directions. All are compressible. When heated, gases expand or exert a greater pressure when confined in a vessel of fixed volume.

Plasmas form in the interior of stars, in outer space, in neon lamps and fluorescent lamps, and in some laboratory experiments. Plasmas result when the atoms in a gas become ionized (electrically charged). Electrical forces between the gas atoms give the gas new physical properties.

Dark Matter

The visible universe is made up chiefly of the two lightest elements. It consists of about 75 per cent hydrogen and 24 per cent helium, with the heavier elements making up the remainder. However, there is convincing evidence that most of the matter in the universe is not visible. This invisible matter is called dark matter. Many scientists believe that dark matter may not be composed of atoms, or even of electrons, protons, neutrons, or quarks. Instead, it may be composed of yet undiscovered types of particles. The nature of dark matter is one of the most important questions in science today.