Chemistry

Atoms

CONCEPT Our world is made up of atoms, yet the atomic model of the universe is nonetheless considered a “theory.” When scientists know beyond all reasonable doubt that a particular principle is the case, then it is dubbed a law. Laws address the fact that certain things happen, as well as how they happen. A […]

ATOMS AND MOLECULES

Gases

CONCEPT The number of elements that appear ordinarily in the form of a gas is relatively small: oxygen, hydrogen, fluorine, and chlorine in the halogen “family”; and a handful of others, most notably the noble gases in Group 8 of the periodic table. Yet many substances can exist in the form of a gas, depending […]

PROPERTIES OF MATTER

CONCEPT Matter is physical substance that occupies space, has mass, is composed of atoms—or, in the case of subatomic particles, is part of an atom—and is convertible to energy. On Earth, matter appears in three clearly defined forms—solid, liquid, and gas—whose varying structural characteristics are a function of the speeds at which its molecules move […]

MATTER

MASS, DENSITY, AND VOLUME

CONCEPT Among the physical properties studied by chemists and other scientists, mass is one of the most fundamental. All matter, by definition, has mass. Mass, in turn, plays a role in two properties important to the study of chemistry: density and volume. All of these—mass, density, and volume—are simple concepts, yet in order to work […]

TEMPERATURE AND HEAT

CONCEPT Temperature, heat, and related concepts belong to the world of physics rather than chemistry; yet it would be impossible for the chemist to work without an understanding of these properties. Thermometers, of course, measure temperature according to one or both of two well-known scales based on the freezing and boiling points of water, though […]

MEASUREMENT

CONCEPT Measurement seems like a simple subject, on the surface at least; indeed, all measurements can be reduced to just two components: number and unit. Yet one might easily ask, “What numbers, and what units?”—a question that helps bring into focus the complexities involved in designating measurements. As it turns out, some forms of numbers […]

MEASUREMENT

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