Meeting to 9 : Classification of matter

Classifications of Matter

    Let's begin our study of chemistry by examining some fundamental ways in which matter is classified and described. Two principal ways of classifying matter are according to its physical state (as a gas, liquid, or solid) and according to its composition (as an element, compound, or mixture).

 States of Matter

A sample of matter can be a gas, a liquid, or a solid. These three forms of matter are called the states of matter. The states of matter differ in some of their simple observable properties. A gas (also known as vapor) has no fixed volume or shape; rather, it conforms to the volume and shape of its container. A gas can be compressed to occupy a smaller volume, or it can expand to occupy a larger one. A liquid has a distinct volume independent of its container but has no specific shape: It assumes the shape of the portion of the container that it occupies. A solid has both a definite shape and a definite volume: It is rigid. Neither liquids nor solids can be compressed to any appreciable extent.

The properties of the states can be understood on the molecular level In a gas the molecules are far apart and are moving at high speeds, colliding repeatedly with each other and with the walls of the container. In a liquid the molecules are packed more closely together, but still move rapidly, allowing them to slide over each other; thus, liquids pour easily. In a solid the molecules are held tightly together, usually in definite arrangements, in which the molecules can wiggle only slightly in their otherwise fixed positions. Thus, solids have rigid shapes.

Pure Substances

Most forms of matter that we encounter—for example, the air we breathe (a gas), gasoline for cars (a liquid), and the sidewalk on which we walk (a solid)—are not chemically pure. We can, however, resolve, or separate, these kinds of matter into different pure substances. A pure substance (usually referred to simply as a substance) is matter that has distinct properties and a composition that doesn't vary from sample to sample. Water and ordinary table salt (sodium chloride), the primary components of seawater, are examples of pure substances.

All substances are either elements or compounds. Elements cannot be decomposed into simpler substances. On the molecular level, each element is composed of only one kind of atom . Compounds are substances composed of two or more elements, so they contain two or more kinds of atoms  Water, for example, is a compound composed of two elements, hydrogen and oxygen.  shows a mixture of substances. Mixtures are combinations of two or more substances in which each substance retains its own chemical identity.

Elements

At the present time 114 elements are known. These elements vary widely in their abundance, as shown in  For example, only five elements account for over 90% of the Earth's crust: oxygen, silicon, aluminum, iron, and calcium. In contrast, just three elements (oxygen, carbon, and hydrogen) account for over 90% of the mass of the human body.

Some of the more familiar elements , along with the chemical abbreviations—or chemical symbols—used to denote them. All the known elements and their symbols are listed on the front inside cover of this text. The table in which the symbol for each element is enclosed in a box is called the periodic table. In the periodic table the elements are arranged in vertical columns so that closely related elements are grouped together. We describe this important tool in more detail in .

The symbol for each element consists of one or two letters, with the first letter capitalized. These symbols are often derived from the English name for the element, but sometimes they are derived from a foreign name instead You will need to know these symbols and to learn others as we encounter them in the text.

Compounds

Most elements can interact with other elements to form compounds. Hydrogen gas, for example, burns in oxygen gas to form water. Conversely, water can be decomposed into its component elements by passing an electrical current through it, as shown in  Pure water, regardless of its source, consists of 11% hydrogen and 89% oxygen by mass. This macroscopic composition corresponds to the molecular composition, which consists of two hydrogen atoms combined with one oxygen atom. As seen in  the properties of water bear no resemblance to the properties of its component elements. Hydrogen, oxygen, and water are each unique substances.