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 (Figure 1.4). 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.
