The familiar compound H₂O provides the evidence that substances occur in three different physical classes called states of matter. At room temperature, H₂O is a dense fluid called a liquid. When this liquid is chilled to 0°C, it changes to a rigid solid. If the liquid is heated to 100°C, however, it abruptly expands to a tenuous fluid called vapor or gas.

Such different states of matter are not unique to H₂O. Almost all substances can exist in two or three of the fundamental states. Table 1 defines the states in terms of the shape and volume of substances. Because both liquids and gases flow readily, they are collectively referred to as fluids.

These states have different properties because they have distinct structures on the atomic or molecular scale. In a solid, the atoms are bonded strongly to the surrounding atoms so each is in a fixed position; if the solid structure has a regular pattern that is repeated throughout the solid, it is described as a crystalline structure. The atoms or molecules in a liquid are less strongly bonded to one another than in a solid of the same chemical composition, and consequently, they may shift their positions. The bonds between molecules in a liquid are, nevertheless, strong enough so that the molecules stay in contact with surrounding molecules. In a gas, the bonding between individual molecules is essentially zero, and individual molecules may move in all directions, allowing the vapor to expand throughout any container.

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