Chemistry: Solids, Liquids, and Gases

Nearly every substance can exist as a solid, a liquid, or a gas. These are the three common states of matter. Whether a substance is a solid or a liquid or a gas depends on its temperature and the pressure placed on it. At room temperature (about 22° C) and at the normal pressure exerted by the atmosphere, water exists as a liquid, which can flow from one container to another. But if its temperature is lowered to about −10° C, liquid water freezes to solid ice. Going the opposite direction in temperature and at this same pressure, water changes to a gas when the temperature exceeds 100° C. Changes in state can also occur by changing the pressure while holding temperature constant. The relationship between temperature and pressure and the three states of matter is easier to see when displayed in a phase diagram. Because phase diagrams provide so much information, they are known for thousands of substances.

Any change in phase is accompanied by the taking in or release of heat energy because, as change takes place, the attractive forces between molecules are being broken down or being formed. As solid water converts to liquid water, heat is absorbed as the forces between water molecules weaken, allowing the liquid to flow. The energy involved in phase changes is accurately known for many substances. The heat energy needed to warm or cool solids, liquids, and gases without changing phase is also accurately known.

The familiar compound H₂O provides the evidence that substances occur in three different physical classes called states. 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.

TABLE 1

Definitions of the States of Matter

*State of Matter*	*Shape of Substance*	*Volume of Substance*
Solid	Definite	Definite
Liquid	Indefinite	Definite
Gas	Indefinite	Indefinite

These states have different properties because they have distinct structures on the atomic or molecular scale. In a solid, the atoms are strongly bonded 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 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.