12.1: A Molecular Comparison of Gases, Liquids, and Solids The state of a substance depends on the balance between the kinetic energy of the individual particles (molecules or atoms) and the intermolecular forces. The kinetic energy keeps the molecules apart and moving around, and is a function of the temperature of the substance and the intermolecular forces try to draw the particles together. 12.2: Kinetic-Molecular Theory The kinetic-molecular theory is a theory that explains the states of matter and is based on the idea that matter is composed of tiny particles that are always in motion. The theory helps explain observable properties and behaviors of solids, liquids, and gases. However, the theory is most easily understood as it applies to gases. The theory applies specifically to a model of a gas called an ideal gas. 12.3: The Ideal Gas Equation Properties of gases such as pressure (P), volume (V), temperature (T), and moles(n) are relatively easy to measure. Unlike with liquids and solids, the particles (molecules or atoms) in a gas phase sample are very far apart from one another. As a result, their behavior is much more predictable because intermolecular forces become insignificant for most samples in the gas phase even over a wide range of conditions. The presence of intermolecular forces makes their behavior harder to predict. 12.4: The Kinetic-Molecular Theory Explains the Behavior of Gases The kinetic molecular theory is a simple but very effective model that effectively explains ideal gas behavior. The theory assumes that gases consist of widely separated molecules of negligible volume that are in constant motion, colliding elastically with one another and the walls of their container with average velocities determined by their absolute temperatures. The individual molecules of a gas exhibit a range of velocities. 12.5: Dalton's Law (Law of Partial Pressures) Dalton’s Law, or the Law of Partial Pressures, states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the gases in the mixture. 12.6: Evaporation and Condensation Evaporation is the conversion of a liquid to its vapor below the boiling temperature of the liquid. Condensation is the change of state from a gas to a liquid. As the temperature increases, the rate of evaporation increases. 12.7: Intermolecular forces