Chemistry Notes: Phases of Matter

States of Matter and Phase Changes  The three most common phases, or states of matter, are solids, liquids, and gases.  We can describe each state of matter in several ways. Compressibility is a way of measuring how little or how much a substance can be compressed. The picture on the right shows that a gas can be compressed significantly, while a liquid can only be compressed slightly.

Structure: the arrangement of particles in a substance, and their ability to move Relative kinetic energy of the particles in a substance (kinetic energy = energy of motion) Shape (definite or not definite) Volume (definite or not definite) Particle diagrams are a way of representing the structure and motion of particles in a substance. The picture on the right shows particle diagrams for a solid, a liquid, and a gas.

Solids Very low compressibility Structure is generally very organized; some solids have an orderly crystalline structure, while others are more amorphous. The particles within a solid are stationary (they cannot move from one place to another). Relatively low kinetic energy compared to liquids and gases Definite shape and volume

Liquids Very low compressibility Liquids have a more chaotic structure than solids, because their particles can “slide” past each other. Liquids have higher kinetic energy than solids, but less kinetic energy than gases. Liquids have definite volume, but their shape changes to fit the shape of their container.

Gases High compressibility (can be compressed and can expand) Gases are composed of particles that move past each other, and collide with each other, at high speeds. Gases have higher kinetic energy than liquids or solids. Gases do not have definite shape or volume.

Phase Changes A phase change is the transition of a substance from one phase to another. Because phase changes only change the phase of a substance and not its composition, they are physical changes. Types of phase changes Boiling/vaporization/evaporation: transition from liquid to gas Condensation: transition from gas to liquid Melting: transition from solid to liquid Freezing: transition from liquid to solid Sublimation: transition from solid to gas (ex. dry ice, which is solid CO2) Deposition: transition from gas to solid

Phase changes can be classified as endothermic (if they absorb or require energy) or exothermic (if they release energy). Endothermic changes occur when a substance transitions from a lower-energy state to a higher-energy state (boiling, vaporization, evaporation, melting, sublimation). Exothermic changes occur when a substance transitions from a higher-energy state to a lower-energy state (condensation, freezing, deposition)