Chemistry Notes: Phases of Matter Chemistry

 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. States of Matter and Phase Changes

 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.

 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 Solids

 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. Liquids

 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. Gases

 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 CO 2 )  Deposition: transition from gas to solid Phase Changes

 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)