Changes of State Section 10.4

Vocabulary Phase: any part of a system that has uniform composition and properties Condensation: the process by which a gas changes to a liquid Equilibrium: a dynamic condition in which two opposing changes occur at equal rates in a closed system

More Information Vapor: a gas in contact with its liquid or solid phase

Equilibrium Vapor Pressure of a Liquid Equilibrium vapor pressure: the pressure exerted by a vapor in equilibrium with its corresponding liquid at a given temperature Increase temperature = increase in kinetic energy = increase in the number of molecules with enough energy to become a vapor = increase in equilibrium vapor pressure

At a Given Temperature Because all liquids have characteristic forces of attraction between their particles, every liquid has a specific equilibrium vapor pressure at a given temperature Stronger forces = smaller number of particles that can evaporate = low equilibrium vapor pressure

Relative Equilibrium Vapor Pressure Volatile liquids: liquids that evaporate readily Volatile liquids have weak forces of attraction between their particles A typical volatile liquid is ether Nonvolatile liquids: liquids that do not evaporate readily

Nonvolatile liquids have relatively strong attractive forces between their particles Molten ionic compounds would be nonvolatile liquids

Boiling Boiling allows the conversion of a liquid to a vapor within the liquid as well as at its surface Increase in temperature = increase in equilibrium vapor pressure Boiling point: the temperature at which the equilibrium vapor pressure of a liquid equals the atmospheric pressure

If you increase the pressure, then the boiling point is higher If you decrease the pressure, then the boiling point is lower At high altitudes, the atmospheric pressure is less, so you have to cook your food longer, or at a higher temperature Energy must be added continuously for a liquid to keep boiling

The temperature at the boiling point remains constant despite the continuous addition of energy The added energy is used to overcome the attractive forces between the particles in the liquid This energy is stored in the vapor as potential energy

Molar Enthalpy of Vaporization Molar enthalpy of vaporization: the amount of energy as heat required to evaporate 1 mol of a liquid at constant pressure and temperature Abbreviated as This is actually a measure of the strength of the attractive forces between particles Each liquid has a characteristic molar enthalpy of vaporization

Freezing Normal freezing point: the temperature at which the solid and liquid are in equilibrium at 1 atm (101.3 kPa) Particles of the liquid and the solid have the same average kinetic energy Energy lost during freezing is the loss of stored energy (potential)

Melting Melting is the reverse of freezing It ocurs at a constant temperature As a solid melts, it absorbs energy as heat For pure crystalline solids, the melting point and the freezing point are the same At equilibrim, melting and freezing proceed at equal rates

Molar Enthalpy of Fusion Molar enthalpy of fusion: the amount of energy as heat required to melt one mole of solid at the solid’s melting point Abbreviated as The energy absorbed overcomes the attractive forces and decreases particle order (dependent on the strength of the forces)

Sublimation and Deposition Sublimation: the change of state from a solid directly to a gas Examples: solid CO2 and iodine Deposition: the change of state from a gas directly to a solid Example: formation of frost on a cold surface

Phase Diagrams Phase diagram: a graph of pressure versus temperature that shows the conditions under which the phases of a substance exist Triple point: (of a substance) indicates the temperature and pressure conditions at which the solid, liquid, and vapor states can coexist at equilibrium

Critical point: of a substance, indicates the critical temperature and critical pressure Critical temperature: the temperature above which the substance cannot exist in the liquid state Critical pressure: the lowest pressure at which the substance can exist as a liquid at the critical temperature