and the Equilibrium of Substances Phase Diagrams and the Equilibrium of Substances 1

Phase Diagrams and Equilibrium • phase diagram = a graphical way to depict the effects of pressure and temperature on the phase of a substance. each point on the phase diagram represents a specific temp and pressure the curves indicate the conditions of temperature and pressure under which equilibrium between different phases of a substance can exist critical point melting LIQUID freezing evaporation SOLID Pressure condensation triple point sublimation GAS deposition Temperature 2

Phase Diagrams and Equilibrium—con’t. • equilibrium = a dynamic (constantly changing) condition in which two opposing phase changes occur at equal rates in a closed system. • Ex: envision a sealed bottle half full of water, with a vacuum above. 1. at first, no vapor is present, but some molecules are beginning to escape the surface of the liquid (they are starting to evaporate). 2. evaporation then continues at a constant rate. Also, some vapor molecules are starting to re-enter the liquid state (they are condensing). 3. after some time, equilibrium is reached between the liquid and vapor states, meaning: rate of evaporation = rate of condensation. Equilibrium! 3

Phase Diagrams and Equilibrium—con’t. Parts of the Phase Diagram • vapor pressure = pressure exerted by a vapor onto its corresponding liquid at a given temperature. Shown on the border between the liquid and gaseous (vapor) states of the substance (AD curve). • the position (on the x-axis) of this AD curve is dependant on the degree of volatility of the substance. LOW volatility substance HIGH volatility substance • volatile substances = liquids that evaporate readily (at “low” temperatures), such as gasoline, acetone, etc. • the AD curve ends at the critical point. Liquid Solid Pressure Gas Temperature Liquid Liquid Solid Solid Gas Gas 4

Phase Diagrams and Equilibrium—con’t. • critical point = temperature above which a gas cannot be liquefied no matter how much pressure is applied (the kinetic energy is too great for attractive forces to overcome, regardless of the applied pressure). Point D on graph. • the line between the gas and solid phase (AB curve) indicates the vapor pressure of the solid as it sublimes at different temps • the line between the solid and liquid phases (AC curve) indicates the melting temp. of the solid as the pressure changes. • triple point = the condition of temperature and pressure where all three phases are in equilibrium (point A on graph). • regions not on a line represent conditions of temperature and pressure where only one phase is present. • gases are most likely under conditions of high temperature • solids are most likely under conditions of high pressure Liquid Gas Liquid Solid Pressure Gas Temperature 5

Phase Diagrams and Equilibrium—con’t. Other Info Found on the Phase Diagram • a line drawn across the diagram at 1 atm (normal, sea level pressure) will show the melting point and boiling point of a substance. • melting point (same as freezing point) = temp at which a substance changes phase from solid to liquid at 1 atm of pressure. • boiling point = temp at which a substance changes phase from liquid to vapor not only at the surface, but also within the liquid itself at 1 atm of pressure. D. Phase Diagram for Water • water is an atypical substance in that the melting point of ice can be lowered by applying pressure (the AC curve has a negative slope). • this is why ice skaters are able to glide so easily on ice—the pressure from the skater’s weight melts the ice under their skates! melting point 1 atm boiling point 0°C 100°C 6