A glass of ice water contains both ice and water. After a few minutes of settling, how do the temperatures of the ice and the water compare?
A. The ice is colder than the water B. The water is colder than the ice C. They’re at the same temperature
Phases of Matter
Solid – ice: fixed volume and fixed shape. Typically present below 32 oF or 0 oC
Liquid – water: fixed volume, but variable shape Typically present above 32 oF or 0 oC
Gas – steam: variable volume and variable shape Typically present at high temperatures
Phase Equilibrium
When two (or more) phases are present molecules continually shift between the phases one phase may grow at the expense of another phase that growth often requires or releases thermal energy
At phase equilibrium, two (or more) phases can coexist indefinitely neither phase grows at the expense of the other Ice and Water
Ice has a melting temperature (0 °C) below which solid ice is the stable phase, above which liquid water is the stable phase, and at which ice and water can coexist
To melt ice at 0 °C, destabilize ice relative to water add heat or increase pressure (very atypical!)
To freeze water at 0 °C, destabilize water w.r.t. ice remove heat or decrease pressure (very atypical!)
Melting ice requires the latent heat of melting
Water and Steam
Liquid water and gaseous steam can coexist over a broad range of temperatures but equilibrium steam density rises with temperature
To evaporate water, destabilize water r.t. steam add heat or reduce steam density
To condense steam, destabilize steam r.t. water remove heat or increase steam density
Evaporating water requires latent heat of evaporation Boiling
Evaporation bubbles can form inside water Pressure in steam bubble depends on steam density When steam pressure exceeds ambient pressure, the steam bubble survives and grows
Boiling occurs when bubbles can nucleate (seed bubbles form) bubbles can grow via evaporation
Need for latent heat stabilizes temperature
Boiling temperature depends on ambient pressure Elevated pressure raises boiling temperature Diminished pressure lowers boiling temperature
Cooking uses boiling to set a stable temperature Foods cook fast at high pressures (sea level) Foods cook slow at low pressures (high altitudes)
Ice and Steam
Solid ice and gaseous steam can coexist over a broad range of temperatures but equilibrium steam density rises with temperature
To sublime ice, destabilize ice r.t. steam add heat or reduce steam density
To deposit steam, destabilize steam r.t. ice remove heat or increase steam density
Subliming ice requires latent heats of melting and evaporation Relative Humidity
At 100% relative humidity, ice is in phase equilibrium with steam (< 0 °C) water is in phase equilibrium with steam (> 0 °C)
Below 100% relative humidity, ice sublimes (< 0 °C) (goodbye ice cubes!) water evaporates (> 0 °C)
Above 100% relative humidity, frost forms (< 0 °C) steam condenses (> 0 °C)