Water, Steam, and Ice 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) .