Water, Steam, and Ice 1 Water, Steam, and Ice

Water, Steam, and Ice 2 Introductory Question 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 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

Water, Steam, and Ice 3 Observations about Water, Steam, and Ice Water has three forms or phases Water has three forms or phases Ice is typically present below 32 °F or 0 °C Ice is typically present below 32 °F or 0 °C Water is typically present above 32 °F or 0 °C Water is typically present above 32 °F or 0 °C Steam is typically present at high temps Steam is typically present at high temps The three phases sometimes coexist The three phases sometimes coexist

Water, Steam, and Ice 4 4 Questions about Water, Steam, Ice How can water and ice coexist in a glass? How can water and ice coexist in a glass? Can steam exist below 212 °F (100 °C)? Can steam exist below 212 °F (100 °C)? Where do ice cubes go in a frostless freezer? Where do ice cubes go in a frostless freezer? Is salt the only chemical that helps melt ice? Is salt the only chemical that helps melt ice?

Water, Steam, and Ice 5 Question 1 How can water and ice coexist in a glass? How can water and ice coexist in a glass?

Water, Steam, and Ice 6 Phases of Matter Ice is solid: fixed volume and fixed shape Ice is solid: fixed volume and fixed shape Water is liquid: fixed volume but variable shape Water is liquid: fixed volume but variable shape Steam is gas: variable volume and variable shape Steam is gas: variable volume and variable shape

Water, Steam, and Ice 7 Phase Equilibrium When two (or more) phases are present When two (or more) phases are present molecules continually shift between the phases molecules continually shift between the phases one phase may grow at the expense of another phase one phase may grow at the expense of another phase that growth often requires or releases thermal energy that growth often requires or releases thermal energy At phase equilibrium, At phase equilibrium, two (or more) phases can coexist indefinitely two (or more) phases can coexist indefinitely neither phase grows at the expense of the other neither phase grows at the expense of the other

Water, Steam, and Ice 8 Ice and Water Ice has a melting temperature (0 °C) Ice has a melting temperature (0 °C) below which solid ice is the stable phase, below which solid ice is the stable phase, above which liquid water is the stable phase, above which liquid water is the stable phase, and at which ice and water can coexist and at which ice and water can coexist To melt ice at 0 °C, destabilize ice relative to water To melt ice at 0 °C, destabilize ice relative to water add heat or increase pressure (very atypical!) add heat or increase pressure (very atypical!) To freeze water at 0 °C, destabilize water r.t. ice To freeze water at 0 °C, destabilize water r.t. ice remove heat or decrease pressure (very atypical!) remove heat or decrease pressure (very atypical!) Melting ice requires the latent heat of melting Melting ice requires the latent heat of melting

Water, Steam, and Ice 9 Introductory Question (revisited) 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 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

Water, Steam, and Ice 10 Question 2 Can steam exist below 212 °F (100 °C)? Can steam exist below 212 °F (100 °C)?

Water, Steam, and Ice 11 Water and Steam Liquid water and gaseous steam Liquid water and gaseous steam can coexist over a broad range of temperatures can coexist over a broad range of temperatures but equilibrium steam density rises with temperature but equilibrium steam density rises with temperature To evaporate water, destabilize water r.t. steam To evaporate water, destabilize water r.t. steam add heat or reduce steam density add heat or reduce steam density To condense steam, destabilize steam r.t. water To condense steam, destabilize steam r.t. water remove heat or increase steam density remove heat or increase steam density Evaporating water requires latent heat of evaporation Evaporating water requires latent heat of evaporation

Water, Steam, and Ice 12 Boiling (Part 1) Evaporation bubbles can form inside water Evaporation bubbles can form inside water Pressure in steam bubble depends on steam density Pressure in steam bubble depends on steam density When steam pressure exceeds ambient pressure, the steam bubble survives and grows When steam pressure exceeds ambient pressure, the steam bubble survives and grows Boiling occurs when Boiling occurs when bubbles can nucleate (seed bubbles form) bubbles can nucleate (seed bubbles form) bubbles can grow via evaporation bubbles can grow via evaporation Need for latent heat stabilizes temperature Need for latent heat stabilizes temperature

Water, Steam, and Ice 13 Boiling (Part 2) Boiling temperature depends on ambient pressure Boiling temperature depends on ambient pressure Elevated pressure raises boiling temperature Elevated pressure raises boiling temperature Diminished pressure lowers boiling temperature Diminished pressure lowers boiling temperature Cooking uses boiling to set a stable temperature Cooking uses boiling to set a stable temperature Foods cook fast at high pressures (sea level) Foods cook fast at high pressures (sea level) Foods cook slow at low pressures (high altitudes) Foods cook slow at low pressures (high altitudes)

Water, Steam, and Ice 14 Question 3 Where do ice cubes go in a frostless freezer? Where do ice cubes go in a frostless freezer?

Water, Steam, and Ice 15 Ice and Steam Solid ice and gaseous steam Solid ice and gaseous steam can coexist over a broad range of temperatures can coexist over a broad range of temperatures but equilibrium steam density rises with temperature but equilibrium steam density rises with temperature To sublime ice, destabilize ice r.t. steam To sublime ice, destabilize ice r.t. steam add heat or reduce steam density add heat or reduce steam density To deposit steam, destabilize steam r.t. ice To deposit steam, destabilize steam r.t. ice remove heat or increase steam density remove heat or increase steam density Subliming ice requires latent heats of melting and evaporation Subliming ice requires latent heats of melting and evaporation

Water, Steam, and Ice 16 Relative Humidity At 100% relative humidity, At 100% relative humidity, ice is in phase equilibrium with steam (< 0 °C) ice is in phase equilibrium with steam (< 0 °C) water is in phase equilibrium with steam (> 0 °C) water is in phase equilibrium with steam (> 0 °C) Below 100% relative humidity, Below 100% relative humidity, ice sublimes (< 0 °C) (goodbye ice cubes!) ice sublimes (< 0 °C) (goodbye ice cubes!) water evaporates (> 0 °C) water evaporates (> 0 °C) Above 100% relative humidity, Above 100% relative humidity, frost forms (< 0 °C) frost forms (< 0 °C) steam condenses (> 0 °C) steam condenses (> 0 °C)

Water, Steam, and Ice 17 Question 4 Is salt the only chemical that helps melt ice? Is salt the only chemical that helps melt ice?

Water, Steam, and Ice 18 Effects of Impurities Dissolved impurities stabilize liquid water, Dissolved impurities stabilize liquid water, its melting temperature drops its melting temperature drops its boiling temperature rises its boiling temperature rises These shifts are proportional to solute density, These shifts are proportional to solute density, such as salt ions such as salt ions or sugar molecules or sugar molecules Any soluble material can help ice to melt Any soluble material can help ice to melt Insoluble materials don’t cause ice to melt Insoluble materials don’t cause ice to melt

Water, Steam, and Ice 19 Summary about Water, Steam, and Ice Phase transitions reflect relative phase stabilities Phase transitions reflect relative phase stabilities Phases in equilibrium are equally stable Phases in equilibrium are equally stable Temperature and pressure affect phase stabilities Temperature and pressure affect phase stabilities Phase transitions usually require or release heat Phase transitions usually require or release heat