Water, Steam, and Ice the temperatures of the ice and the water - PDF document

Water, Steam, and Ice 1 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 Water, Steam, and Ice the temperatures of the ice and the water compare? The ice is colder than the water A. The water is colder than the ice B. They’re at the same temperature C. Water, Steam, and Ice 3 Observations about Water, Steam, and Ice 4 4 Questions about Water, Steam, and Ice Water, Steam, Ice � Water has three forms or phases � How can water and ice coexist in a glass? � Ice is typically present below 32 °F or 0 °C � Can steam exist below 212 °F (100 °C)? � Water is typically present above 32 °F or 0 °C � Where do ice cubes go in a frostless freezer? � Steam is typically present at high temps � Is salt the only chemical that helps melt ice? � The three phases sometimes coexist Water, Steam, and Ice 5 Water, Steam, and Ice 6 Question 1 Phases of Matter � How can water and ice coexist in a glass? � Ice is solid: fixed volume and fixed shape � Water is liquid: fixed volume but variable shape � Steam is gas: variable volume and variable shape 1

Water, Steam, and Ice 7 Water, Steam, and Ice 8 Phase Equilibrium Ice and Water � Ice has a melting temperature (0 °C) � When two (or more) phases are present � below which solid ice is the stable phase, � molecules continually shift between the phases � above which liquid water is the stable phase, � one phase may grow at the expense of another phase � and at which ice and water can coexist � that growth often requires or releases thermal energy � To melt ice at 0 °C, destabilize ice relative to water � At phase equilibrium, � add heat or increase pressure (very atypical!) � two (or more) phases can coexist indefinitely � To freeze water at 0 °C, destabilize water r.t. ice � neither phase grows at the expense of the other � remove heat or decrease pressure (very atypical!) � Melting ice requires the latent heat of melting Water, Steam, and Ice 9 Water, Steam, and Ice 10 Introductory Question (revisited) Question 2 A glass of ice water contains both ice and � Can steam exist below 212 °F (100 °C)? � water. After a few minutes of settling, how do the temperatures of the ice and the water compare? The ice is colder than the water A. The water is colder than the ice B. They’re at the same temperature C. Water, Steam, and Ice 11 Water, Steam, and Ice 12 Water and Steam Boiling (Part 1) � Liquid water and gaseous steam � Evaporation bubbles can form inside water � can coexist over a broad range of temperatures � Pressure in steam bubble depends on steam density � but equilibrium steam density rises with temperature � When steam pressure exceeds ambient pressure, the � To evaporate water, destabilize water r.t. steam steam bubble survives and grows � add heat or reduce steam density � Boiling occurs when � To condense steam, destabilize steam r.t. water � bubbles can nucleate (seed bubbles form) � remove heat or increase steam density � bubbles can grow via evaporation � Evaporating water requires latent heat of � Need for latent heat stabilizes temperature evaporation 2

Water, Steam, and Ice 13 Water, Steam, and Ice 14 Boiling (Part 2) Question 3 � Boiling temperature depends on ambient pressure � Where do ice cubes go in a frostless freezer? � 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) Water, Steam, and Ice 15 Water, Steam, and Ice 16 Ice and Steam Relative Humidity � Solid ice and gaseous steam � At 100% relative humidity, � can coexist over a broad range of temperatures � ice is in phase equilibrium with steam (< 0 °C) � but equilibrium steam density rises with temperature � water is in phase equilibrium with steam (> 0 °C) � To sublime ice, destabilize ice r.t. steam � Below 100% relative humidity, � add heat or reduce steam density � ice sublimes (< 0 °C) (goodbye ice cubes!) � To deposit steam, destabilize steam r.t. ice � water evaporates (> 0 °C) � Above 100% relative humidity, � remove heat or increase steam density � Subliming ice requires latent heats of melting � frost forms (< 0 °C) and evaporation � steam condenses (> 0 °C) Water, Steam, and Ice 17 Water, Steam, and Ice 18 Question 4 Effects of Impurities � Is salt the only chemical that helps melt ice? � Dissolved impurities stabilize liquid water, � its melting temperature drops � its boiling temperature rises � These shifts are proportional to solute density, � such as salt ions � or sugar molecules � Any soluble material can help ice to melt � Insoluble materials don’t cause ice to melt 3

Water, Steam, and Ice 19 Summary about Water, Steam, and Ice � Phase transitions reflect relative phase stabilities � Phases in equilibrium are equally stable � Temperature and pressure affect phase stabilities � Phase transitions usually require or release heat 4