7.2 Water, Steam, and Ice Phases of matter
Why is cooking with steam better than 100 C hot air? Why do foods get freezer burn? Why does the freezer get frosty? How does snow disappear when it’s still cold outside?
New ideas for today • Phases of water (liquid, gas, solid) • Phase equilibrium • Phase transitions (melting…) • Latent heat • Relative humidity
Water exists in a few phases • Liquid • Solid (ice) • Gas (steam/water vapor) Bonds to form different structures (phases) Polar molecule
Gas Water vapor in can • Mostly independent molecules • In motion because of thermal energy • Compressible • Fluid (changes shape easily) • Fills volume Liquid N2 cannon
Liquid • Molecules loosely bound into loops and chains • In motion because of thermal energy • Incompressible • Fluid (changes shape easily)
Solid • Molecules bound into orderly structure • In motion because of thermal energy • Incompressible • Cannot change shape
Ice bomb Generally: • Liquids denser than gas • Solids denser than liquids Not water! Ice floats = life possible on earth! Water most dense at 4 C
Ice water Phase equilibrium • Multiple phases exist at one temperature • Molecules leave and enter phases at equal rates • Ice / liquid water: 0 °C • Liquid water / steam: 100 °C
Phase transitions • Transformation from one phase to another • Absorbs/releases latent heat (energy in bonds) • Represents a change in order
Clicker question Which is colder: the glass with lots of ice or the glass with a little ice? (A) a little ice (B) lots of ice (C) both are the same temperature
Latent Heat E. gas C. liquid A. solid • Latent heat of fusion / melting: 330,000 J/kg • Latent heat of vaporization / evaporation: 2,300,000 J/kg
Evaporation Evaporation A cooling process steam Latent heat liquid Your body regulates its temperature this way!
Condensation – cooking with steam! steam Latent heat liquid
Clicker question Orange growers in Florida spray their trees with water when they expect a freeze. First, figure out why this might work. Then, pick the true statement: (A) This trick works only for a cold snap. (B) This trick works even for an extended freeze. (C) This trick only works with warm water.
sublimation condensation freezing melting evaporation ice steam liquid water deposition
CO2 balloon Deposition– frost on your windows Frost in the freezer
Dry ice Sublimation– freezer burn
landing rate Relative Humidity = leaving rate Lower temperature favors condensation and increased relative humidity
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)
Clicker question Why can you see your breath on a cold day? (A) When it’s cold, water vapor can absorb latent heat and condense. (B) Lower temperature favors high relative humidity and condensation. (C) Lower temperature favors low relative humidity and condensation. (D) Cold temperatures make the water vapor in your breath more dense and easier to see.
Boiling water • Pressure of steam below 100 C < atmospheric pressure (14 lb / in2) • Steam bubble below 100 C is crushed! T<100 C
Boiling water Boiling water Boiling water in vacuum • Pressure of steam at 100 C = atmospheric pressure (14 lb / in2) • Steam bubble above 100 C can grow through evaporation T=100 C
Ice water and salt Dissolving salt in water • Lowers the vapor pressure (increases the boiling temperature) • Lowers the melting temperature Solvation shells
For next class: Read Section 7.3 See you next class!