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) Polar molecule Bonds to form different structures (phases)

Gas Mostly independent molecules In motion because of thermal energy Compressible Fluid (changes shape easily) Fills volume Liquid N 2 cannon Water vapor in can

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

Generally: Liquids denser than gas Solids denser than liquids Not water! Ice floats = life possible on earth! Water most dense at 4 C Ice bomb

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 Ice water

Phase transitions Transformation from one phase to another Absorbs/releases latent heat (energy in bonds) Represents a change in order

Example: Phase transition - melting heat

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 of fusion / melting: 330,000 J/kg Latent heat of vaporization / evaporation: 2,300,000 J/kg Latent Heat A. solid C. liquid E. gas

Evaporation A cooling process steam Latent heat liquid Your body regulates its temperature this way! Evaporation

Condensation Condensation – cooking with steam! steam liquid Latent heat

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.

melting freezing evaporation condensation sublimation deposition ice liquid water steam

Deposition Deposition – frost on your windows Frost in the freezer CO 2 balloon

Sublimation Sublimation – freezer burn Dry ice

Lower temperature favors condensation and increased relative humidity Relative Humidity landing rate leaving rate =

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 / in 2 ) Steam bubble below 100 C is crushed! T<100 C

Boiling water Pressure of steam at 100 C = atmospheric pressure (14 lb / in 2 ) Steam bubble above 100 C can grow through evaporation T=100 C Boiling water in vacuum Boiling water

Solvation shells Dissolving salt in water Lowers the vapor pressure (increases the boiling temperature) Lowers the melting temperature Ice water and salt

See you next class! For next class: Read Section 7.3