Presentation on theme: "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."— Presentation transcript:
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.
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