Change of Phase (state) SolidLiquidGas SolidXmeltingsublimation LiquidfreezingXevaporation Gas “anti- sublimation” deposition condensationX

Demo Put ice in a beaker on a stirring hot plate Using Logger Pro, begin recording the temperature of the substance in the beaker – a time scale of 3000 seconds might be good. (heating ice demo)

Evaporation When evaporation occurs from a liquid, like water, particles of the substance (like the H20 molecules) become part of the gas above the liquid. What happens to the temperature of the liquid, assuming the liquid is otherwise isolated from the environment ? Why? Do the evaporation pair learning exercise if you have not already done so

Evaporation When a liquid changes to a gas, it is called evaporation when the change occurs only near the surface of the liquid. Recall that temperature is related to the average velocity of the molecules. The fastest of the molecules have enough speed to escape the fluid, or to knock other molecules out. The remaining molecules have a lower average kinetic energy - making evaporation a cooling process. Then why is the water vapor not hotter than the liquid?

Evaporation Molecules are always leaving the liquid (evaporating) and returning from the gas to the liquid (condensing). If you put room temperature water in a super insulated container with dry air, also at room temperature, and seal the container, what will happen to the temperature? Why?

Condensation When particles of gas, like H2O molecules in air, condense from a gas state into a liquid, like into liquid water, they become part of the liquid. What happens to the temperature of the liquid, assuming the liquid is otherwise isolated from it’s environment ? Why ?

Condensation Condensation happens when a gas changes to a liquid. It is a heating process because the molecules leaving the gas are accelerated as they become attracted to the liquid surface – potential energy becomes random kinetic energy.

Boiling When a liquid changes to a gas and the change happens throughout the liquid, it is called boiling. Bubbles form in the water when the vapor pressure equals that of the water pressure. Why do the bubbles rise? Would you expect the size of the bubbles to increase or decrease as they rise?

Boiling: can think of as internal evaporation, a cooling process.

Boiling is a type of internal evaporation (a cooling process). Within a liquid, a tiny vapor bubble of the substance forms (typically around pieces of dust or other impurities). If the pressure of the surrounding liquid is greater than the internal pressure of the bubble, the bubble is crushed. When the liquid is heated, the internal pressure of the vapor bubble increases allowing it to rise to the surface instead of being crushed. While a liquid is boiling, the rate of heat transfer into the liquid is the same as the rate at which heat is carried away by the evaporating water. Boiling: can think of as internal evaporation, a cooling process.

Evaporation vs. boiling Bubbles can form when the vapor pressure inside the bubbles is large enough to withstand the pressure of the water. Air pressure pushes down on the water, making the water pressure higher. At high altitude, air pressure is lower, so bubbles can form at lower temperature. This means that food cooked using boiling water will take longer at high altitude.

Vapor Pressure Liquid Water Water Vapor Air is removed, container then plugged Vapor Pressure is the pressure of the vapor above the liquid after sufficient time

Vapor Pressure Liquid Water Water Vapor Air is removed, container then plugged Vapor Pressure is the pressure of the vapor above the liquid after sufficient time

Boiling The saturated vapor pressure of the fluid at its temperature must be higher than the pressure of its surroundings Demo – water in watch glass & pumped Simulation:

Phase diagram for water

Triple point of water Video demo – elatedhttp:// elated Supercritical fluid –

Freeze drying If we lower the pressure enough, we can boil water even at room temperature. Remember, boiling is a cooling process. If we keep lowering the pressure further, we can reach a state where the boiling water is cold enough to freeze! This is what happens in the process of freeze drying.

Regelation With enough pressure, solid crystals can be crushed into liquid. This process is called regelation.

Pressure Macroscopically: Force on the substance is equal to the weight of the material above it in height. For a fluid, the walls of the container also push in and contribute to the pressure. Microscopically: Pressure is caused by molecular impacts.

Sublimation CO2, commonly called dry ice, typically goes from a solid directly to a gas at room temperature. This process is known as sublimation.

Carbon dioxide phase diagram

Sublimation Comets also undergo sublimation when they get near enough to the sun.

Change of state - energy viewpoint Recall graph of heating ice Latent heat of transformation Latent heat of vaporization For water kJ/kg (or 540 cal/gram) Latent heat of fusion For water kJ/kg (or 80 cal/gram)

It takes 80 calories to melt 1g of ice into 1 g liquid water and an additional 540 calories to convert 1g of liquid water 1g of steam Thats 80 cal cal cal = 720 calories It takes an additional 100 calories to warm 1g of liquid water from 0 to 100 degrees Celcius

Activities Can you boil water in the inner container of a double boiler? Why or why not? Do the “Ice Melt” lab

Geysers Geothermal heating (at high pressure) causes water to boil at high temperature. Bubble form and start to float, expanding as they rise. This tends to lower the local pressure, so more water can now start to boil Do the Geyser demo?

Humidity Saturation level Dependence on temperature Absolute and relative humidity

Weather (some aspects) Fog and clouds Rain, snow and hail Weather east and west of mountains of West Coast, North America

Weather Water evaporates from the ocean, condenses into clouds and fog, rains or snows down onto the land, and then returns to the sea via rivers, streams and underground seepage or passageways.

Weather Clouds and fog form when water vapor condenses in the air to form droplets. When the droplets gain enough mass they become heavy enough to fall.

Weather If the temperature is low enough for raindrops to freeze, and the wind is strong enough to lift the frozen drops up so that they build up layers of ice, a hailstorm results.

Weather High mountains can obstruct clouds, forming a rain shadow inland from the coast. sonoma.edu