VAPOR PRESSURE & BOILING POINT
a super demo by the Boom
This is a fun demo that takes a full period and covers the following concepts: Vapor pressure, Boiling Point, Cooling by Evaporation, Liquid to Gas Phase Change, and the Equilibrium between liquid and gas in a closed system at constant temperature.
Ring stand, 2 utility clamps, a 500 ml thick-walled round-bottom flask, two glass bends, two pieces of thick walled rubber tubing (about a meter in length), rubber stoppers for the flask [a solid, a one-hole, and a three-hole (I had to drill my third hole)], a tall glass cylinder, a straight glass tube at least 80 cm long (a 90 deg bend at one end is nice), a meter stick, a 100 ml beaker containing about 30 ml of mercury, an aspirator or other vacuum pump, a thermometer (0 - 110 deg), a pinch clamp, and a burner.
Set up a 500 ml thick-walled round-bottom flask on a ring stand over a burner. Insert a 3-hole rubber stopper therein. One hole gets a thermometer, the other two are for glass bends for attaching rubber tubing.
Using the same ring stand, set up a glass tube at least 80 cm long with one end resting in a 100 ml beaker containing about 30 ml of mercury. Mount a meter stick along side the tube. This will be a manometer for measuring the vapor pressure.
Fill the 500 ml flask about half-full with water and preheat the water to about 50 deg C.
Attach an aspirator or other vacuum pump via rubber tubing to the top of the manometer tube. Start the pump and note how many millimeters the mercury in the glass tube is pushed up above the mercury in the beaker by the atmospheric pressure. Call this reading "atmospheric pressure".
Explain that when there is very little air left in the tube, the atmosphere will push the mercury up until the pressure of the mercury equals the pressure of the atmosphere.
Allow air to enter the manometer by removing the rubber tubing from the pump. (Warning: do not lift the glass tube out of the mercury or the mercury in the tube will flow to the pump. If you are using an aspirator, place a safety beaker under the water flow to catch any mercury that comes through).
Now, attach the manometer to the flask with a rubber tube, and attach the pump to the flask with the other rubber tube.
Explain the following two definitions:
Vapor pressure is the force exerted by evaporating molecules escaping through the surface tension of the liquid.
Boiling point is the temperature at which the vapor pressure equals the atmospheric pressure (or the gas pressure within a closed flask).
Explain now that we shall measure the vapor pressure in the flask by pumping the air out, pinching off the aspirator, and waiting for the system to reach equilibrium, reading the manometer, and obtaining the vapor pressure by subtracting the manometer reading from the atmospheric pressure reading.
Note: with "no air" in the manometer, the mercury goes so many mm high. Then, when vapor molecules from the flask enter the manometer, they will push the mercury down so far. The difference between the two manometer readings is the vapor pressure in mm of Hg.
Start the pump... Oh Oh, What's this, the water is BOILING! WOW! A STATE OF SHOCK!
Now explain what boiling is and how it is possible to boil a liquid by TWO methods: by either raising the vapor pressure to the atmospheric pressure or by lowering the atmospheric pressure to the vapor pressure (say by a pump).
Back to the vapor pressure measurements. Pump the system until boiling occurs, then pinch off the rubber tube to the pump, stop the pump and allow the system to reach equilibrium (boiling stops at equilibrium). Read the manometer and calculate the vapor pressure by subtracting the manometer reading form the atmospheric pressure reading.
Heat the system and note how raising the temperature increases the vapor pressure and pushes the mercury further down in the manometer.
Take several readings at different temperatures and note the vapor pressures. Then note the boiling temperature at one atmosphere of pressure when the mercury is pushed all the way down the manometer.
Here's a good place to discuss why it takes so long to boil an egg on a high mountain (At 5000 meters elevation, it takes a half hour or so), and the principle of the pressure cooker.
Now let air back into the system, and heat the water to boiling with the burner. Remove the burner, and note for a minute that the water cools very slowly. Then start the pump, and read off how fast the temperature DROPS as the water is boiling!
Here is a good place to discuss that evaporation is endothermic, and boiling is a cooling process. Give examples of cooling by evaporation, the wind-chill factor, and refrigeration. (Inasmuch as the typical teenager will attack the fridge frequently, he can be told to listen for the sound of boiling liquid around the ice trays when the fridge is running).
Water + 538 cal/g <----> Vapor
GREAT PHASE CHANGE:
To demonstrate the tremendous volume change when water goes from liquid to gas, remove the stopper from the flask and boil the water with the burner. Remove the burner, and place a solid rubber stopper in the flask before the steam stops coming out.
Carefully cool the flask with a damp cloth and note how COOLING makes the water BOIL FASTER! GASP! What's happening?? When the flask is cool enough to handle, remove it from the ringstand and hold it under the cold tap. It really boils now! The steam within is condensing and lowering the pressure inside to boil the water by reduced atmospheric pressure.
Now replace the solid rubber stopper with a one-hole stopper and attach a piece of rubber tubing. Boil the water with the burner, and show that LIVE STEAM is an INVISIBLE GAS by noting that the visible CONDENSATE, which is composed of tiny droplets of LIQUID water, shooting out of the tubing is preceded by a few millimeters of invisible region. Also the vapor inside the flask is invisible.
Place the end of tubing with the steam shooting out into a tall cylinder of cold water. ROAR! BIG NOISE! The bubbles of steam are condensing in the cold water with much rattling. Here, again, you may discuss the big volume change when a gas becomes a liquid, and the water is collapsing onto itself.
For the next mystification, and another illustration of the big volume change, keep the steam tube at the bottom of the cylinder of cold water and remove the burner from the flask. Soon the water will be pushed with much vigor by the atmospheric pressure into the flask.
This demonstration is a blast to do and the kids love it. Good luck!
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