PURPOSE: This lab
is used to introduce the concept of boiling
EQUIPMENT: The following equipment is needed
2. Temperature probe (either direct connect or standard )
3. Flat bottom round flask (500 mL) MUST BE PYREX
4. One hole stopper to fit flask
5. Ring stand and clamps
6. Plastic waste basin
8. Ring stand and clamps
9. Safety goggles and lab aprons
10. Source of heat (Bunsen burner or hot plate)
Equipment setup: make
a setup like the following diagram.
Computer Setup: Perform the following steps in setting up the ULI and computer
1. Connect the ULI and temperature probe
2. Open the MacTemp program
3. Load the correct temperature probe calibration for your computer
4. Make the following changes in the MacTemp setup
a. Under the Collect menu, /Select inputs change to Port 1 input only
b. Under the Display menu/Units , change to minutes as the unit for time
c. Change the range of the time to 0 to 25 minutes
and the range of the temperature to 25 to 105 degrees Celsius
PART 1. BOILING WHILE SUPPLYING CONSTANT HEAT
1. Fill the 500 mL flask approximately 1/2 full with hot tap water
2. Suspend the temperature probe from the upper clamp and place it into the water so that it is approximately 1-2 cm into the water
3. Make sure that the temperature readings are responding
4. Allow the temperature reading to stabilize
5. Click on the START button to start recording the temperatures
6. Turn on your hot plate to a high setting
7. Allow the computer to record the temperatures until the water has started boiling
8. Let the water to boil while graphing temperature for about 4 additional minutes. Note when vigorous boiling started to occur
9. While the water is heating, get ready to do
part 2 of the experiment
PART 2. BOILING WITHOUT A HEAT SOURCE -
1. Remove the temperature probe from the boiling water ( allow the water to continue to boil)
2. Carefully place the temperature probe into a one hole stopper
3. Have a plastic basin and container of ice ready
4. Stop the previous data collection (if you have not already done so)
5. Transfer your data from Data A to Data B
6. Hide Data A
7. Change the range for the time from O to 10
DO THE FOLLOWING STEPS ONLY WITH THE ASSISTANCE OF YOUR INSTRUCTOR
MAKE SURE YOU ARE WEARING YOUR GOGGLES AND LAB APRONS
8. Carefully remove the hot plate
9. Your instructor will place your stopper/temperature probe on the flask and invert it
10. Immediately Click on the Start button and make sure it is recording
NOTE : your equipment will now look like this
11. Place ice on top of the flask and rub it around the areas of the flask containing air
12. Continue rubbing ice on the flask until the computer stops taking data. Make sure you pay attentions to what occurs in the flask.
13. Show your graph so that it shows both data
A and data B
Discuss the following questions with your lab
partner. Look at your graphs to support your answers.
1. When you were performing part one of this experiment,
the temperature of the water while boiling remained constant even though
we were adding heat energy. What happened to the heat that was being added?
2. During part two of the experiment, the water
starting boiling while rubbing ice on top of the flask. What did that ice
do to "cause" the water in the flask to boil? Why did the water
start to boil?
3. What happened to the temperature of the boiling
water in Part 2 of the experiment? Why?
Rationale: This experiment is used as an introduction
to the process of boiling. It gives students a visual representation of
the concept of boiling as a cooling process. It also emphasizes the importance
of atmospheric pressure in considering the boiling process.
Data/Graphs: The following graphs are representative
of expected graphs
If the students believe that the water temperature is decreasing for other reasons than the boiling process (which it is to a small extent) have them reheat their water to boiling then insert a temperature probe. Then have them remove the heat source and follow the temperature over approximately the same time period without cooling the flask with ice. A graph of that cooling process would look like this:
Superimposing these graphs (which can be done
within the MacTemp program if you have moved Data A to Data B) makes it
easy to see that the temperature drops much more rapidly when ice is applied.
Safety notes: This experiment can be used with upper level responsible students. If you do let the students do the complete experiment, please be aware of the following:
The danger of the flask breaking (even though I have never had it occur) is definitely present. Make sure the flask is Pyrex, thick walled and free of scratches. Still Extreme care is needed.
If a Bunsen burner is used as a heat source, you will need to support the flask with a ring and wire gauze. After removing the Bunsen burner, the ring and wire gauze must be lowered. This ring will be hot and should be handled only with gloves or a wet rag.
The main danger is of the liquid superheating (which I have had occur) Allowing the water to boil several minutes should eliminate this problem but some students forget to keep the water boiling. They then reheat it as fast as possible. Giving the flask a swirl before stoppering it should also help. If the water is super heated, it will blow off the stopper/temperature probe and force hot boiling water all over you and or your students. To avoid this problem I always do this step myself.
Possible additional methods: If doing this experiment
as a demonstration, Part two may be set up with two probes connected to
the computer with two setups. After boiling, treat both setups exactly
the same except one setup has ice applied while the other does not. The
graphs of both are then collected and displayed simultaneously.