## The Gas Laws in Action

Objective

Gases are a form of matter with much lower density than solids of liquids. In this lab you will observe how the volume of a gas in a balloon changes as the pressure of the air around it is reduced at constant temperature. You will also observe the change in volume of a gas at constant pressure as it is warmed and cooled.

The procedure of this lab is step-by-step. Your analysis of your observations and your calculations and your conclusions should be step-by-step, too. I have written the procedure. It is up to you to put the rest into an orderly whole. I ask questions in the procedure to guide your analysis. I also provide some generalized notes about how to put together your lab report. The key to writing a good lab report is taking good notes while you are in the lab.

Materials

no-hole stopper to fit
safety goggles
lab notebook & pen
ice & water
thermometer
ruler
vacuum tubing
sink aspirator
ring stand
clamps
3 small balloons
400 mL beaker
centrally located: 2 500 mL beakers
electrically heated to 35°C - 40°C
(magnetically stirred)

Background

You will look at two of the basic proportional relationships introduced at the beginning of your lessons on the ideal gas laws. The first, Boyle’s Law, relates pressure and volume at constant temperature:

```
```
``` P · V = k  or    P1 · V1 = P2 · V2
```
```
```

As you know, this relationship represents an inverse proportionality. When pressure is increased, volume must decrease. When pressure is decreased, volume must increase. This is all fine and good to imagine but it is much easier to understand if you can see it for yourself. Science is more clear to the mind when it is clear to the senses.

The second relationship you will explore is the one commonly called Charles’s Law:

```
```
``` V              V1    V2
```
``` — = k    or    —  =  —
```
``` T              T1    T2
```
```
```

Clearly, this is a direct proportionality and says that as temperature rises, so must volume. Similarly, you can say that as temperature falls, the volume must decrease. Keep in mind that this proportionality is only accurate when the pressure of the gas remains constant.

Safety

Tie back loose hair and clothing. If you are not wearing shoes with closed toes and heels then you should be! The aspirator you will use in this lab provides a powerful source of suction and can reduce pressure inside sealed vessels to a remarkable degree. The sealed vessel you will use is designed for the purpose of supporting the immense pressure of the Earth’s atmosphere when the air inside is drawn out but that does not mean there is no danger of implosion. For this reason you must wear safety goggles at all times! No chemicals will be used in this lab but be careful anyway.

Procedure

1. Part One: Boyle’s Law
1. Inflate a balloon to a diameter of about 2 cm and tie off. This balloon must be very small in order to fit through the opening in the vacuum flask.
2. I will make one observation for you: balloons contain gas at the same pressure as the pressure pushing on them from the outside. The balloons you inflate will be at atmospheric pressure as long as they are in the atmosphere. If there is a barometer in the room find out what the atmospheric pressure is today and record this data. Convert the reading you record to atmospheres.
3. Measure as accurately as you can the diameter of the balloon. Put it into the filter flask and stopper the flask tightly.

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1. I have already connected the flask to an aspirator. When the flask is stoppered tightly enough and you turn the water on full blast in the faucet it draws air through the tube, evacuating the flask of air. Turn the faucet on full blast.
2. It may be a few minutes before you notice anything happening. If nothing does happen for about 5 or 6 min, check the connections: ask for help with this.
3. In all of this I am assuming that you are using your lab notebook as described in the candle observation lab: write down the name of the lab and the materials; take notes on each step of the procedure as you perform it. Write down observations of the unexpected and the obvious. In short, at this point please consider taking the necessary steps to address the problem that you have not done any of this yet. Or did you read ahead? Good, then you are already doing this. By the way, are you using a pen?
4. Write down in your lab notebook what you see once the pressure inside the flask goes down. What happens to the balloon? What was the pressure inside the balloon before you started the aspirator? Is the pressure inside the balloon greater or less now that the aspirator has been working for a few minutes? How do you know?
1. Try to measure the new size of the balloon inside the flask using the ruler. Treat all measured diameters as the diameters of perfect spheres (a decent enough approximation). Find the radius and use V = (4/3)πr3 to find the respective volumes. Use the same units for each calculation and show the correct units for the result. If the balloon is more cylidrical than sperical use V = πr2h.
2. In your lab notebook use Boyle’s Law to estimate the pressure inside the balloon according to your measurement of its volume before and after subjecting it to reduced pressure. Assume that the pressure in the balloon was 1 atm before you put it in the flask. Alternately, if there is a barometer in the room, find out what the atmospheric pressure actually is and convert that number to atmospheres. Use that number in the proportionality calculation.
1. Part Two: Charles’s Law
1. Fill the 400 mL beaker just over half full with ice. Add water until the beaker is about ¾ full. Swirl or stir to mix thoroughly. If at any time the ice seems to be melting away, replenish it.
2. Use a thermometer to measure the temperature of the ice bath. Note this temperature in °C and convert that value to K (K = °C + 273). How does the water feel now that you have added ice to it? Does condensation form on the outside of the beaker? Notice anything else?
3. Inflate two balloons and measure their diameters. Make them both the same size (about 2.5"). Calculate their volumes.
4. Put a balloon into the ice bath and hold it down.
5. Keep the balloon submerged for a few minutes then pull it out. Compare it to the other balloon that you have not subjected to a tortuously cold ice bath. Is it sensibly smaller or larger than the reference balloon? What did you expect to happen? Measure the diameter and calculate the corresponding volume.
6. Put the cold balloon back in the ice bath so that it stays at the temperature you measured above.
7. One lab partner should now take the other balloon over to one of the warm water baths maintained by your teacher. This bath will be somewhere between 35°C and 40°C. For reference, 37°C is human body temperature (check if that’s true by converting 98.6°F to °C). This will not be hot enough to hurt you. It will, however, be nearly 40° warmer than the ice bath. Let the balloon warm up fully to the temperature of the warm water (hold it submerged for about 3 or 4 minutes). Note the exact temperature of the warm water bath in your notebook.
8. Compare the balloon to the one being held in the ice bath. Measure the diameter of the balloon you just heated and compare its volume to the volume of the balloon you chilled. Now you have direct measurments of all four quantities in the second Charles’s Law formula given in the background section. The volume and temperature of the cold balloon and the volume and temperature of the warm balloon. Are the balloons different sizes? How well do your data fit the model? Did you remember to do your calculations using K temperature units?
9. Compare the V & T data collected for the heated and cooled balloons to their original V & T from before heating/cooling. Note: the T from before heating/cooling was the same as the room temperature. How do all of these data fit the Charles’s Law mathematical model?

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Lab Report

Write a neat, grammatically correct, stylistically sophisticated lab report. Use the first person (I, me, my), the present tense, and the active voice: “I held the balloon below the surface of the ice water,” not “The balloon was held below the surface of the ice water.” Please write in 12 pt Times, 1.5-spaced, typed; written: write neatly! Tell me what you did, why you did it and what you found out.

Your observations in this lab were meant to lead you to a better understanding of what the mathematical models, represented by the ideal gas laws, look like in ‘real life’. Relate the mathematical models (the formulas) to what you found in the course of the observations made during this lab activity. Science proceeds by observing nature, recording observations, building a model (in this case mathematical), and checking that model against nature. It is important to connect your imaginations about nature (the model) against the real thing. When the model doesn’t match up with what you observe, then the model must be modified. That is, unless you can think of some reason why your observations were not exact enough to verify or disprove the model. Organize your report using the following outline:

Introduction: This is, ironically, the last section you will write. In this section you should briefly summarize (50 words or less) the results of your observations. Just the results, nothing else. What did you find out? What do you know now that you did not know before? How does it relate to the conceptual/factual learning you are doing in class?

Materials: Copy (yes, copy) the list of materials from the procedure I wrote. Well, not exactly copy. Please do not write down anything you did not use. Also, add any items you used but which were not listed on the procedure that I wrote. This will help me to make the lab better.

Procedure: In your own words (this is very important) in your own, your very own words, write down what you did. It does not have to be a lot of writing but it does have to give a reasonably complete picture of your activity in the lab. Use the procedure I wrote and your own labs notes to create this picture. Think of it as a story you are writing about what you did in the lab. Keep it short and to the point and for this lab it should not require more than about 450 - 500 words. No comments on results are expected or desired in this section. Observations are welcome and should be reported here.

Data: This section is where you should present all numerical data you collected. For this lab you measured the diameters of several balloons under different conditions. Create a table showing the conditions under which each diameter was measured. Specifically note the temperature of each measurement, and the pressure, if possible. For each balloon I asked you to calculate the volume before and after changes. Include this in the data table as well. If there was a barometer in the room and you figured out how to read it, include the reading in this section.

Results: This is where it all comes together. You gave your observations in the procedure section, you provided your data in the data section. This is where you write about what it means and what you learned.

The following is a list of the observations, measurements, and calculations that I expect you to write about in this report:

1. Observations
1. The sizes of the balloons you inflated.
2. What happens to the balloon inside the filter flask after you turn on the aspirator? To the cooled and heated balloons?
3. The visual comparison of all before/after pairs.
4. All physical changes and how things feel to you.
2. Measurements
1. The atmospheric pressure as found on the barometer in the room (if there is one)
2. The diameter of each balloon at room temp/pressure and the relevant temperature and pressure before you do anything to it
3. The diameter of the balloon inside the evacuated filter flask and its temperature
4. The diameter of the balloon you cooled in the ice bath and its temperature and pressure (room pressure)
5. The diameter of the balloon you heated and its temperature and pressure (room pressure)
3. Calculations
1. The volume of each balloon, based on the diameter you measured and the assumption that it is roughly spherical (V = (4/3)πr3) or cylindrical (V = πr2h). You should have this volume calculation for each balloon before and after each change.
2. The pressure inside the filter flask based on your measurement of its volume, the volume of the unchanged balloon, and the atmospheric pressure in the room. This is just a Boyle’s Law calculation.
3. The predicted volume of each balloon when cooled or when heated. Compare the predicted volume (found using the initial volume, the room temperature, and the cold/hot water bath temperatures) to the volume you measured. Are they close enough that you could say that Charles’s Law is confirmed? Or was there too much uncertainty in your measurements and calculation of volumes?

If you have already written your introduction (remember, I said to write it last) then go back and look at it after you finish writing the rest of the report. Re-write it if it seems necessary.

Last updated: Jun 30, 2006      Keller Home  |   Home