What happens to the pressure inside the syringe and volume of the balloon when the plunger of the syringe is pulled?

September 2013

Emil Lozanov, Wayne State University, Detroit MI

The small balloon in a syringe for demonstrating Boyle’s law can be substituted with an air-containing, cut and knotted finger of an exam glove. The glove’s “finger balloon” is pushed into the open end of a 60-mL plastic syringe, and  adhesive tape is attached to its opening to work as a removable stopper, Fig. 1. Decreasing the air pressure in a syringe is achieved by moving the plunger.

Figure 1.

Figure 2.

Figure 3.

The position of the plunger (decreased pressure, increased balloon volume) can be secured by making a hole in the plunger for a screw/nail, and the syringe can be handed to students for closer observation, Fig. 2.

Taking off the tape will increase air pressure on the balloon, decreasing its volume, Fig. 3. It's interesting because people usually do the experiment in a closed syringe, increasing the air pressure by pushing down the plunger. In this case, by taking off the tape, air rushes inside, and the volume of the balloon decreases.

A 60-mL syringe with a closed end (heated, softened and pressed with pliers) can also be used to demonstrate Charles’s law. If 40 mL of air is heated by a hair dryer, the volume of air will increase by 5-6 mL. At room temperature, the plunger of the syringe returns to the starting position — the volume decreases. The change will be faster if the syringe is cooled with running tap water or an ice bath with dissolved table salt. The volume changes are even faster if oil is applied on the rubber part of the plunger. In addition, there is an opportunity to discuss condensation and pressure change when the water vapour in hot air condenses to form droplets inside the syringe.

25th Dec 2019 @ 4 min read

Boyle's law is a pressure versus volume relationship. The law was discovered by Robert Boyle in the 17th century. It states the pressure of a fixed amount of a gas is inversely proportional to its volume at a constant temperature. The law can be empirically proven. The article discusses an experimental method to verify the law using a syringe.

Experiment: Sealed syringe

The experiment is very simple. It can be performed at home. When the tip of a syringe is sealed with a cap, the air inside the syringe is isolated from the atmosphere. This will fix the amount of the gas. The weights (books) are added upon the plunger of the syringe. It will push the plunger downwards; in other words, the air in the syringe is compressed. By recording the weights of the books added and the volume reading from the syringe, we can establish the pressure-volume relationship.

Objective

To verify Boyle's law and to plot the pressure-volume graph

Materials

1. A 140 mL disposable syringe
2. A seal cap
3. Two wooden blocks: one with the central hole on which the syringe will be mounted and the other which will be attached to the plunger
4. Books that can comfortably place on the wooden block
5. A lubricant
6. A wooden split or tongue depressor

Nomenclature

1. Vi is the volume reading.
2. wi is the weight on each book.
3. w0 is the initial weight, which is the sum of the weight of the wooden piece resting on the plunger and the weight of the plunger.
4. Wi is the total weight on the air inside the syringe.

Procedure

1. Take the syringe and paste a thin layer of the lubricant to the rubber gasket of it with the help of a wooden split or tongue depressor. This will reduce friction.
2. Pull the plunger of the syringe upwards—around 110 mL.
3. Now, attach the seal cap to the syringe.
4. When a small amount of downward force is applied to the plunger, it should revert to the original position. If not, the more lubrication is necessary or the seal cap is not properly attached.
5. Mount the tip of the syringe to the cavity of the wooden block and place it in the upside-down position as shown in the above figure.
6. Fix the other block to the plunger of the syringe such that the syringe is perpendicular to the blocks.
7. Measure the initial volume reading.
8. Place a book on the wooden piece and record the volume reading.
9. Repeat the previous step for two books, three books, four books, and five books.
10. Remove all the books and weigh each. Also, weigh the wooden block with the plunger; it will give w0.
11. Reset the apparatus. Repeat all the above steps twice. Take the average of all three sets.

Precautions

1. The proper lubrication is necessary to eliminate friction.
2. The end of the syringe should tightly fix by a sealed cap. Otherwise, the experiment will fail.
3. The syringe must be properly fixed, so it can firmly withstand the weights.

Observation

The initial weight (w0) is 92 g.

The total weight is

The observation table is as follows:

Calculation

The pressure on the air inside the syringe is the pressure exerted by the weights plus atmospheric pressure.

The pressure exerted by the weights is the force exerted by the weights divided the inner area of the syringe.

Now, Force (Fw) is mass (Wi) times acceleration (a).

Here, r is the inner radius of the syringe, which can be measured; r = 0.005 m. a is the acceleration due to gravity; a = 9.81 m s−2.

For Wi = 92 g,

Assume atmospheric pressure (Patm) as 101.325 kPa.

Similarly, we can calculate the total pressure for the rest.

The calculation table is as follows:

We have to plot the graph of Pi vs Vi and PiVi vs Vi.

Results

The Pressure vs volume graph is as follows:

The pressure-volume vs volume graph is as follows:

Conclusion

The PV curve from the above figure is satisfactory. As the pressure of the air increases, its volume decreases. The air obeys Boyle's law. Also, the product of pressure and volume approximately constant and its value is independent of volume or pressure.

Also, check a laboratory method: To verify Boyle's law»

Associated articles

Copy Article Cite

Thanks for your response!

Zephaniah Lapa
07th Jun 2021

Very helpful, Thankyou so much..

Henry
30th Jun 2020

Awesome! work, i like your examples, thank you sir.