When pulling the plunger the volume inside a syringe increases and as a result there is a decrease of pressure inside of it which gas law is emphasized in this example?

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»

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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.

Boyle’s law is a gas law that describes the relationship between the pressure and volume of gas for a mass and temperature. This law is the mechanism by which the human respiratory system functions. Boyle’s law is equivalent to PV = K (P is pressure, V is volume, K is a constant), or one may state that pressure is inversely proportional to the volume.[1]

The lungs do not follow Boyle’s law at all volumes.  In a resting state with a normal tidal volume, when the alveoli are not collapsed nor are the lungs at maximal capacity, the lungs follow proportional changes of volume and pressure in accordance with Boyle’s law. At low lung volumes, it takes a large pressure change to make small changes in the volume (low compliance of lung tissue). At high volumes within the lung, it takes a more negative pressure to expand the tissue, once again not in compliance with a direct relationship as Boyle’s law dictates. At low and high volumes, the lung has low compliance meaning that the ability of the tissue to expand or its elasticity decreases (compliance = [change in volume]/[change in pressure]).[2]

The primary organ system involved in the usage of Boyle’s law is the respiratory system. The human body brings air into the lungs by negative pressure. At baseline, the thoracic cavity is in static equilibrium with an intrapleural pressure near -5 cmH2O. During inspiration, there is a contraction of inspiratory muscles (diaphragm, external intercostal muscles; additional muscles such as the scalene and sternocleidomastoid can take part under specific circumstances) that increases intrathoracic volume. Due to the combined motion of the lungs and the chest wall, the lungs will begin to expand as the thorax expands during inspiration. According to Boyle’s law, as the volume increases, the pressure must decrease; therefore, as the intrapleural volume increases, the intrapleural pressure decreases to about -8 cm H2O occurs at end inspiration.[2]

At baseline (rest), the alveolar pressure is equal to the atmospheric pressure (0 cm H2O), and during inspiration, this pressure will go to -1cmH2O as the volume expands within the alveoli. When the alveolar pressure drops below the atmospheric pressure, air will flow into the lungs for gas exchange.[2]

When the inspiratory muscles relax, the volume within the thorax will decrease; thus, the pressure increases and forces out alveolar air back into the atmosphere. With inspiration: lung volume increases, intrapleural pressure decreases. With expiration: lung volume decreases, intrapleural pressure increases.[2]

Intrapleural pressure is the term for pressure within the intrapleural space; alveolar pressure is pressure within the alveoli. As both the intrapleural and alveolar pressures become increasingly negative due to the expansion of the chest cavity during inspiration, air from the atmosphere flows into the lungs, which allows the lung volume to increase and participate in gas exchange.[3]

Testing related to the mechanism that Boyle’s law works can be applied to the volume within the lung and equations to describe how much air is moving.

The minute ventilation, calculated as the product of tidal volume and respiratory rate, essentially is how much air is inhaled every minute. These two factors control ventilation, which directly depends on the thoracic cavity volume expanding and the decrease in pressure within the intrapleural space and alveoli, allowing the lungs to fill with air, producing the tidal volume. If there is an adequate tidal volume, a normal respiratory rate will ensure. If the tidal volume is insufficient, there will be a compensatory increase in the respiratory rate in an attempt to maintain normal minute ventilation.[4] 

Minute alveolar ventilation is an equation that also depends on Boyle’s law and the inverse relationship of pressure and volume of the thoracic cavity. Alveolar ventilation is the amount of air that reaches the alveoli for gas exchange in each breath; calculated by subtracting the dead space from the tidal volume and then multiplying by the frequency of ventilation.[4]

With a pneumothorax or a hemothorax, there is increased pressure within the intrapleural space. Because of this increased pressure, it moves the resting state of about -5 cmH2O to a higher value depending on the degree of disease. As this occurs, it would take a much more significant expansion of the thoracic cavity to create a negative pressure to bring air in from the atmosphere. In a tension pneumothorax, the pressure in the pleural space continually raises the intrapleural pressure, thus decreasing the volume in the lungs. Tension pneumothorax can generate enough pressure to cause a mediastinal shift which eventually interferes with the venous return to the right side of the heart and cardiovascular demise.[5][6][7]

At birth, newborns are born with no air within their alveoli; thus, the volume is zero. The compliance (elasticity of lung tissue) is low at birth. Therefore, the effort to create negative intrapleural pressure during the initial breaths is high; however, the lungs fill with air and become more compliant with successive breaths. As the lungs become more compliant, the newborn's lungs will follow Boyle's law of the inverse relationship of pressure and volume.[2]

Pneumothorax is a clinical condition that can either be primary (typically from trauma) or secondary (patient has a predisposing condition such as COPD). Boyle's law dictates how air draws into the lungs. As the intrathoracic pressure becomes increasingly negative, the intra-alveolar pressure decreases below atmospheric pressure, causing air to flow into the lungs. In a pneumothorax, there is increased pressure within the intrapleural space, thus causing the need for an increased force to create enough negative pressure for air to come into the lungs.[5][6][7]

Boyle's law also applies when using a medical syringe. When the cylinder on the syringe is empty, it is said to be in a neutral state as there is no air in the syringe. As one pulls back on the plunger, the volume in the cylinder increases, therefore by Boyle's law, the pressure decreases. The liquid is thus drawn into the cylinder to balance the pressure within the syringe and outside of the syringe.

SCUBA divers must be cognizant of Boyle's law as they descend and ascend to great depths. As a diver descends in the water, the pressure on the person's lungs increases, and therefore according to Boyle's law, the volume of air inside the lungs must decrease. As the diver ascends in the water and the pressure on the thoracic cage decreases, the volume of air increases.[3] It is important to exhale steadily to release the volume of the gas; if this does not occur, the diver can experience pulmonary barotrauma, which is overexpansion and alveolar rupture. The diver may have a pneumothorax (chest pain, dyspnea, unilateral decreased breath sounds) or pneumomediastinum (neck pain, pleuritic chest pain, dyspnea, coughing; there may be subcutaneous emphysema causing a crepitation on palpation).[8]

Review Questions

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Mortola JP. How to breathe? Respiratory mechanics and breathing pattern. Respir Physiol Neurobiol. 2019 Mar;261:48-54. [PubMed: 30605732]

Conkin J, Abercromby AF, Dervay JP, Feiveson AH, Gernhardt ML, Norcross JR, Ploutz-Snyder R, Wessel JH. Hypobaric Decompression Sickness Treatment Model. Aerosp Med Hum Perform. 2015 Jun;86(6):508-17. [PubMed: 26099121]

Tantucci C, Bottone D, Borghesi A, Guerini M, Quadri F, Pini L. Methods for Measuring Lung Volumes: Is There a Better One? Respiration. 2016;91(4):273-80. [PubMed: 26982496]

Imran JB, Eastman AL. Pneumothorax. JAMA. 2017 Sep 12;318(10):974. [PubMed: 28898380]

Swierzy M, Helmig M, Ismail M, Rückert J, Walles T, Neudecker J. [Pneumothorax]. Zentralbl Chir. 2014 Sep;139 Suppl 1:S69-86; quiz S87. [PubMed: 25264729]

Arshad H, Young M, Adurty R, Singh AC. Acute Pneumothorax. Crit Care Nurs Q. 2016 Apr-Jun;39(2):176-89. [PubMed: 26919678]

Walker, III JR, Hexdall EJ, Murphy-Lavoie HM. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 10, 2022. Diving Gas Embolism. [PubMed: 29493946]