What happens to the volume and pressure inside the syringe when pulling the plunger to draw fluid?

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

Experimental diagram

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:

Observation tableNo. of booksVolume reading in mL (Vi)Average (Vi)Weight in g (wi)Total weight in gSet 1Set 2Set 3

0	102	100	104	102	0	92
1	60	58	62	62	505	597
2	50	56	44	50	503	1100
3	32	38	34	34	503	1603
4	26	32	32	30	499	2102
5	24	28	26	26	501	2603

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:

Calculation tableNo. of booksPw in kPaPi in kPaVi in mLPiVi

0	11.5	112.8	102	11500
1	74.6	175.9	62	13100
2	137.4	238.7	50	11900
3	200.2	301.5	34	10200
4	262.5	363.8	30	10900
5	325.1	426.4	26	11100

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

Results

The Pressure vs volume graph is as follows:

Pressure vs volume

The pressure-volume vs volume graph is as follows:

Pressure-volume vs volume

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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"To Demonstrate Boyle's Law by Syringe Experiment" ChemistryGod, 25th Dec 2019, //chemistrygod.com/demonstrate-boyle-law

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.

Not only does the water in your syringe appear to be boiling, it is boiling.

Living as we do at typical atmospheric pressures, we tend to think that water has to be hot to boil. But the transition from liquid to gas can occur not just as the result of increased temperature, but also as the result of decreased pressure.

Pulling on the plunger reduces the pressure on the gases inside the syringe by increasing the volume—a relationship given by Boyle’s Law: For a gas in an enclosed space at a constant temperature, volume and pressure vary inversely. In other words, doubling the volume halves the pressure. 

Tap water has air dissolved in it. When you reduce the pressure in the syringe by pulling out the plunger, the dissolved air comes out of solution and forms an air pocket at the tip of the syringe. When you slowly allow the plunger to slide back in, the air that has come out of solution stays out of solution. That’s why there may seem to be more air in the syringe than when you started.

But something else happens when you pull back on the syringe: Under reduced pressure, water changes from liquid to gas. Bubbles of water vapor form inside the liquid, and the water in the syringe boils at room temperature.

In a clean liquid, it’s not easy for small bubbles to form, or nucleate. However, when you pull out the plunger and allow it to snap back in, you create many tiny “seed” bubbles throughout the water. The next time you pull back the plunger, boiling happens more easily, thanks to the nucleation sites provided by these seed bubbles.

One way to understand the importance of seed bubbles is to remember the last time you tried to blow up a balloon. At first, a balloon can be difficult to get started. But once started, it’s easier to inflate. The same rule applies to bubbles of water vapor forming in water. If you reduce the pressure and temperature even further, either by using a vacuum pump or by traveling to the surface of Mars, it’s possible to have water both boil and freeze at the same time. At this combination of low temperature and pressure—0.01 °C and 0.006 atmospheres, also known as the triple point of water—all three phases of water can exist at the same time.