# Boyle's Law

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## Objective

SWBAT identify and explain the relationship between gas pressure and gas volume

#### Big Idea

Use this hands-on activity to help students understand the inversely proportional relationship between gas pressure and gasp on

## Getting Started

In this activity students will explore the relationship between grass pressure and gas volume using 50 mL plastic syringes and bathroom scales to measure the force required to change the volume inside the syringe.

Materials

• 50 mL plastic syringes w/o no cap (1 per pair)
• 50 mL syringes with cap (1 per group)
• bathroom scale

Setup

You will need two types of syringes for this lab: uncapped syringes for Part 1 of the lab and capped syringes for Part 2. It is important to check the capped syringes to make sure the caps are tightly screwed on or firmly secured with superglue. Instruct students to wear safety glasses and to always point the syringe with cap down as they push on it. The caps can fly off at very high pressures. The number of groups you have for Part 2 will depend on the number of bathroom scales you have. Thus, you could do Part 2 in larger groups or as a student-assisted demo.

## Engage

10 minutes

To capture students attention, show them an empty 2 L bottle with a cork inserted into the opening. Ask the students to predict what will happen if you jumped onto the bottle if it were laying on its side on the floor. Have them include volume and pressure in their explanations.

Now comes the fun part. Rather than you jumping on the bottle ask for a student volunteer. Place the bottle on its side on the floor making sure to point the opening away from the other students. With lots of dramatic flair and theatricality ask your volunteer to jump on top of the bottle trying to land right in the middle and ask the rest of the class pay attention to the cork and where and what happens.

Once the excitement calms down, ask the some guiding questions:

• Was the bottle empty to start? If not what did it contain? How do you know?
• What is happening in terms of pressure and volume?

## Explore

20 minutes

Introduce the lab:

Write the terms temperature, volume, pressure, and number of molecules on the board.

Let them know that the number of molecules doesn’t change and the gas is assumed to remain at room temperature. They will focus on pressure and volume and how these two variables interact.

For the first part of the activity, students work in pairs with one plastic syringe that does not have a cap on it.

Display the following procedure and ask them to record their observations in their science journals.

• Start the plunger at 50 mL. Cover the tip of the syringe with your fingertip. Be sure to make a good seal.
• Push the plunger until it reads 40 mL.
• Continue to cover the tip with your finger. Apply more pressure until the inside volume reads 30 mL, then 20 mL, and so on.
1. What did you experience when you pushed in the plunger from 40 mL to 30 mL, and then from 30 mL to 20 mL?
2. Are you able to push the plunger all the way in? Explain why or why not.
3. Explain why the number of air molecules in the syringe doesn’t change but the volume does.

## Explain

10 minutes

Once finished, discuss their observations

• What did you observe when you explored the syringe with no cap on it?
• Does the amount of air change when you push in the plunger?
• Can you decrease the volume of the syringe to zero? Why or why not?
• Why does it get harder and harder to depress the plunger as the volume gets smaller and smaller?

Introduce second part of the lab, which they will complete each in their groups of 4 to 5 students.

## Explore

15 minutes

For the second part of this lab,  students work in groups of four or five with a 50 mL syringe that is sealed at the end and a bathroom scale. If you are unable to get enough after the scales for each table group this can be completed with one scale and one syringe it will just take a bit longer to gather all the data.

Procedure.

1. Start with the syringe at 50 mL. Make sure the cap is on tight.
2. Hold the syringe vertically with the tip on top of a bathroom scale.
• One person should depress the plunger by a few milliliters.
• A second person should read the exact volume.
• A third person should read the number of pounds that is exerted on the bathroom scale.
• Everyone should record the volume and weight data in a table like the one shown.
• Repeat these steps for at least five different volumes. Depress the plunger a little more each time. Be sure to include a reading where you depress the plunger as far as you can.

Have students record their data in a table like the one below. I like to create this table in an Excel spreadsheet and projected then have students come up and enter the data so that we can easily create a graph of our class data when finished.

 Trial Volume (mL) Volume (mL) Weight or force you apply (lb) Pressure you apply (lb/in2) Atmospheric pressure (lb/ in2) Total pressure (lb/in2) 1 14.7 lb/in2 2 14.7 lb/in2 3 14.7 lb/in2 4 14.7 lb/in2 5 14.7 lb/in2 6 14.7 lb/in2

Once you have collected the volume and weight date, tell the students that the number of pounds on the bathroom scale is related to the pressure inside the syringe. Ask them to explain what that means.

Have them estimate the cross-sectional area inside the syringe in square inches.

Calculate the pressure you applied in pounds per square inch. Enter these values in the “pressure you apply” column.

The atmosphere is also applying a pressure on the gas in the syringe. This pressure is equal to 14.7 lb/in2.

Add 14.7 lb/in2 to the pressure you apply to obtain the total pressure on the gas. Enter these values in the last column.

Once the data is entered plot the total pressure in pounds versus volume and display the graph.

## Explain

10 minutes

Looking at the graph, ask students to explain how the pressure and volume of a gas change in relation to each other as each one increases or decreases.

• What does the graph show about the relationship between pressure and volume?
• Why isn’t the graph a straight line?
• Does the curve ever touch the axis? Explain.
• Does the graph of pressure versus volume describe a proportional relationship? Why or why not?

The mathematical relationship between pressure and volume is described by the equation

PV or k/V

This relationship is known as Boyle’s law, after Robert Boyle, the British scientist who discovered the relationship. If you were able to make very accurate measurements in your syringe and scale experiment, the proportionality constant, k, would be the same number for every value of times V. You can have students calculate the product PV for their data. The products will be nearly identical, but not exactly, due to experimental error.