Pressure Change & Volume of a Gas: Physics Lab

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  • 0:01 What is Pressure and Volume?
  • 1:17 Physics Lab Steps
  • 2:12 Data Analysis
  • 3:23 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lab, you will be able to explain what pressure and volume are and how pressure, volume, and temperature change in response to each other - with the aid of marshmallows! A short quiz will follow.

What is Pressure and Volume?

In today's lab, we're going to play with marshmallows. I promise you, there is a good reason for it. Though really, who needs a reason to play with marshmallows?

This lab's goal is to investigate pressure and volume. Pressure is a continuous physical force exerted on or against an object by something in contact with it. But in physics and chemistry, gas pressure has a more specific definition. It is the pressure that gas molecules exert on the walls of their container. It's generally measured in pascals, Pa, or sometimes atmospheres, atm.

Volume is also a quantity that can be used to describe gases. Volume is the physical space that an object or substance takes up. It's usually measured in meters squared.

Along with temperature, pressure and volume can change in response to each other. For example, if you increase the temperature of a gas, either its volume has to increase in response or the pressure has to increase - or some mixture of both.

Today, we're going to use marshmallows to investigate how these three quantities affect each other. Although marshmallows aren't gases, the way they behave will still be a good analogy for the way gases behave, because they contain little pockets of air inside them.

Physics Lab Steps

In this physics lab, you'll need:

  • A bag of marshmallows
  • A jar with a lid
  • A tool to make a hole in the jar lid, such as a hand drill
  • A kitchen vacuum pump or wine preserver pump
  • A microwave-safe plate

Step 1: Put several marshmallows in the jar. Make a hole in the lid and then screw it back on.

Step 2: Put the vacuum pump nozzle against the hole or through the hole, depending on the design of the pump.

Step 3: Pump out as much air as you can, and note down what you see happen to the marshmallows.

Step 4: Get a fresh marshmallow and put it on a microwave-safe plate.

Step 5: Microwave on high for around a minute.

Step 6: Note down your observations.

If you haven't already, now it's time to pause the video and get started. Good luck!

Data Analysis

In the first part of the experiment, you removed air from a jar containing marshmallows. What happened to the marshmallows? You should have found that they expanded. Marshmallows contain lots of pockets of air. You would think that removing this air would, therefore, make them shrink, not expand. So why does that happen?

Well, we have to think in terms of gases. By removing air from the jar, we are decreasing the gas pressure inside it. It turns out that if you keep the temperature the same, pressure and volume are inversely proportional to each other. So, less pressure means more volume. This is called Boyle's Law. Another way to think about this is that with less air inside the jar, the gas molecules spread out to fill a larger space.

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