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# Compressibility

## Introduces relationship between changes in volume due to applied pressure

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Compressibility

Credit: Image copyright Jandrie Lombard, 2014
Source: http://www.shutterstock.com
License: CC BY-NC 3.0

Will it all fit?

When we pack to go on vacation, there is always “one more” thing that we need to get in the suitcase. Maybe it’s another bathing suit, pair of shoes, book – whatever the item, we need to get it in. Fortunately, we can squeeze things together somewhat. There is a little space between the folds of clothing, we can rearrange the shoes, and somehow we get that last thing in and close the suitcase.

### Compressibility

Scuba diving is a form of underwater diving in which a diver carries his own breathing gas, usually in the form of a tank of compressed air. The pressure in most commonly used scuba tanks ranges from 200 to 300 atmospheres. Gases are unlike other states of matter in that a gas expands to fill the shape and volume of its container. For this reason, gases can also be compressed so that a relatively large amount of gas can be forced into a small container. If the air in a typical scuba tank were transferred to a container at the standard pressure of 1 atm, the volume of that container would need to be about 2500 liters.

Credit: Ian Myles
Source: http://www.flickr.com/photos/imphotography/3754144111/
License: CC BY-NC 3.0

Scuba diver. [Figure2]

Compressibility is the measure of how much a given volume of matter decreases when placed under pressure. If we put pressure on a solid or a liquid, there is essentially no change in volume. The atoms, ions, or molecules that make up the solid or liquid are very close together. There is no space between the individual particles, so they cannot pack together.

The kinetic-molecular theory explains why gases are more compressible than either liquids or solids. Gases are compressible because most of the volume of a gas is composed of the large amounts of empty space between the gas particles. At room temperature and standard pressure, the average distance between gas molecules is about ten times the diameter of the molecules themselves. When a gas is compressed, as when the scuba tank is being filled, the gas particles are forced closer together.

Compressed gases are used in many situations. In hospitals, oxygen is often used for patients who have damaged lungs to help them breathe better. If a patient is having a major operation, the anesthesia that is administered will frequently be a compressed gas. Welding requires very hot flames produced by compresses acetylene and oxygen mixtures. Many summer barbeque grills are fueled by compressed propane.

Credit: Flickr: rick
Source: http://www.flickr.com/photos/spine/309730216/
License: CC BY-NC 3.0

Oxygen tank. [Figure3]

#### Summary

• Gases will compress more easily that solids or liquids because here is so much space between the gas molecules.

#### Practice

Questions

Use the link below to answer the following questions:

1. What brings the fuel-air mixture into the cylinder?
2. What is the role of the compression cycle?
3. Does the exhaust cycle compress the gases produced by ignition?

#### Review

Questions

1. Why is there no change in volume when pressure is applied to liquids and solids?
2. Why do gases compress more easily than liquids and solids?
3. List uses for compressed gases.

1. [1]^ Credit: Image copyright Jandrie Lombard, 2014; Source: http://www.shutterstock.com; License: CC BY-NC 3.0
2. [2]^ Credit: Ian Myles; Source: http://www.flickr.com/photos/imphotography/3754144111/; License: CC BY-NC 3.0
3. [3]^ Credit: Flickr: rick; Source: http://www.flickr.com/photos/spine/309730216/; License: CC BY-NC 3.0

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