# 7.3: Compressibility

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When we pack to go on vacation, there seems to always be "one more" thing that we need to get in our suitcase. Maybe it's another bathing suit, pair of shoes, book—whatever the item, we need to get it in. Fortunately, we can usually squeeze things together somehow. Perhaps there is a little space between folds of clothing, or we can rearrange the shoes; somehow we can get that last item 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 $$1 \: \text{atm}$$, the volume of that container would need to be about 2500 liters.

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 compressed acetylene and oxygen mixtures. Many summer barbeque grills are fueled by compressed propane.

## Summary

7.3: Compressibility is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.