Question

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 gasLinks to an external site., 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 matterLinks to an external site. 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.

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 liquidLinks to an external site., there is essentially no change in volume. The atoms, ionsLinks to an external site., 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.





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

Answer 1

1. Why is there no change in volume when pressure is applied to liquids and solids?
- In solids and liquids the particles (atoms, ions, molecules) are already packed very close together with almost no empty space between them. Applying pressure cannot significantly push them closer, so their volume changes negligibly. (In technical terms they have a very large bulk modulus compared with gases.)

2. Why do gases compress more easily than liquids and solids?
- Gas particles are far apart and most of a gas’s volume is empty space. When pressure is applied the particles can be pushed closer together, substantially reducing the volume. (This behavior is described by the gas laws such as PV = nRT.)

3. List uses for compressed gases.
- Scuba tanks for diving
- Medical oxygen for patients and compressed-gas anesthetics in surgery
- Welding (acetylene + oxygen mixtures)
- Propane for barbecue grills
- (Other common uses include compressed air for pneumatic tools, aerosol propellants, fire extinguishers, and compressed natural gas for vehicles.)