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Introduces relationship between changes in volume due to applied pressure

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Behavior of Gases Kinectic Molecular Theory

The Kinetic Molecular Theory

The kinetic-molecular theory is a theory that explains the states of matter and is based on the idea that matter is composed of tiny particles that are always in motion. The theory helps explain observable properties and behaviors of solids , liquids , and gases . However, the theory is most easily understood as it applies to gases and it is with gases that we will begin our detailed study. The theory applies specifically to a model of a gas called an ideal gas . An ideal gas is an imaginary gas whose behavior perfectly fits all the assumptions of the kinetic-molecular theory. In reality, gases are not ideal, but are very close to being so under most everyday conditions.

The kinetic-molecular theory as it applies to gases has five basic assumptions.

  1. Gases consist of very large numbers of tiny spherical particles that are far apart from one another compared to their size . The particles of a gas may be either atoms or molecules. The distance between the particles of a gas is much, much greater than the distances between the particles of a liquid or a solid. Most of the volume of a gas, therefore, is composed of the empty space between the particles. In fact, the volume of the particles themselves is considered to be insignificant compared to the volume of the empty space.
  2. Gas particles are in constant rapid motion in random directions . The fast motion of gas particles gives them a relatively large amount of kinetic energy. Recall that kinetic energy is the energy that an object possesses because of its motion. The particles of a gas move in straight-line motion until they collide with another particle or with one of the walls of its container.
  3. Collisions between gas particles and between particles and the container walls are elastic collisions . An elastic collision is one in which there is no overall loss of kinetic energy. Kinetic energy may be transferred from one particle to another during an elastic collision, but there is no change in the total energy of the colliding particles.
  4. There are no forces of attraction or repulsion between gas particles . Attractive forces are responsible for particles of a real gas condensing together to form a liquid. It is assumed that the particles of an ideal gas have no such attractive forces. The motion of each particle is completely independent of the motion of all other particles.
  5. The average kinetic energy of gas particles is dependent upon the temperature of the gas. As the temperature of a sample of gas is increased, the speeds of the particles are increased. This results in an increase in the kinetic energy of the particles. Not all particles of gas in a sample have the same speed and so they do not have the same kinetic energy. The temperature of a gas is proportional to the average kinetic energy of the gas particles.

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.


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. [Figure1]

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 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. [Figure2]


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


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?


  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.

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Image Attributions

  1. [1]^ Credit: Ian Myles; Source: http://www.flickr.com/photos/imphotography/3754144111/; License: CC BY-NC 3.0
  2. [2]^ Credit: Flickr: rick; Source: http://www.flickr.com/photos/spine/309730216/; License: CC BY-NC 3.0

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