# 6.1: Gases: Introduction and Review

In the gaseous, $$\left( g \right)$$, state of matter, the individual particles of a substance are able to fully separate from one another and move randomly in three-dimensional space.  As a result, gases, such as molecular chlorine, Cl2, have neither a defined volume nor a defined shape and will disperse until their constituent particles are uniformly distributed throughout the container in which they are placed, as shown below in Figure $$\PageIndex{1}$$.

Furthermore, because gaseous chemicals exist as independent particles, the volume of a gaseous chemical is largely comprised of the "empty" spaces between its particles.  Therefore, if pressure is applied to a gas, its constituent particles respond by shifting their positions to occupy these "empty" spaces.  This spatial redistribution effectively decreases, or compresses, the overall volume of the gas by increasing the physical proximity of its particles.  In contrast, the particles that exist within a solid or a liquid must, by definition, maintain physical contact with one another, and, consequently, there is no "empty" space between these particles.  As a result, the application of pressure does not impact the volume of a solid or liquid substance, as the direct physical contact of the corresponding constituent particles precludes these molecules or atoms from becoming any closer to one another.  Therefore, the ability to forcibly alter the volume of a chemical is unique to substances that exist in the gaseous state of matter.  As a result, gases must be quantified using a unique set of measurable properties, which will be described in the following section.