Gases are a state of matter characterized by particles that have no definite shape or volume, and tend to expand to fill any container in which they are placed. They are composed of molecules that are free to move independently from one another, resulting in their ability to diffuse and mix easily with other gases. Gases are typically compressible, meaning that their volume can be changed by applying pressure. The study of gases encompasses topics such as their behavior under different temperatures and pressures, the laws that govern their properties, their interactions with other substances, and their applications in various industries such as chemistry, physics, environmental science, and engineering.