The difference between a gas and a compressed gas lies in the pressure at which the gas is contained.
Gas: In general, a gas refers to a state of matter where the substance has no fixed shape or volume. Gases have molecules or atoms that are in constant random motion and are widely spaced apart. They fill the container they are placed in and can expand to occupy larger volumes. Examples of gases include oxygen, nitrogen, and carbon dioxide.
Compressed Gas: A compressed gas, on the other hand, is a gas that has been subjected to increased pressure by compressing it into a smaller volume. Compressing a gas increases the density and pressure of the gas. These gases are stored under pressure in cylinders or containers to keep them in a condensed state. Compressed gases are commonly used in various applications, such as industrial processes, medical gases, welding, and scuba diving.
The distinction between a gas and a compressed gas is primarily based on the pressure at which the gas is maintained. A gas can exist at ambient pressure, whereas a compressed gas is intentionally pressurized to achieve a higher density and store it in a smaller space.