Ideal gas is a theoretical model of a gas in which the molecules are assumed to be point particles with negligible volume, no intermolecular forces, and perfectly elastic collisions. In this model, gas molecules move randomly and independently, and their behavior is governed solely by their kinetic energy, which depends on temperature. Because interactions between molecules are ignored, the physical properties of an ideal gas are described exactly by the ideal gas law, relating pressure–volume–temperature, and amount of gas. While no real gas behaves as an ideal gas under all conditions, many real gases closely approximate ideal behavior at low pressures and high temperatures, making the ideal gas model a fundamental and highly useful reference in thermodynamics and fluid mechanics.
An ideal gas does not actually exist in the physical world, it is a theoretical concept used to simplify the study of gas behavior. Real gases always have molecules with finite volume and experience intermolecular forces such as attraction and repulsion, which cause deviations from ideal behavior, especially at high pressures or low temperatures. However, under conditions of low pressure and high temperature, these molecular effects become very small, and many real gases behave very closely to an ideal gas. Because of this, the ideal gas model is extremely useful in engineering, physics, and chemistry as an accurate approximation and a baseline for understanding and comparing real gas behavior.