compressibile

Compressibile is the adjective used in several languages, including Italian, to describe something that can be compressed or reduced in volume under applied pressure. In physics and engineering, the related concept is compressibility, a measure of how readily a substance changes its volume when pressure is applied.

Compressibility is commonly quantified by the isothermal compressibility κ_T, defined as κ_T = −(1/V)(∂V/∂P)_T. The reciprocal of κ_T is the bulk modulus B = 1/κ_T = −V(∂P/∂V)_T, which characterizes the stiffness of a material under pressure at constant temperature. There are other forms, such as the isentropic (or adiabatic) compressibility κ_S = −(1/V)(∂V/∂P)_S, which accounts for thermal effects during pressure changes. For many substances, κ_S ≈ κ_T / γ, where γ = C_p/C_v for ideal gases.

Material behavior varies widely. Gases are highly compressible, liquids have moderate compressibility, and solids are the least compressible, though none are perfectly incompressible. Temperature, phase, and state strongly influence κ_T.

In fluids, compressibility influences the propagation of disturbances. The speed of sound is related to the bulk modulus and density by c = sqrt(B/ρ) for small perturbations. In ideal gases, B equals the pressure, which leads to characteristic relationships between compressibility, pressure, and temperature. In engineering, compressible flow becomes important when speed or pressure changes are large enough to cause significant density variations, such as in high-speed aerodynamics or shock-wave phenomena.

The term compressibile thus informs discussions across material science, acoustics, and fluid dynamics, linking microscopic stiffness to macroscopic volume change under pressure.