Everyday experience indicates that it is much easier to compress gases than liquids. This property is measured by the isothermal compressibility, β =−1/V ∂V/∂p|_T , the fractional change in a substance’s volume with pressure at constant temperature. What is the isothermal compressibility of an ideal gas? The smaller β is, the less work must be done to pressurize a substance. Compare the work necessary at 20 ^◦C to raise the pressure of a kilogram of air and a kilogram of water (βwater ≅ 4.59 × 10⁻¹⁰ Pa⁻¹) from 1 atm to 10⁴ atm.