Boyle point, the point of minimum on the isotherm of a real gas in the coordinates p—pV (Fig.; p — the pressure of the gas, V is the volume occupied by the gas); named in honour of the English scientist R. Boyle. Near the Boyle point small portions of the isotherms of a real gas can be treated approximately as segments of horizontal straight lines which, according to the Clapeyron equation pV=(M/μ)RT, represent the isotherms of an ideal gas (R — the gas constant, M is the mass of the gas, μ is the molar mass). In other words, the Boyle point defines the temperature at which the equation of an ideal gas is applicable to the given real gas. The portion ab of the isotherm (to the left of the Boyle point) corresponds to conditions in which the real gas is more compressible than the ideal one; the portion be (to the right of the Boyle point) corresponds to conditions of lower compressibility of the real gas compared with the ideal one. To the left of the Boyle point the predominant influence is that of the forces of attraction between the molecules, which make the gas easier to compress; to the right of the Boyle point — that of the molecules' own volume, which hinders compression. Near the Boyle point these factors, which distinguish a real gas from an ideal one, cancel each other out.
The isotherms of a real gas in the coordinates p—pV. On the isotherms with temperature T>TB there are no Boyle points.
The line joining the Boyle points of the isotherms is called the Boyle curve. The point of this curve that lies on the axis of ordinates defines the so-called Boyle temperature TB. For a gas obeying the van der Waals equation, TB=3.375 Tk, where Tk is the critical temperature, At T<Tk complete liquefaction of the gas under pressure is possible, at T<Tk partial liquefaction of gases is possible on throttling (see Joule–Thomson effect).