Andrews' Carbonic Acid

Plate 13 speaks for itself: “Above 31 degrees, liquid and gas are one fluid.”

The Physics

Andrews mapped the isotherms of carbon dioxide and found that above a critical temperature no pressure liquefies it: liquid and gas become one continuous fluid. The critical isotherm has an inflection with zero slope.

(∂p/∂V)_T = 0 and (∂²p/∂V²)_T = 0

• p: pressure, Pa
• V: volume, m³
• T: temperature, K
• T_c, p_c: critical temperature (K) and pressure (Pa)

CO₂: T_c = 304.13 K (30.98 °C), p_c = 7.3773 MPa.

Sources

• T. Andrews, Phil. Trans. R. Soc. 159, 575 (1869).
• R. Span, W. Wagner, J. Phys. Chem. Ref. Data 25, 1509 (1996).

On the poster

Plate 013, “Andrews' Carbonic Acid”: the pixel-art plate with its slogan “Above 31 degrees, liquid and gas are one fluid.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.