Physical Properties of Potassium

Potassium is a soft, silver-white metal of high lustre. Its melting-point is given as 60° C., 62.4° C., 62.5° C., and 63.5° C. A wide discrepancy exists between the values given for the boiling-point, among them being 667° C.; 667° C. at 760 mm. pressure, 719° to 731° C., 757.5° C., 365° C. at 0 mm., and about 90° C. in the vacuum of the cathode light. For the density are recorded the values 0.8629 and 0.859 at 0° C., 0.8621 at 20° C., and 0.851 (solid) and 0.8298 (liquid) at 62.1° C. The atomic volume calculated from the value at 20° C. is 45.35. The metal has a lower density and a higher atomic volume than sodium.

At ordinary temperature potassium has a wax-like consistence, its hardness being 0.5, the corresponding values for the other alkali-metals being lithium 0.6, sodium 0.4, rubidium 0.3, and caesium 0.2. At low temperatures the metal becomes hard and brittle. Like sodium, it crystallizes in cubes and also in quadratic octahedra. In reflected light it has a greenish-blue colour, that of light transmitted through very thin layers being violet-blue. At the boiling-point its vapour is green, at bright redness violet.

Between -78.5° and 23° C. the specific heat is given as 0.1662, corresponding with the atomic heat 6-5. The specific heat increases rapidly with rise in temperature, the value recorded between 22.3° and 56.5° C. being 0.1922. For the specific heat of the solid at the melting-point, Rengade gives 0.1914. The latent heat of fusion has been found by Joannis to be 15.7 Cal. per kilogram, and by Rengade to be 14.63 Cal. The metal is a good conductor of heat and electricity, and displays slight magnetic properties. The "velocity of sound" method indicates the monatomicity of potassium vapour.

Potassium and many of its salts are said to exhibit radioactivity, the amount of radium present in a gram of the chloride, sulphate, carbonate, or nitrate being given as 3×10^-14. Some authorities deny the existence of radioactive properties.