Rubidium is a very soft, ductile, silvery-white metal. It is the second most electropositive of the stable alkali metals and melts at a temperature of 39.3 °C (102.7 °F). Like other alkali metals, rubidium metal reacts violently with water. As with potassium (which is slightly less reactive) and caesium (which is slightly more reactive), this reaction is usually vigorous enough to ignite the hydrogen gas it produces. Rubidium has also been reported to ignite spontaneously in air. It forms amalgams with mercury and alloys with gold, iron, caesium, sodium, and potassium, but not lithium (even though rubidium and lithium are in the same group)， It is widely distributed throughout nature, but only occurs in small amounts (its abundance in the earth's crust is 310 ppm), the main source being carnallite, the hydrated chloride of magnesium and potassium. The metal is obtained by the electrolysis of the fused halide, although it can be obtained on a laboratory scale by heating the chloride with calcium metal and distilling out the metal. The metal ignites spontaneously in air and reacts violently with water and applications are limited.