Rubidium is a very soft, ductile, silvery-white metal.[2] It is the second most electropositive of the non-radioactive alkali metals and melts at a temperature of 39.3 °C (102.7 °F). Similar to other alkali metals, this metal reacts violently with water, forms amalgams with mercury and alloys with gold, iron, caesium, sodium, and potassium, but not lithium (despite the fact that rubidium and lithium are in the same group).[3] 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 liberates. Rubidium has also been reported to ignite spontaneously in air.[2] Rubidium has a very low ionization energy of only 406 kJ/mol.[4] Rubidium and potassium show a very similar violet color in the flame test, which makes spectroscopy methods necessary to distinguish the two elements. ]