Caesium is a very soft (it has the lowest Mohs hardness of all elements), very ductile, silvery-white metal, which develops a silvery-gold hue in the presence of trace amounts of oxygen. It has a melting point of 28.4 °C (83.1 °F), making it one of the few metals that are liquid near room temperature. Mercury is the only metal with a known melting point lower than caesium.[note 3] In addition, the metal has a rather low boiling point, 641 °C (1,186 °F), the second lowest of all metals. Its compounds burn with a blue color.
Caesium forms alloys with the other alkali metals as well as with gold, and amalgams with mercury. At temperatures below 650 °C (1,202 °F), it alloys with cobalt, iron, molybdenum, nickel, platinum, tantalum or tungsten. It forms well defined intermetallic compounds with antimony, gallium, indium and thorium, which are photosensitive. It mixes with the other alkali metals (except with lithium), and the alloy with a molar distribution of 41% caesium, 47% potassium, and 12% sodium has the lowest melting point of any known metal alloy, at −78 °C (−108 °F). A few amalgams have been studied: CsHg2 is black with a purple metallic lustre, while CsHg is golden-coloured, also with a metallic lustre.
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