Cesium's atomic number is 55 and its symbol is Cs. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. It ignites spontaneously in air, and reacts explosively with water even at low temperatures, more so than the other alkali metals. It even reacts with ice at temperatures as low as −116 °C (−177 °F).Because of the bright blue lines in its emission spectrum, its name was derived from the Latin word caesius, meaning sky-blue. To obtain the first pure sample of Cesium, 44,000 liters (9,700 imp gal; 12,000 US gal) of mineral water had to be evaporated to yield 240 kilograms (530 lb) of concentrated salt solution, which were required to start the purification process. This display is contained within crystal clear Lucite acrylic.