Niobium is a silver-gray, soft, and ductile transition metal with a melting point of 2468°C, a boiling point of 4742°C, and a density of 8.57 g/cm3. Niobium is stable in the air at room temperature, and is not completely oxidized when it is red hot in oxygen. It can directly combine with sulfur, nitrogen, and carbon at high temperatures, and can form alloys with titanium, zirconium, hafnium, and tungsten. It does not interact with mineral acids or alkalis, nor is it soluble in aqua regia, but it is soluble in hydrofluoric acid. Because the physical and chemical properties of niobium are similar to those of tantalum, and the two elements are often co-existing, it is difficult to separate pure niobium without impurity tantalum.