Tungsten (also known as wolfram) is a refractory metal, and as such is characterized by a very high melting point (~3420°C, the highest of all metals), high density (19.25 g/cm3, over twice than that of iron), low coefficient of thermal expansion (4.5µm/(m•K) at 25 °C) and superior mechanical properties at elevated temperatures. Elemental tungsten resists attack by oxygen, acids, and alkalis. Bulk tungsten samples are subjected to severe oxidation at temperatures above 500°C. Tungsten powder, with an average grain size above 1µm, reacts like bulk tungsten. In the case of submicron powders and depending on the grain size and preparation method, the powder can be pyrophoric.