Arsenic is a grey, metallic looking solid. It has a density of 5.727 g/cm and is brittle, and moderately hard (more than aluminium; less than iron). It is stable in dry air but develops a golden bronze patina in moist air, which blackens on further exposure. Arsenic is attacked by nitric acid and concentrated sulfuric acid. It reacts with fused caustic soda to give the arsenate Na 3 AsO 3 and hydrogen gas. Arsenic sublimes at 615 °C. The vapour is lemon-yellow and smells like garlic. Arsenic only melts under a pressure of 38.6 atm, at 817 °C. It is a semimetal with an electrical conductivity of around 3.9 × 10 S•cm and a band overlap of 0.5 eV. Liquid arsenic is a semiconductor with a band gap of 0.15 eV.