Sulphur or Sulfur is an element which is known since ancient times. Sulphur sometimes naturally occurs in its elemental form and as such often it is emitted in volcanic eruptions. Sulfur has a complex chemistry and it is essential to life. On the other hand, it has some very toxic and environmentally hazardous compounds. Notable are hydrogen sulfide, which gives rotten eggs their smell and sulfur dioxide and trioxide, which, when dissolved in water, give sulfurous acid and sulfuric acid.