Lanthanum oxide is a white solid that can be prepared by direct reaction of its constituent elements. Due to the large size of the La3+ ion, La2O3 adopts a hexagonal 7-coordinate structure that changes to the 6-coordinate structure of scandium oxide (Sc2O3) and yttrium oxide (Y2O3) at high temperature. When it reacts with water, lanthanum hydroxide is formed: a lot of heat is evolved in the reaction and a hissing sound is heard. Lanthanum hydroxide will react with atmospheric carbon dioxide to form the basic carbonate.