Water is at once simple and complex. Its study begins with a single molecule. A water molecule consists of two hydrogen atoms and one oxygen atom, as noted by its chemical formula H2O. In water’s gaseous state, thermal energy—heat—enables single water molecules to float freely, mostly independently of one another. In water’s liquid state, hydrogen atoms constantly form and break bonds with other hydrogen atoms, as exhibited by water’s characteristic fluidity. In the solid phase, water’s molecular bonds are dictated by the oxygen atoms, which
form crystalline shapes, or ice.