O and a relative molecular mass of only 18.0, the water molecule is deceptively simple, and yet, as far as we can tell, it is essential for life. In fact, the search for life on other planets and planetary satellites has centred on looking for bodies where liquid water may exist now or may have existed in the past. In this chapter you will learn about some of the physical and chemical properties of water that make it such an important molecule, and some of the chemistry relevant to environmental systems.