For questions relating to water (H₂O), specifically its structure, properties, and uses in chemistry. For questions not about water but rather about solutions in water, use tags [aqueous-solution] or [solubility] instead.
Water, chemical formula H2O, is a common chemical compound on Earth, covering over two-thirds of the surface area of the planet (in either frozen or liquid form) and comprising 0.02% of the total mass of the Earth. The majority of lifeforms on the Earth depend on water and are primarily composed of it, and many natural mineral compounds contain water molecules in their basic structure. As such, water is a key component of many physical and chemical processes of interest to modern science.
Water is naturally produced by the oxidation of pure hydrogen gas by pure oxygen gas. This reaction, like many oxidation reactions, produces heat. It is also a byproduct of the reaction of most strong acids, containing an H+ ion, and strong bases, containing a hydroxide (OH-) ion. The remaining components of the acid and base generally produce a salt.
Because of the arrangement of the electrons of an oxygen atom, a water molecule has a "bent" structure, with the hydrogen atoms at a 105° angle to the oxygen and each other, instead of a linear 180° arrangement. Because of this, a partial positive charge can be observed on the side of the molecule containing the two hydrogens, and a partial negative charge directly opposite. These partial charges make each water molecule, in effect, a tiny magnet, which gives water several interesting properties.
Water is a polar solvent; the molecular structure and partial polar charges can break weaker chemical bonds, such as hydrogen bonds and some ionic bonds. This causes materials which have these bonds, such as many salts including table salt, to readily dissolve in water. This property makes water crucial for most biological processes, which use the water to carry nutrients and electrolytes dissolved within it.
Another interesting property caused by its molecular shape is the tendency to form a crystalline structure when frozen which is less dense (occupying more space) than the same mass of liquid water. This causes ice to float, but also causes problems when living organic tissues, which contain water, are frozen; the water expands, bursting and destroying the cells, damaging the tissue.
These properties, paired with water's abundance on Earth, cause many other compounds to incorporate water into their structures. Many minerals, composed of various metallic and nonmetallic solid elements, are "hydrated"; their molecular arrangement incorporates water molecules in the structure. The water can often be released by heating the mineral, which produces a material with different properties. The production of Portland cement, for instance, involves "dehydrating" limestone, creating anhydrous calcium carbonate powder. Re-adding water to the cement during the formation of concrete in effect "reconstitutes" the limestone.