Chemical Properties of Water

Chemical Behaviour

Water participates in a wide range of chemical reactions. The main categories are outlined below.

1. Amphoteric nature. Water can act as either an acid or a base depending on what it reacts with — a Brønsted acid toward $NH_3$, and a Brønsted base toward $H_2S$:

Water even reacts with itself this way, in auto-protolysis (self-ionisation):

with one molecule acting as the acid (donating a proton to become its conjugate base, $OH^-$) and the other as the base (accepting a proton to become its conjugate acid, $H_3O^+$).

2. Redox reactions. Water is readily reduced to dihydrogen by strongly electropositive metals:

making it a valuable source of hydrogen gas. Conversely, water is oxidised to $O_2$ during photosynthesis:

and also by fluorine, one of the very few substances reactive enough to oxidise water directly:

3. Hydrolysis. Water's high dielectric constant gives it a strong hydrating tendency, and it dissolves many ionic compounds outright. It also chemically hydrolyses certain covalent and ionic compounds:

$N^{3-}(s) + 3H_2O(l) \rightarrow NH_3(g) + 3OH^-(aq)$ …