Water is a vital part of [life] on this [planet]: it makes up about 70% of all living things. It has several important properties that make it this important:
- It is a [liquid] at room temperature.
- It is a good [medium] for [chemical reactions].
- It has a high [specific heat capacity], so it is a good [insulator] and is [thermally stable].
- It expands as it cools. If it did not, the cold water would sink to the bottom of a [lake] and freeze there, so a [lake] or [pond] would freeze from the bottom up, killing its inhabitants, whereas actually [ice] forms in a layer over the top of the water and [organisms] can survive underneath.
- It is a small enough [molecule] that it can diffuse freely through [cell membranes]. This is important for vital biological processes such as [osmosis].
- It is dense enough to support [aquatic creatures](one of the reasons [life] evolved in the [sea]), but not too dense.
- It is a good [solvent].
Many of these properties are caused by [hydrogen bonds], which dramatically increase the attraction between [water molecules], increasing its [boiling point] so that it is a liquid at room temperature.
Because water is so unique, it is considered vital for [life], and is one of the more important [chemicals] looked for by many [space probes].