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.