All living beings, from microbes to humans, live in water or contain water. All chemical reactions that take place inside cells are carried out in water.
Liquid water is an extraordinary compound, with
strange properties. It has one
of the highest specific heats of all substances. This means that when heat is added to or removed
from a body of water, the temperature varies more slowly than in any other
liquid. This is important for living beings, as water acts as an environmental
stabilizer. Also, it is the
non-metallic liquid with the highest thermal conductivity, so local temperature variations are balanced very
quickly.
Most liquids contract when they solidify, but water is an exception. It has its maximum density at a temperature of 4°C. The density of ice is lower, 0.92 times lower, so ice floats on water. So, when the temperature drops, water freezes from top to bottom, while other liquids solidify from bottom to top. This also has important biological consequences. In polar seas and in fresh waters of cold regions, when the temperature drops below the freezing point, the surface layer of ice insulates the water below from the cold, so that it never freezes and the beings that live there can remain alive and active, despite the harsh environmental conditions.
