What characteristics of water make it so essential for life on Earth?
Some of the properties of water that make it essential for life are the following:
As far as inorganic substances are concerned apart from water only mercury and liquid ammonia can exist as liquids at the temperature and pressure that prevails on Earth. The fact that water can remain as a liquid is essential for its use as a medium for the transfer of ions within living cells.
Water has a very high specific heat. As the amount of heat required to change the temperature of water is very large it allows water to remain in the liquid phase over a very wide temperature range. Under normal pressures water can remain in the liquid phase from 0 degree Celsius to 100 degree Celsius. No other known substance can remain a liquid in this wide a temperature band.
Another unique property of water is that unlike most other substances, the density of water as a solid is less than that of water in the liquid phase. This makes ice float on water. The low conductivity of ice allows water below it to remain liquid and at a temperature that can sustain life. In regions of the Earth where temperatures fall far below 0 degree Celsius, life is able to exist in water due to the sheet of ice floating above it.