ABSTRACT
Life on our planet is supported mainly by water, which exists in three states-solid ice, liquid water, and gaseous vapor-in what is termed as the hydrosphere of the earth. In particular, the liquid form that is referred to as water is considered to be responsible for life on the planet, supporting its evolution from its beginning and maintaining it throughout. Water transports energy in the form of nutrients to the various parts of a living organism. It has unique characteristics. Living organisms have been able to adapt themselves to varying conditions on our planet. Some plants are able to grow in saline soils and water. Some animals and plants have adapted themselves to very arid conditions existing in some parts of the earth. However, no organism can live without water. Communities of plants and animals thrive in the proximity of water. Human civilizations flourished close to freshwater resources such as rivers and lakes. The special chemical characteristics of water, especially the strong hydrogen bonding between atoms in the water molecule, give this substance some very important and unique properties. For example, water has very strong surface tension; its freezing point and boiling point are very high for its molecular weight; its specific heat is very high; and it anomalously expands upon freezing, forming ice that is less dense than liquid water. Consequently, aquatic life remains safe in extremely cold climates in liquid water under which exist the insulating sheets of the cover.
