Distribution of water on Earth The presence of water on Earth in the solid, liquid and gaseous states is unique within the solar system. The fact that the Earth has retained this water is a result of the planet’s gravitational force, determined by its mass and the Earth’s temperature, determined by its distance from the Sun. Water plays a central role in the evolution of life on Earth, and remains critical to the maintenance of life on the planet. Water has some remarkable properties that are crucial to the maintenance of all living systems. Such properties include: Water is the only pure substance on Earth commonly found in all three states (solid, liquid and gaseous). Ice is less dense than water at 0°C, so as a result, ice floats allowing many aquatic life-forms to surviv e below the surface in cold climates. Water is transparent to visible light, which allows photosynthesis to occur in aqueous systems. Water has a high surface tension, which is important in some physiological processes and also capillary action in soils and plants. Water is an excellent solvent, allowing biological processes to occur in aqueous solutions, and also allows water to serve as a transportation system for nutrients and waste products in living organisms. Water in the spheres 97% of the water on the planet is present in the hydrosphere. Liquid water covers nearly 75% of the Earth’s surface in the form of oceans, seas, lakes, ground water and rivers, which are collectively referred to as the hydrosphere. The polar ice caps cover another portion of planet Ear th, dominating the regions north of the Ar ctic Circle and south of the Antarctic Circle. The atmosphere also contains varying amounts of water vapour (ranging from 0 –5%). Water in the lithosphere can occur as moisture or permafrost within soils and rocks or as water of crystallisation within minerals. Waterofcrystallisation(or waterofhydration) refers to the water molecules that form part of the crystal structure of many ionic substances, such as hydrated copper (II) su lfate –CuSO 4 .5H 2 O Water is the major constituent of li ving matter, representing approximately 70% of the biosphere. It is a component of all cells, the transport system for nutrients and waste products in living organisms, a reaction medium for many biochemical processes. Together with carbon dioxide, water is a reactant in photosynthesis. Solutions The oceans and seas dominate planet Earth. They are not pure substances, but rather mixtures containing large quantities of dissolved substances, called solutions. Solutions are defined as homogenous mixtures. In a solution, one substance, called the soluteis dissolved in another substance, called the solvent. Aqueous solutions Solution in which the solute is dissolved in water, are called aqueoussolutions. Common nat urally occurring aqueous solutions include sea water, blood plasma, stomach acid, sap in plants and drinks like coffee and lemonade. Aqueous solutions are indicated in chemical equations using the (aq) solution to the right of the solute. For example an aqueous solution of glucose is C 6 H 12 O 6 (aq) and an aqueous solution of sodium chloride i s NaCl (aq) .
