.................................................... The Earth's hydrosphere

Introduction. We all know that in our country the lack of water has caused serious conflicts between affected populations. The organizations that are in a closer contact with these problems are making constant appeals for savings in water consumption. A European citizen consumes about 300 liters of water, from which 40% are spent in the toilet cisterns and 35% on washing and bathing. We have to have in mind, that if we do not close the tap, washing hands for one minute consumes 10 liters of water, brushing teeth about 20 liters and washing dishes can reach up to 200 liters. It is necessary to turn off the tap while doing those actions, try to buy toilet cisterns with regulation of water consumption and showering instead of bathing.

This new water culture, which basically means to save its consumption, may give enough time until source of clean energy is found, certainly called hydrogen fusion energy which will allow us to produce fresh water from seawater. At that time, all problems of lack of water would be over, water would be carry even to deserts, which would multiply food production and end hunger around the world. In this chapter, you will find a lot of information about the importance of the hydrosphere on Earth.

Activities to do. Read the explications on the general characteristics of water and types of water and do the exercise "Multiple Choice Test 8". Then read the text on the water cycle and water purification and do the exercises "Identify water-related process 8" and "Crossword 8".

1. The Earth's hydrosphere. It is the layer of water that covers 70% of the Earth's surface. It was formed from the water steam existing in the primeval atmosphere when the planet's surface, formed 4.6 billion years ago, cooled sufficiently.

2. Water in the Universe. Liquid water is the essence for the existence of life. Currently no liquid water has been detected on other planets, but has been, indeed, detected water steam on Venus and in interstellar space, and there has been found an ice on Mars (in the polar icecaps) and in the rings of Jupiter and Saturn. Also, on Mars traces of old water erosions activity has been observed.

3. The water molecule. Is a molecule composed of two hydrogen atoms and one of oxygen (H2O). It has two positive electric poles where are the two atoms of hydrogen and a negative pole where is the oxygen. As the different electric sign poles attract, water molecules attract each other and form groups of 2 to 10 molecules. Therefore, despite of being a group of very small molecules, water is a liquid and not a gas at room temperature.

4. Water properties. The main properties are:

• Pure water is a colorless, transparent, odorless and tasteless liquid that at sea level boils at 100 ºC and freezes at 0 ºC.

• Thanks to its electric charges, it dissolves a large number of substances.

• It is an essence for the existence of life, since all of the chemical reactions that are necessary for the realization of the three vital functions of organisms (nutrition, reproduction and relation) occur only in the aqueous medium.

• It has a great resistance to the temperature changes, which is why the oceans are a heat stabilizer at a planetary level.

• It presents a greater density than the ice. This results in the fact that ice on the surface is acting as a thermal insulator. If ice sink, first the water on the bottom of the reservoir would freeze and then all of the water, which would make a life, in cold areas, impossible.

5. Water type and abundance. Although water is very abundant, the majority of it (97,2%) is the salt water, accumulated in the oceans and seas, that serves neither to plants nor animals on land. This depend on the limited amount of fresh water that is in lakes and rivers (0,01%) and in underground aquifers (0,6%). The large reservoir of fresh water are the polar icecaps and glaciers (2,1%).

6. The seawater. It has an average salinity of 35g of salt per liter of water. This salinity ranges from 7g/l of the Baltic Sea (sea with little evaporation and a great inflow) and 41g/l of the Red Sea (sea with high evaporation level and without river inputs). The salinity of the sea comes from the numerous times that the rainwater falling on the continents has dissolved its soluble rocks.

7. The water of the continents. The continental water comes from rain and snowfall. Basically it is very low salted water, which is why it is called a fresh water. It may be in the surface, whether it is moving and forming rivers and torrents or accumulated forming lakes, marshes and wetlands, or infiltrate into the ground to reach a waterproof layer, usually made of clay, and stay there soaking into the ground and forming aquifers. In few cases, underground lakes may be formed. The top level of an aquifer is called the water table (see the details in the picture below).

8. Water vapor in the atmosphere. It comes from the evaporation of seas and lakes and plant transpiration. Basically it is located in the troposphere. The higher air temperature is, the more water vapor may contain. When the air cannot hold more water vapor, it is called the humidity saturated air. The excess of the water vapor contained in air takes place by condensing onto dust particles (called condensation cores) and resulting in microdroplets, which with the friction with air, prevents them from falling, giving rise to clouds. If hundreds of thousands of these droplets aggregate, it originates a drop of rain or if the temperature is below 0 ºC, a glass of ice is formed along with snowflakes.

9. The cycle of water. On our planet due to the Sun's heat each year 517.000 km3 evaporate (The equivalent of a pool of the size of Spain and 1 km deep). This annual staem comes back to the surface as rain and snow. The different starges of the cycle are:

10. Water and health. Water is essential for all living things. A human being needs about two liters of water daily, to prevent dehydration. Water fit for human consumption is called drinking water. It is a water free of pathogens (protozous, bacteria and viruses), toxic substances (lead, mercury, cadmium, fertilizers, insecticides, etc.) and its salinity does not exceed the limits set by health authorities.

11. Water pollution and purification. There are three types of pollutants:

• Chemicals. The main ones are: fertilizers, insecticides, herbicides, detergents, plastics, petroleum products, such as fuels and organic solvents, oils and heavy metals.

• Biological. The main ones are: feces and excretions of animals and humans, food debris and all pathogenic microorganisms that feed on them.

• Physical. The main ones are: rock fragments and dust, heat and nuclear radiation.

12. Water purification. To avoid contamination of natural waters, which is where plants and aquatic animals live, purification of wastewater, which is generated in the stocks, is needed, before letting them into the natural environment. This is done in purification plants and the process comprises: