Water is the most essential of all substances, upon which all life dependsWater is odourless and tasteless. It has a bluish tint that can be seen in very deep layers.At Standard Atmospheric Pressure (760mm of mercury) water has a freezing point of 0Â°C (32F) and a boiling point of 100Â°C (212F). Water is at its maximum density at 4Â°C (39F).Water can remain a liquid even below its freezing point, up to -25Â°C, if it is not disturbed and if the temperature does not drop further and no particle or ice crystal is added to it.Water is used in the metric system to define the gram.Water is the only substance that occurs at ordinary temperatures in all three phases: liquid, gas and solid.75% of the earth’s surface is covered by water.The oceans contain 97.5% of the earth’s water, the land 2.4%, and the atmosphere holds less than .001%Only 1% of the earth’s water is available for drinking; 2% is frozen.50-90% of the weight of living organisms is made up of water. Blood in animals and sap in plants is mostly water.The adult human body is composed of approximately 55 to 60% water–the brain is composed of 70% water, as is skin, blood is 82% water, and the lungs are nearly 90%t water.The world average rainfall is 860 mm.You can survive about a month without food, but only 5-7 days without water.It is possible to drink water today that was here in the dinosaur age.The average urban home of 4.6 people uses 640 litres of water per day.A dripping tap can waste as much as 60 litres per day or 1 800 litres per month.A leaking toilet can waste up to 100 000 litres of water per year, enough to take three full baths every day.It takes about 2.5 litres of water to cook pasta and about 5 litres to clean the pot.The average bath holds between 150 and 200 litres of water when filled to the brim.A toilet is the biggest user of indoor water. On average, it uses 11 litres of water when flushed.The hydrological cycle is the continuous movement of water between the earth and the atmosphere. Water evaporates from water and land surfaces and transpires from living cells. This vapour circulates through the atmosphere, condensing to form clouds and precipitating as rain or snow. When water hits the earth’s surface it either runs into streams and ends up in oceans or lakes, or seeps into the soil. The water that seeps into the soil is then either absorbed by the roots of vegetation, or it sinks into the groundwater reservoir.Â Water is made up of hydrogen and oxygen. Two hydrogen atoms are linked by a single chemical bond to one oxygen atom. Its chemical formula is H2O.The water molecule is angular in shape, forming negative and positive charges on opposite sides. This means that the H2O molecule is highly polar. Because of this high polarity H20 molecules form hydrogen bonds which are very strong. This is when hydrogen atoms in one water molecule are attracted to the non-bonding electron pairs of the oxygen atom in another H20 molecule. These strong hydrogen bonds mean that water has a very high boiling point because it takes a lot more energy to overcome them and release H2O molecules from the liquid into the gaseous phase.The H2O molecules in ice are highly ordered, although loosely structured. When ice melts this orderly arrangement breaks and so H2O molecules can be packed closer together. Therefore the liquid is denser than the solid and this explains why ice floats on water.One of the most important properties of water is that it can dissolve many other substances to form aqueous solutions. This happens because of the H2O molecule’s high polarity. If, for example, an ionic compound such as sodium chloride (NaCl) is added to water, the positively charged Na atoms will be attracted to the negative end of the H2O molecules, and the negatively charged Cl atoms will be attracted to the positive end of the H20 molecules. Therefore the Na and Cl ions will be pulled apart and hydrated, meaning they will be surrounded by H2O molecules. This keeps the Na and Cl- ions from recombining.Water can act as an acid or a base because it can dissociate to some extent into H (hydrogen) ions which are acidic, and OH- (hydroxyl) ions which are alkaline (basic).Most hydrogen atoms consist of only 1 proton, but the isotopes deuterium and tritium have one and two neutrons in their nuclei respectively. Deuterium oxide (D2O) is called heavy water because it has a greater molecular weight than H2O due to the extra neutron in the deuterium nucleus. D2O can be produced through electrolysis and fractional distillation of water. It is used as a moderator of neutrons in nuclear power plants and in biological research as an isotopic tracer.Ice occurs when water vapour or liquid water freeze. At temperatures below 0Â°C (32F) water vapour becomes frost at ground level and snowflakes (each one a single ice crystal) in clouds, while liquid water becomes solid ice in the form of river ice, sea ice, hail etc. Each H2O molecule forms hydrogen bonds with four neighbouring molecules, creating a tetrahedral shape. An ice crystal is made up of rings of these tetrahedrons forming at various angles to each other.At first sight ice seems brittle and shatters like glass when it is struck. However it flows under low stresses over long periods of time or under high stresses where pressure stops the ice from splintering. This happens because the layers of ice crystal can glide over and past each other without the hydrogen bonds being broken. Flow is very slow because the different crystals glide in different directions and tend to interfere with one another. Glaciers are an example of this flow.Ice is used as a refrigerating agent because it takes more energy to melt it than most other substances, due to strong hydrogen bonds. Melting ice remains at a constant 0Â°C (32F).Because ice is less dense than water at 0Â°C (32F) a mass of ice occupies 9% more volume than an equal mass of water. This is why when water in pipes freezes it can cause the pipes to burst. When water enters tiny cracks in rocks and freezes, the expansion creates great pressure that will split the rocks, causing erosion.Another important point about ice being less dense than water is that it floats. In rivers, lakes and oceans this means that the ice traps the warmer water below, allowing for fish and other creatures to survive the freezing temperatures of the surface.Steam is vapourised water and is an odourless, invisible gas. It often looks white and cloudy because there are tiny water droplets present. Steam is created in nature from volcanic processes heating underground water and is released through hot springs and geysers, for example. The temperature at which water will boil depends on its pressure. If pressure is reduced, the boiling point is also reduced. If pressure is increased, more energy is required to allow the liquid molecules to escape into the gaseous phase, and therefore the temperature at which water boils also increases.Modern industrial society makes extensive use of steam power. Virtually all the world’s electricity is created through steam power. Power plants heat water into steam which, under pressure, drives turbines that produce electrical current. Steam is also used in the manufacture of steel, aluminium, copper and nickel, and the production of chemicals, the refinement of petroleum and for cooking and heating in the home.Most of the earth’s water is undrinkable. If a large bucket of water were to represent the sea water on the planet, an egg cup full would represent the amount of water locked in ice caps and glaciers and a teaspoonful would be all that was available as drinking water. Human use of natural waters, especially freshwater resources such as rivers and lakes, has steadily increased over the centuries. With population growth and increasing use of water for agriculture, industry and recreation, water is becoming an incredibly valuable resource. It is not only the scarcity of water that is becoming an issue, but also quality. Mineral fertilizers, pesticides and herbicides have seeped into surface and subsurface waters contaminating them beyond human consumption and disrupting delicate ecosystems. Dumping of sewage and industrial wastes and toxins pollute rivers and lakes and threaten the world’s most important resource. Will their be enough water to accommodate the needs of future generations?