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This article focuses on water as we experience it every day, and on its importance to life. The water (molecule) article describes pure water, from a scientific and technical perspective.
Water is an abundant substance on Earth. It exists in many forms, such as sea, rain, and rivers : water is continuously flowing through a cycle of evaporation, precipitation and runoff.
All forms of life need water. Humans consume "drinking water", i.e. water with qualities compatible with our metabolism. This natural resource becomes scarce with growing world population, and its availability is on the agenda of many governmental organisations.
Water takes many different shapes on earth : clouds in the sky, waves and icebergs in the sea, glaciers in the mountain, aquifers in the ground, to name but a few. Through evaporation, precipitation and runoff, water is continuously flowing from one form to another, in what is called the great water cycle.
Because of the importance of precipitation to agriculture, and to mankind in general, we give different names to its various forms : while rain is common in most countries, other phenomena are quite surprising when seen for the first time: hail, snow, fog or dew for example. When appropriately lit, rain takes many colours and form a wonderful rainbow.
Similarly, water runoffs have played major roles in our history: rivers and irrigation brought the water needed for agriculture and the seas offered opportunity for commerce. Less common forms of runoffs are glacier and waterfalls. Through erosion, runoffs played also a major part in shaping our environment: valleys and river delta hosts many people.
Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells.
Because water can contain many different substances, it can taste or smell very differently. In fact, we have developed our senses to be able to evaluate the drinkability of water: we avoid the salty seas and the putrid swamps, and we like the water that is adequate for our body.
Water has three unusual properties that are critical for life: it is a good solvent, it has high surface tension, and it becomes less dense as it freezes.
Water dissolves many types of substances, such as sugar, and facilitates their chemical interaction, which allows complex metabolisms.
Some substances, however, do not mix well with water, including oils and other hydrophobic substances. Cell membranes, composed of lipids and proteins, take advantage of this property to carefully control interactions between their contents and external chemicals. This is facilitated somewhat by the surface tension of water. Fatty substances can also create membranes, separating an "inside" from an "outside". This allows the creation of individual cells, their competition for resources, and the process of evolution through natural selection.
Water drops are stable due to the high surface tension of water. This can be seen when small quantities of water are put onto a nonsoluble surface such as glass: the water stays together as drops. This property is also important for life, and plays a key role in transpiration.
The third important property of water distinguishes it from nearly all other compounds: its solid phase, ice, is less dense than liquid water and will float. For most substances, the solid form is denser than the liquid form.
Although water is densest at 4°C, and will sink by convection as it cools to that temperature, if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4°C. This effectively insulates a lake floor from the cold.
Life on earth has evolved with and fine tuned itself to the important features of water. The existence of abundant liquid, vapor and solid forms of water on Earth has no doubt been an important factor in the abundant colonization of Earth's various environments by life-forms adapted to those varying and often extreme conditions. The abundance of water on Earth is a result of the Sun-Earth distance, the mass of Earth, the amount of incoming solar radiation, and the trapping of some of that radiation by Earth's atmosphere. The long term essentially constant surface temperature of Earth with varying solar flux may indicate that life itself is controlling the surface temperature and therefor the water content and forms. (see the Gaia hypothesis)
All known forms of life depend on water. Water is a vital part of many metabolic processes within the body. Significant quantities of water are used during the digestion of food. (Note however that some bacteria and plant seeds can enter a cryptobiotic state for an indefinite period when dehydrated, and come back to life when returned to a wet environment)
About seventy percent of the human body is made of water. To function properly the body requires between one and seven litres/quarts of water per day to avoid dehydration, the precise amount depending on the level of activity, temperature, humidity, and other factors. Water is lost from the body in urine and faeces, through sweating, and by exhalation of water vapour in the breath.
Humans require water that does not contain too much salt or other impurities. Common impurities include chemicals or harmful bacteria. Some solutes are acceptable and even desirable for perceived taste enhancement. Water that is suitable for drinking is termed potable water.
Because of the growth of world population and other factors, the availability of drinking water per person is shrinking. This issue can be solved through more production, better distribution, or through less waste.
Water is a strategic resource for many countries. Many battles and wars, such as the Six-Day War in the Middle East, have been fought to gain access to it. Experts predict more trouble ahead because of the world's growing population, increasing contamination through pollution and global warming.
40% of the world's inhabitants currently have insufficient fresh water for minimal hygiene. More than 2.2 million people died in 2000 from diseases related to the consumption of contaminated water or drought. In 2004, the UK charity WaterAid reported that a child dies every 15 seconds due to easily preventable water-related diseases.
UNESCO's World Water Development Report (WWDR, 2003) from its World Water Assessment Program indicates that, in the next 20 years, the quantity of water available to everyone is predicted to decrease by 30%.
There are 3 ways to provide drinking water to everyone: produce it more, distribute it better to the needy, and waste it less.
Drinking water is often collected at spring or extracted from artificial borings in the ground, or wells. Building more wells in adequate places is thus a possible way to produce more water. Other water sources are the rain or the seas. This water however is not adequate for human consumption, and water purification is needed. Popular methods for purifying water are filtering, boiling and distillation. More advanced techniques exist, such as reverse osmosis.
The distribution of drinking water is done through water pipes or as bottled water. Government in many countries have programs to distribute water to the needy at no charge. Others argue that the market mechanism and free enterprise are best to manage this rare resource, and to finance the boring of wells or the construction of dams and reservoirs.
Reducing waste, i.e. using drinking water only for human consumption, is another option. In some cities, such as Hong Kong, sea water is extensively used for flushing toilets citywide in order to conserve fresh water resources. Polluting water may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the polluter.
Water is considered a purifier in most religions, including Christianity, Islam, Judaism, and Sikhism. For instance, baptism in Christian churches is done with water. As well, a ritual bath in pure water is done for the dead in many religions including Judaism and Islam. And in Islam, the daily Salah can only be done after Ablution (Wodoo) that is washing parts of the body in clean water. In Shinto, water is used in almost all rituals to cleanse a person or an area.
Water is often given spiritual powers. In Celtic mythology, Sulis is the local goddess of thermal springs; in Hinduism, the Ganga is also personified as a goddess. Alternatively, gods can be patrons of particular springs, river or lakes: for example in Greek and Roman mythology, Peneus was a river god, one of the three thousand Oceanids.
Empedocles, a Greek philosopher maintained that water is one of the four classical elements along with fire, earth and air, and was regarded as the ylem, or basic stuff of the universe. Water was considered cold and moist. In the theory of the four bodily humours, water was associated with phlegm. Water was also one of the Five Elements in Chinese Taoism, along with earth, fire, wood, and metal.
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