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Water
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An important feature of water is its polar nature. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge, relative to the hydrogen side. A molecule with such a charge difference is called a dipole. The charge differences cause water molecules to be attracted to each other (the relatively positive areas being attracted to the relatively negative areas) and to other polar molecules. This attraction is known as hydrogen bonding.
This relatively weak (relative to the covalent bonds within the water molecule itself) attraction results in properties such as a very high boiling point, because a lot of heat energy is necessary to break the hydrogen bonds between molecules, and also a large specific heat capacity.
Also due to hydrogen bonding, water molecules have the peculiar property that their density in the liquid state is higher than in the crystalline (solid) state. The highest density of water occurs in the liquid form at a temperature of 4 °C. This has the effect that the water at the bottom of lakes in winter typically has a temperature of 4 °C, allowing fish to survive. Another consequence is that ice will melt if sufficient pressure is applied.
Water is also a good solvent due to its polarity. When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute. The partially negative dipoles of the water are attracted to positively charged components of the solute, and vice versa for the positive dipoles.
In general, ionic and polar substances such as acids, alcohols, and salts are easily soluble in water, and nonpolar substances such as fats and oils are not. Nonpolar molecules stay together in water because it is energetically more favorable for the the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with nonpolar molecules.
An example of an ionic solute is table salt; the sodium chloride, NaCl, separates into Na+ cations and Cl- anions, each being surrounded by water molecules. The ions are then easily transported away from their crystaline latice into solution. An example of a nonionic solute is table sugar. The water dipoles hydrogen bond to the dipolar regions of the sugar molecule and allow it to be carried away into solution.
The solvent properties of water are vital in biology, because many biochemical reactions take place only in solution (e.g., reactions in the cytoplasm and blood).
The strong hydrogen bonds give water a high cohesiveness and, consequently, surface tension. This is evident when small quantities of water are put onto a nonsoluble surface and the water stays together as drops. This feature is important when water is carried through xylem up stems in plants; the strong intermolecular attractions hold the water column together, and prevent tension caused by transpiration pull. Other liquids with lower surface tension would have a higher tendency to "rip", forming vacuum or air pockets and rendering the xylem vessel inoperative.
Pure water is actually an insulator, meaning that it doesn't conduct electricity well. Because water is such a good solvent, it often has some solute dissolved in it, most frequently salt. If water has such impurities, then it can conduct electricity well.
Water can be split into its constituent elements, hydrogen and oxygen, by passing a current through it. This process is called electrolysis. Water molecules naturally disassociate into H+ and OH- ions, which are pulled toward the cathode and anode, respectively. At the cathode, two H+ ions pick up electrons and form H2 gas. At the anode, four OH- ions combine and release O2 gas, molecular water, and four electrons. The gases produced bubble to the surface, where they can be collected.
Purified water is needed for many industrial applications, as well as for consumption. Humans require water that doesn't have too much salt or other impurities in it. Common impurities include toxic chemicals or harmful bacteria. Some solutes are acceptable and even desirable for perceived taste enhancement.
Six popular methods for purifying water are:
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 asssociated with phlegm[?].
Water was also one of the Chinese five elements along with air, fire, wood, and metal.
UNESCO's World Water Development Report (WWDR, 2003) from its World Water Assessment Program indicates that in the next 20 years the world is facing an unprecedented lack of drinking water. The quantity of water available to everyone is predicted to decrease by 30%. The causes are contamination, global warming and political problems.
40% of the world's inhabitants have insufficient fresh water for minimal hygiene. More than 2.2 million people died in 2000 from illnesses related to the consumption of contaminated water.
The report indicates large global disparities in the raw volume of available water: from 10 m³ per person per year in Kuwait to 812.121 [m³?] in French Guiana. However, richer countries such as Kuwait can more easily cope with low water availability.
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