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UCLA DESMA 170 - Water's Chemical Properties

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Water's Chemical PropertiesWhat are the physical and chemical properties of water that make it so unique andnecessary for living things? When you look at water, taste and smell it - well, what couldbe more boring? Pure water is virtually colorless and has no taste or smell. But the hiddenqualities of water make it a most interesting subject. All these water molecules attracting each other mean they tend to clump together. This iswhy water drops are, in fact, drops! If it wasn't for some of Earth's forces, such as gravity,a drop of water would be ball shaped -- a perfect sphere. Even if it doesn't form a perfectsphere on Earth, we should be happy water is sticky.Water is called the "universal solvent" because it dissolves more substances than anyother liquid. This means that wherever water goes, either through the ground or throughour bodies, it takes along valuable chemicals, minerals, and nutrients.Pure water has a neutral pH of 7, which is neither acidic nor basic.Diagram about pHYou probably know water's chemical description is H2O. Asthe diagram to the left shows, that is one atom of oxygenbound to two atoms of hydrogen. The hydrogen atoms are"attached" to one side of the oxygen atom, resulting in a watermolecule having a positive charge on the side where thehydrogen atoms are and a negative charge on the other side,where the oxygen atom is. Since opposite electrical chargesattract, water molecules tend to attract each other, makingwater kind of "sticky." As the right-side diagram shows, theside with the hydrogen atoms (positive charge) attracts theoxygen side (negative charge) of a different water molecule. (Ifthe water molecule here looks familiar, remember thateveryone's favorite mouse is mostly water, too).Water's Physical Properties• Water is unique in that it is the only natural substance that is found in all threestates -- liquid, solid (ice), and gas (steam) -- at the temperatures normally foundon Earth. Earth's water is constantly interacting, changing, and in movement.• Water freezes at 32o Fahrenheit (F) and boils at 212o F (at sea level, but 186.4° at14,000 feet). In fact, water's freezing and boiling points are the baseline withwhich temperature is measured: 0o on the Celsius scale is water's freezing point,and 100o is water's boiling point. Water is unusual in that the solid form, ice, isless dense than the liquid form, which is why ice floats. • Water has a high specific heat index. This means that water can absorb a lot ofheat before it begins to get hot. This is why water is valuable to industries and inyour car's radiator as a coolant. The high specific heat index of water also helpsregulate the rate at which air changes temperature, which is why the temperaturechange between seasons is gradual rather than sudden, especially near the oceans.• Water has a very high surface tension. In other words, water is sticky and elastic,and tends to clump together in drops rather than spread out in a thin film. Surfacetension is responsible for capillary action, which allows water (and its dissolvedsubstances) to move through the roots of plants and through the tiny blood vesselsin our bodies. • Here's a quick rundown of some of water's properties: o Weight: 62.416 pounds per cubic foot at 32°F o Weight: 61.998 pounds per cubic foot at 100°F o Weight: 8.33 pounds/gallon, 0.036 pounds/cubic inch o Density: 1 gram per cubic centimeter (cc) at 39.2°F, 0.95865 gram per cc at212°FBy the way: 1 gallon = 4 quarts = 8 pints = 128 ounces = 231 cubic inches1 liter = 0.2642 gallons = 1.0568 quart = 61.02 cubic inches 1 million gallons = 3.069 acre-feet = 133,685.64 cubic feetThe hydrologic cycleEvaporation:As water is heated by the sun, it's surface molecules become sufficiently energized tobreak free of the attractive force binding them together, and then evaporate and rise asinvisible vapour in the atmosphere.Transpiration:Water vapour is also emitted from plant leaves by a process called transpiration. Everyday an actively growing plant transpires 5 to 10 times as much water as it can hold atonce.Condensation:As water vapour rises, it cools and eventually condenses,usually on tiny particles of dustin the air. When it condenses it becomes a liquid again or turns directly into a solid (ice,hail or snow). These water particles then collect and form clouds.Precipitation:Precipitation in the form of rain, snow and hail comes from clouds. Clouds move aroundthe world, propelled by air currents. For instance, when they rise over mountain ranges,they cool, becoming so saturated with water that water begins to fall as rain, snow or hail,depending on the temperature of the surrounding air.Runoff:Excessive rain or snowmelt can produce overland flow to creeks and ditches. Runoff isvisible flow of water in rivers, creeks and lakes as the water stored in the basin drains out.Percolation:Some of the precipitation and snow melt moves downwards, percolates or infiltratesthrough cracks, joints and pores in soil and rocks until it reaches the water table where itbecomes groundwater.Groundwater:Subterranean water is held in cracks and pore spaces. Depending on the geology, thegroundwater can flow to support streams. It can also be tapped by wells. Somegroundwater is very old and may have been there for thousands of years.Water table:The water table is the level at which water stands in a shallow well.The sun-powered cycleThe endless circulation of water from the atmosphere to the earth and its return to theatmosphere through condensation, precipitation, evaporation and transpiration is calledthe hydrologic cycle.Heating of the ocean water by the sun is the key process that keeps the hydrologic cycle inmotion. Water evaporates, then falls as precipitation in the form of rain, hail, snow, sleet,drizzle or fog. On its way to earth some precipitation may evaporate or, when it falls overland, be intercepted by vegetation before reaching the ground. The cycle continues inthree different ways:• Evaporation/transpiration – On average, as much as 40% of precipitation inCanada is evaporated or transpired.• Percolation into the ground – Water moves downward through cracks and pores insoil and rocks to the water table. Water can move back up by capillary action or itcan move vertically or horizontally under the earth's surface until it re-enters asurface water system.• Surface runoff – Water runs overland into nearby streams and lakes; the steeperthe land and the less porous the


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