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Water cycling on Earth
Water is the only substance that exist naturally on Earth in all three physical states of the matter - gas (vapor), liquid, and solid (ice). Water in all these three states makes a large contribution to the planet's climate. Clouds in the atmosphere and ice sheets on the surface reflect a good deal of the solar radiation preventing it from warming the planet. On the other hand, where the earth's surface has been heated, clouds help trap energy radiated from the heated surface and thus have a warming effect as well. In fact, water vapor (a powerful greenhouse gas) is responsible for more than half the Earth's greenhouse gas warming.
Most plants and animals contain more than 60% water by volume. Water has a number of unique chemical and physical properties that make it essential for life. Without water, life would not have evolved on Earth.
Water molecule consists of two hydrogen atoms bonded to one oxygen atom – H2O (H-O-H). The bond between each hydrogen atom and the oxygen atom results from a pair of electrons shared between the two atoms. Because the oxygen atom has a greater affinity for electrons than does the hydrogen atom the electrons in the O–H bond are more attracted to oxygen. The unequal sharing of negatively charged electrons results in oxygen acquiring a partial negative charge (−) and hydrogen a partial positive charge (+). The H–O–H bond angle in water is 104.5°, which means that the molecule has a bent shape. This geometry and the accumulation of electrons on the oxygen side of the molecule cause the water molecule to have a negative charge on one side, the oxygen side, and a positive charge on the other side, the hydrogen side. Such molecules are called polar molecules.
Polar molecules are attracted to each other. The attraction results from the negative region of one molecule, the oxygen atom, being drawn to the positive region of another molecule, the hydrogen atom. Oxygen atoms have a great affinity for electrons, and so the tiny hydrogen atoms bonded to an oxygen atom acquire a significant concentrated positive charge. Because of the significant polarization of water molecules the attraction between them gets particularly strong. It is so strong that it has been given a particular name: hydrogen bonding. The energy associated with hydrogen bonds in water is about 20 kJ·mol–1, which is about 1/10 the strength of a typical shared-electron bond within a molecule. Namely hydrogen bonding between water molecules gives water its unique properties.
Solid state of water (ice) has lower density than liquid water.Almost all liquids contract when they get colder and reach a maximum density when they solidify. Water is different. As water cools, it contracts until it reaches 4°C, then it expands until it freezes at 0°C. Because ice floats on liquid water when ponds and lakes freeze, aquatic plants and animals can survive in the unfrozen liquid below ice.
Water has a very high boiling point for a substance with such small molecules.In order for a substance to boil, the molecules of the liquid must have enough kinetic energy to overcome the forces of attraction between them. Generally, boiling points of related compounds increase with molar mass. When the boiling points of the hydrogen compounds (hydrides) of Group VI elements, H2O, H2S, H2Se, and H2Te are plotted versus molar mass, water is far out of line with the heavier compounds. If water followed the trend for the two heaviest hydrides, its boiling point would be about −90°C. The boiling point of water is 100°C. It requires a high temperature to give water molecules enough kinetic energy to overcome hydrogen bonds between them. For similar reasons, water also has a higher melting point than would be expected for its low molar mass.
Water has the second highest surface tension of all common liquids except mercury.Polar molecules at the surface of a liquid are pulled inward by the molecules on the inside of the liquid, which keeps them firmly attached to the bulk of the liquid. The molecules on the surface are also attracted to each other. The forces between them cause them to behave something like a stretched elastic film that squeezes the liquid into the shape with the smallest possible surface area. The shape with the smallest surface-to-volume ratio is a sphere, so surface tension causes drops of water into as close to a spherical shape as possible.
Surface tension makes it more difficult to move an object through the surface of a liquid than to move an object when it is completely submerged. Surface tension is typically expressed in dynes·cm– 1, the force in dynes required to break a film of length 1 cm. At 20°°C, the surface tension of water is 72.8 dynes·cm–1. For comparison, the surface tension of mercury and ethanol are 465 and 22.3 dynes·cm–1, respectively. Surface tension allows water striders, insects that hunt prey on the surface of still water, to skate across the top of a pond.
Water has an unusually high specific heat.It takes more energy to raise the temperature of one gram of water by 1°C than any other liquid. Temperature is an expression of the amount of kinetic energy in the molecules of a substance; increasing temperature corresponds to increasing kinetic energy. As water is heated, much of the added energy goes to breaking apart its strong hydrogen bonds. This energy is not made available to increase the kinetic energy of the water molecules, so the temperature of water does not rise as much as would a liquid with lower intermolecular affinity. Therefore, water must absorb more heat energy to raise its temperature. Similarly, as water cools, it releases a great deal of heat. The high specific heat of water is responsible for the ocean's ability to act as a huge thermal reservoir.
Water has property of acid as well as base.The acid-base chemical properties of water contribute to the dissolution of solutes that also have acid-base properties. In pure water, about two molecules in every billion react to form a hydroxide ion, OH–(aq), and a hydronium ion, H3O+(aq):
H2O + H2O ⇄ H3>O+ + OH-
In this reaction, a hydrogen ion is released from a bonding pair of electrons in one molecule and transferred to a non-bonding pair of electrons in the other molecules. Releasing a hydrogen ion in this way is characteristic behavior for an acid, and accepting a hydrogen ion is characteristic behavior of a base. In the reverse direction, the hydroxide ion is the base, and the hydronium ion is the acid. In pure water, the number and concentration of hydronium ions is the same as that of the hydroxide ions. This equality means that pure water is neutral in the acid-base sense.
Carbon dioxide, CO2 (O=C=O), is an acidic solute that plays a particularly important role on Earth. The electrons in the carbon-oxygen bonds are attracted more to the oxygen atom giving the oxygen atoms a partial negative charge and the carbon atom a partial positive charge. Because carbon dioxide is a linear molecule these bond dipoles cancel one another, so the molecule has no permanent dipole moment and is usually said to be nonpolar. However, when dissolved in water, the partial positive charge on the CO2 carbon atom and the partial negative charge on the water oxygen attract one another. Similarly, the partial negative charges on the CO2 oxygen atoms and the partial positive charge on the water hydrogen atoms attract one another. One result is that carbon dioxide is about 40 times more soluble in water than the truly nonpolar atmospheric gases, nitrogen, N2, and oxygen, O2.
Another result is that some of the dissolved carbon dioxide reacts with water to form carbonic acid:
CO2 + H2O ⇄ H2CO3 ⇄ H+ + HCO3- ⇄ 2H+ + CO32-
In summary, the reaction of carbon dioxide in seawater proceeds as follows: First the carbon dioxide reacts with water to form carbonic acid. This then reacts with carbonate ions and forms bicarbonate.
Over the past half century, due to our burning of large amounts of fossil fuels, the concentration of carbon dioxide in the atmosphere has increased from about 310 parts per million (ppm, by volume) to about 400 ppm. The consequence of increased atmospheric carbon dioxide is an increase in the amount of carbon dioxide dissolved in the oceans that has caused their acidity to increase and endanger several forms of life, including corals and some microscopic organisms that rely on the appropriate acid-base properties of seawater to build exoskeletons made of calcium carbonate. Cal-cium carbonate is quite insoluble in slightly basic seawater, but as the sea has become less basic (more acidic), calcium carbonate is more soluble. This is because carbonate ions can react with hydronium ion to form the hydrogen carbonate ion and displace the calcium carbonate solubility equilibrium toward the dissolved side.
Many organisms are unable to cope with the increased solubility, are unable to form the necessary structures to maintain their integrity, and die, thus decreasing the amount of food available to higher trophic levels in the ocean ecosystem.
Other acidic solutes, oxides of sulfur and nitrogen, also formed in the combustion of fossil fuels, are quite soluble in water, are incorporated into condensed water vapor in the atmosphere, and fall to the ground as acid rain (and other forms of precipitation). Acid rain falls mainly in areas downwind from power plants that burn large quantities of coal, oil, and gas.
View of the Earth as seen by the Apollo 17 crew traveling toward the moon.
Hydrological cycle is the cycle in which water evaporates from the oceans and the land surface, is carried over the Earth in atmospheric circulation as water vapor, condenses to form clouds, precipitates over ocean and land as rain or snow, which on land can be intercepted by trees and vegetation, provides runoff on the land surface, infiltrates into soils, recharges groundwater, discharges into streams, and ultimately, flows out into the oceans, from which it will eventually evaporate again. The various systems involved in the hydrological cycle are usually referred to as hydrological systems.
Distribution of water around the globe
The hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet.
Sometimes, definition of hydrosphere includes only total mass of liquid water.
- Hydrosphere - 100% (1,300 million km3)
- Salt water - 97.5% (36 million km3)
- Fresh water - 2.5%
- Glaciers and icecaps (cryosphere) - 68.7% (28 million km3)
- Groundwater - 30.1% (8 million km3)
- Permafrost - 0.8%
- Surface and atmospheric water - 0.4%
- Freshwater lakes - 67.4%
- Soil moisture - 12.5%
- Atmosphere - 9.5% (12,000 km3)
- Wetlands - 8.5%
- Rivers - 1.6%
- Biota - 0.8%
"The water cycle is dynamic; it describes the continuous movement of water on, above and below the surface of the
Earth and the transitions from one state to another. Sea surface temperature, surface winds, and air temperature influence the
rate of evaporation at the ocean surface. In the tropics, warm ocean surface temperatures support high rates of evaporation.
Wind also increases evaporation. When the air's temperature is warmer, it can hold more water. While the atmosphere does not
store a large quantity of water compared to the ocean, rivers and lakes, it can transport water quickly from one place to another.
Low-lying regions of the atmosphere with high moisture and strong winds, can form "atmospheric rivers" to transport water horizontally.
Clouds are formed as water vapor cools and condenses into droplets and ice crystals. Clouds and water vapor act as insulators in the atmosphere. Clouds help shield the Earth from the Sun and trap heat from below. When cloud particles grow large enough, they may fall out as rain or snow. Under the right conditions, areas of precipitation can grow into large storms. As storms grow, they transfer heat vertically into the upper atmosphere. The migration of storms helps to distribute heat between the equator and poles - shaping wind patterns globally. How storms grow and intensify depends upon atmospheric moisture, surface temperatures and wind patterns. Precipitation is concentrated in some parts of the world and scarce in others. It can vary substantially from season-to-season and from year-to-year. Water that falls on the land surface as precipitation is stored within snow packs, lakes, reservoirs, soils and underground aquifers. "
- Prof. Shakhashiri. Chemical of the week: Water. 2011, www.scifun.org
- Anderson BT. Chapter 5 - Water in the Atmosphere. 2010
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