Even though most of the surface of the planet is covered with water, more than 97 percent of this water is salty and not readily usable for most human purposes. While the oceans may produce enormous amounts of food, offer transportation, and eventually provide a source of energy, humans require freshwater for drinking, agriculture, and industry. Freshwater is becoming the most valuable resource in the world, and because of its uneven distribution in a world with a rapidly growing population, there will be debate and outright conflict over the rights to and use of freshwater. At present, humans use more than half of all the freshwater that flows in rivers or that is stored in lakes, and this percentage is growing rapidly. other sources of water are being exploited, including extraction from beneath the ground, to building reservoirs behind large dam projects.
The volume of groundwater is 35 times the volume of freshwater in lakes and streams, but overall freshwater accounts for less than 3 percent of the planet's water. The united states and other nations have come to realize that freshwater is a vital resource for their survival and are only recently beginning to appreciate that much of the world's water resources have become contaminated by natural and human-aided processes. Most drinking water in the united states comes from surface reservoirs or is purified from rivers, yet approximately 40 percent of drinking water in the country comes from groundwater reservoirs; about 80 billion gallons of groundwater are pumped out of these reservoirs every day in the united states. Groundwater is a limited resource since it is being pumped out of the ground faster than it is being replenished by natural processes.
Water is one of the most unusual substances in the entire solar system. Its unusual properties are responsible for controlling climate, life, and many
Bar graphs showing the distribution of water in hydrosphere. Ninety-seven percent of the planet's water in the hydrosphere is salt water located in the oceans. Three percent of the planet's water is fresh, but 75 percent of that small fraction is locked up in glacial ice. Most of the remaining freshwater is located underground in the groundwater system, and less than one-tenth of the world's freshwater is readily accessible to people in freshwater streams, rivers, and lakes.
processes on Earth. A water molecule (H20) consists of two hydrogen atoms bonded to one oxygen atom and is a polar molecule with a partial positive charge at the end with hydrogen, and a partial negative charge at the end containing the oxygen atom. This allows different water molecules to form weak bonds known as hydrogen bonds with other water molecules, as the positive end of one molecule bonds with the negative end of an adjacent molecule. The nature of this bond determines the properties of water, such as its melting/freezing point of 32°F (0°C) and boiling point of 212°F (100°C). Water may exist in three different states: as a solid (ice), liquid (water), or vapor (water vapor or steam). Since most of the planet has a temperature between 32° and 212°F (0°-100°C), most water exists in the liquid form.
one of the most unusual properties of water is that, like other compounds, it contracts and becomes denser as it cools, until about 39°F (4°C), at which point the cold water begins to become less dense again, becoming less dense than the warmer water. This property of water allows ice to float and causes water bodies to freeze from the top downward. If water was not so unusual, lakes and oceans would freeze from the bottom up and eventually become solid ice. No life would live beneath the seas, and the planet would become a giant cold iceball.
Water can also absorb a lot of heat or solar energy without becoming much warmer. This is because water has a high heat capacity; large water bodies do not change temperature rapidly and have a moderating climatic effect on nearby landmasses. It also takes a large amount of energy to change water from a liquid state to a vapor. To change liquid water to vapor, the water absorbs a lot of this heat energy from the source (say, the ocean) and carries this heat (the heat of vaporization) to the atmosphere, warming it. This is one of the most important heat-transfer processes on the surface of the planet, and it plays a large role in many atmospheric and climate effects. The processes can be appreciated on a personal level by feeling the cooling effect of allowing perspiration to evaporate from the body. Water may also sublimate, or move directly from the solid to the vapor state, or move from the groundwater system to water vapor in the atmosphere through the aid of transpiration in plants.
Water can also dissolve many substances with time, and can carry many substances in solution. Most water contains many mineral salts (such as sodium chloride, sea salt) derived from erosion of the landmasses and many dissolved gases from the atmosphere. The amount of gases dissolved in seawater is partly a function of temperature and plays a large role in climate and global warming.
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