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THE WATER MOLECULE





Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to one oxygen atom in a V-shape. The oxygen atom attracts the electrons more strongly than the hydrogen. The hydrogen side of the water molecule has a slight positive charge. On the other side of the molecule, a negative charge exists. This gives water an asymmetrical distribution of charge. Molecules that have ends with partial negative and positive charges are known as polar molecules. It is this polar property that allows water to separate polar solute molecules, explains why water can dissolve so many substances, and is responsible for its strong surface tension.

WATER MOLECULE


The nature of the atomic structure of water causes its molecules to have unique physical properties.

Solid, Liquid, Gas
Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth.

Solvency
Water is often called the "Universal Solvent." It is able to dissolve a large number of different chemical compounds. This feature also enables water to carry solvent nutrients in runoff, infiltration, groundwater flow, and living organisms.

Surface Tension
Water has a high surface tension. In other words, water is adhesive and elastic, and tends to aggregate in drops rather than spread out over a surface as a thin film. This phenomenon also causes water to stick to the sides of vertical structures despite gravity's downward pull. Water's high surface tension allows for the formation of water droplets and waves, allows plants to move water (and dissolved nutrients) from their roots to their leaves, and the movement of blood through tiny vessels in the bodies of some animals.

Specific Heat
Water has a high specific heat. Specific heat is the amount of energy required to change the temperature of a substance. Because water has a high specific heat, it can absorb large amounts of heat energy before it begins to get hot. It also means that water releases heat energy slowly when situations cause it to cool. Water's high specific heat allows for the moderation of the Earth's climate and helps organisms regulate their body temperature more effectively.

pH
pH refers to water being either an acid, base, or neither (neutral). A pH of 7 is said to be neutral, a pH below 7 is "acidic'' and a pH above 7 is "basic'' or "alkaline". Water in a pure state has a neutral pH. As a result, pure water is neither acidic nor basic. Water changes its pH when substances are dissolved in it. Rainwater is not pure water. It contains appreciable amounts of sodium, potassium, calcium, magnesium, chloride, and sulfate, as well as the dissolved gases carbon dioxide, oxygen, and nitrogen from the atmosphere. Rain has a naturally acidic pH of about 5.6. Usually this is neutralized as water percolates through the soil and picks up calcium and magnesium ions.

Like the Richter scale used to measure earthquakes, the pH scale is logarithmic. A pH of 5.5 is 10 times more acidic than water at a pH of 6.5. Thus, changing the pH by a small amount (suddenly) is more of a chemical change (and more stressful to fish!) than might first appear.

One of the major non-point source pollution problems with many States is acidic deposition. The primary pollutants are: sulfate and nitrate.

Conductivity
Water conducts heat more easily than any liquid except mercury. This fact causes large bodies of liquid water like lakes and oceans to have essentially a uniform vertical temperature profile.

Other interesting facts about water:

Water exists as a liquid over an important range of temperature from 0 - 100° Celsius. This range allows water to remain as a liquid in most places on the Earth.

The freezing of water causes it to expand. When water freezes, it expands rapidly adding about 9 percent by volume. Fresh water has a maximum density at around 4° Celsius. Water is the only substance on this planet where the maximum density of its mass does not occur when it becomes solidified.

Water freezes at 32° Fahrenheit (F) and boils at 212° F (at sea level, but 186.4° at 14,000 feet). In fact, water's freezing and boiling points are the baseline with which temperature is measured: 0° on the Celsius scale is water's freezing point, and 100° is water's boiling point. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice floats.

For more information do a search on Water Molecule, Water Chemistry, or Physical Properties of Water on a search engine.


Additional Information:
     USGS. Water Properties.
     https://water.usgs.gov/edu/waterproperties.html.
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