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Brittany Roberts AP bio WATER ESSAY The first thing water reminds us of is the clear liquid in our water bottles. However, water is more than a normal substance. Water is the beginning and the continual of life on earth. Because of its many unique properties, water was able to start life on our planet. Water covers about 3/4 of the earth. In nature, water naturally exists in all three physical states of matter—solid, liquid and gas. Water’s extraordinary properties are emergent properties resulting from water’s structure and molecular interactions.

Living cells are 70%-95% H2O The simple structure of H2O is the source of all water’s properties. With two hydrogen atoms sticking to an oxygen atom in a tetrahedron shape, water is considered as a polar molecule. Because oxygen is very electronegative, it pulls hydrogen’s electrons towards it, causing oxygen to become partial negative. The hydrogen atoms then will have fewer electrons towards its own nucleus, therefore making it partial positive. Since positive charge and negative charge attracts each other, the molecular formation of H2O will result hydrogen bonding.

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Each water molecule can form hydrogen bonds to a maximum of four other water four molecules. Organisms Depend on the Cohesion of Water Molecules Hydrogen bonds hold water molecules together. When water is in liquid form, its hydrogen bonds continually break and re form. Molecules are continually forming re-form. new bonds with other water molecules. As a result, most of the water molecules are bonded to their neighbors, which makes water more structured than other liquids. The hydrogen bonds hold the substance together in the phenomenon called cohesion. ohesion Since the hydrogen bonds connect water molecules together, water can reach the bonds leaves of plants through microscopic vessels extending from the roots. When water evaporates from a leaf, it is replaced by water in the vessels of the leaf. Hydrogen bonds cause the water leaving the leaf to pull on molecules further down the vessel. Adhesion , the clinging of one substance to another, is also important. The Adhesion of water to the walls helps to counteract the force of gravity. Cohesion is related to surface tension a measure of how difficult it is to stretch or tension, break the surface of a liquid.

Water Moderates Temperatures on Earth Water can absorb a relatively large amount of heat with only a slight change in its own temperature. sorb This is why water is slower to cool on a cold day and slower to heat up on a warmer day. Water has an unusually high specific heat. Water will change its temperature less when it absorbs or loses a given amount of heat. This characteristic is attributed to hydrogen bonding. Heat must be absorbed in order to break hydrogen bonds, and heat is released when hydrogen bonds form. In order for water molecules to begin moving faster, hydrogen bonds must be broken.

Thus, most of the energy from one calorie of heat is used to break the hydrogen bonds. When the temperature of the water drops slightly, many additional hydrogen bonds form and release a considerable amount of heat. High specific heat is important to life on Earth. A large body of water can absorb and store large amounts of heat from the sun in the daytime and summer while warming up only a few degrees. At night and during winter, the gradually cooling water will warm the air. High specific heat of water also stablizes ocean temperatures and creates a favorable environment for marine life.

The water that covers most of earth plays an important role in keeping temperature fluctuations within a range that allows life. Since organisms are made mostly of water, they can better resist changes in their own temperature If molecules move fast enough, they can overcome attractions to one another and enter the air as a overcome gas. The transformation from liquid to gas is called evaporation. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g to be converted from liquid to the gaseous state. About 580 cal of c heat is needed to vaporize 1 g of heat, which is relatively high compared with most other liquids.

This property is also a direct result of hydrogen bonding. As a liquid evaporates, the surface of the liquid remaining cools down. Evaporative cooling occurs cooling because it is the molecules with the most kinetic energy that leave as a gas. This property of water allows the temperatures in lakes and ponds to remain stable and prevents terrestrial organisms from overheating. Oceans and Lakes Don’t Freeze Solid Because Ice Floats As a solid, water is less dense than as a liquid, because water expands when it solidifies. When molecules are no longer moving quickly enough to break their hydrogen bonds, water begins to freeze.

Each molecule is bonded to the maximum four others at 0°C. As a result, the hydrogen bonds keep the molecules further apart than in liquid water. Water is at its greatest density at 4°C. Because of this property, water in lakes does not freeze solid – only the top layer of water freezes. The floating ice zes. insulates the liquid water below and allows life to exist under the frozen surface Water is the Solvent of Life A liquid with a completely homogenous mixture of two or more substances is a solution The solution. dissolving agent is the solvent , and the substance dissolved is the solute.

When salt is dissolved in solvent ent, solute. water, salt is the solute and water is the solvent. An aqueous solution is one in which water is the solvent. While water is not a truly universal solvent, it is viewed as such be because of its versatility. Because of water’s polarity, it can easily dissolve numerous substances. When sodium chloride is placed in water, the sodium and chloride ions are attracted to the water molecules because of their electrical attraction. Eventually, the water Eventually, molecules surround each individual sodium and chloride ions, creating a hydration shell around each dissolved ion.

The two solutes, sodium and chloride, became homogeneously mixed with water, the solvent. Compounds made of polar molecules also dissolve in water. When acids dissolve in water, they donate additional H+ to the solution. An acid is a substance that increases the hydrogen ion concentration of a substance. Solutions with more hydrogen ions than hydroxide ions are acidic solutions. A substance that reduces the hydrogen ion concentration of a solution is called a base Some bases base. work to reduce the concentration of hydrogen ions by directly accepting them.

Other bases dissociate to form hydroxide ions, which will bond with the hydrogen ions to form water. Solutions with more hydroxide ions than hydrogen ions are basic solutions. Solutions with equal hydrogen ions and hydroxide ion concentrations are neutral. Strong acids and bases dissociate completely. A weak base, such as ammonia, has a reversible reaction regarding the binding and release of its hydrogen ion. Weak acids also reversibly release and reaccept hydrogen ions. Acid precipitation is a serious assault on water quality in some industrialized areas. Uncontaminated rain has a slightly acidic pH of 5. 6.

The acid is a product of the formation of carbonic acid from carbon dioxide and water. Acid precipitation occurs when rain, snow, or fog has a pH that is more acidic than 5. 6. Acid precipitation is caused primarily by sulfur oxides and nitrogen oxides in the atmosphere. These molecules react with water to form strong acids that fall to the surface with rain or snow. The major source of these oxides is the burning of fossil fuels (coal, oil, and gas) in factories and automobiles. The presence of tall smokestacks allows this pollution to spread from its site of origin to contaminate relatively pristine areas thousands of kilometers away.

In 2001, rain in the Adirondack Mountains of upstate New York had an average pH of 4. 3. The effects of acids in lakes and streams are more pronounced in the spring during snowmelt. As the surface snows melt and drain down through the snowfield, the meltwater accumulates acid and brings it into lakes and streams all at once. The pH of early meltwater may be as low as 3. Acid precipitation has a great impact on the eggs and the early developmental stages of aquatic organisms that are abundant in the spring. strong acidity can alter the structure of molecules and impact ecological communities.

Direct impacts of acid precipitation on forests and terrestrial life are more controversial. Acid precipitation can impact soils by affecting the solubility of soil minerals. Acid precipitation can wash away key soil buffers and plant nutrients such as calcium and magnesium ions. It can also increase the concentrations of compounds such as aluminum to toxic levels. This has done major damage to forests in Europe and substantial damage of forests in North America. Progress has been made in reducing acid precipitation.

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