Some Unusual Properties of Water

Some Unusual Properties of Water
Water molecules attract each other as a result of hydrogen bonding. This ionic attraction is 1/20 as strong as covalent bond in water?s liquid form. They form, break and re-form with great frequency; each hydrogen bond last only a few trillionths of a second, but the molecules bond promiscuously to a succession of partners. At any instant, a substantial percentage of all the water molecules are bonded to their neighbors, giving water more structure than other liquids. Collectively, this phenomenon is known as cohesion. A related property of cohesion is adhesion, a water molecule?s attraction to other polar surfaces. This is, again, directly attributed to water?s high polarity. Hydrophilic substances/materials, having similar strong polarity, are attracted to water through polar interactions. If you have ever tried to separate two glass slides stuck together with a film of water, you can appreciate how tightly water adheres to glass, a hydrophilic substance. (Water Module)
Biological Impact:

Water?s cohesive property is especially crucial to the survival of plants. Cohesion due to hydrogen bonding contributes to the transport of water against gravity in plant xylem. As transpiration, water evaporation, in leaves occurs, water in the plant xylem is ?tugged? into the leaves to replace evaporated water. This upward pull is transmitted along the vessel all the way to the roots. This cohesive property allows tall evergreen trees to survive. Water cohesion also leads to another property known as surface tension, a measurement of the strength and toughness of the surface of a liquid to penetration. Because of water?s high surface tension, due to hydrogen bonding, insects such as the water strider are able to walk across water without breaking through the surface.

High Specific Heat

Water stabilizes air temperatures by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler. Water is so effective as a heat bank because of its high specific heat, which means that a slight change in its own temperature is accomplished by the absorption or the release of a large quantity of heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1? C. Actually, a calorie is defined as the specific heat of water.

One can attribute water?s high specific heat to hydrogen bonding. Because of hydrogen bonding, molecules of water must absorb significant amounts of heat before they can gain even a small amount of kinetic energy, the energy of motion. Water molecules are relatively strongly attracted to each other, so independent movement can only be achieved by absorbing larger quantities of heat energy.

Biological Impact:

The high specific heat of water means that relative to other materials, water will change its temperature less when it absorbs or loses a given amount of heat. Specific heat can be considered the measurement of resistance to temperature change of a substance when it absorbs or releases it. This property is especially important in aquatic environments. By warming up only a few degrees, a large body of water can absorb and store a huge amount of heat from the sun during daytime and summer. At night and during winter, the gradually cooling water can warm the air. The high specific heat of water also makes ocean temperatures quite stable, creating a favorable environment for marine life. Also, since organisms are made primarily of water, they are able to resist changing their own temperatures better than they would be if they were made of a liquid with a lower specific heat. (gly Electronic Tutor)

High Heat of Vaporization

Molecules of any liquid are, at least, moderately attracted to each other. This is what gives liquid such a compact volume. However, molecules moving fast enough, possessing enough kinetic energy, can overcome these attractions and enter the gas phase. This change of phase is known as vaporization or evaporation. However, the tendency of a liquid to enter the gas phase varies and is directly dependent upon the strength of intermolecular attractions. In water?s case, hydrogen bonding severely lowers that kinetic energy of water molecules. It takes a significant amount of energy for a given molecule of water to gain a small amount of kinetic energy-specific heat. It takes an even larger amount of energy for a given molecule to completely escape the intermolecular forces to become a gas. This explains water?s high heat of vaporization, the quantity of heat a liquid must absorb for 1 g of it to be converted into the gaseous state. Compared to other liquids, water has a high heat of vaporization, mainly due to the high level of intermolecular forces that must be overcome to reach the gaseous state. (Water in Biological Systems)

Biological Impact:

Water?s high heat of vaporization helps moderate earth?s climate. A considerable amount of solar heat absorbed by tropical seas is dissipated during the evaporation of surface water. Then, as moist tropical air circulates toward the poles, its condensation to form rain releases heat.

As a substance evaporates, the surface of the liquid that remains behind cools down. This evaporative cooling occurs because the ?hottest? molecules, those with the highest kinetic energy, are the molecules with the best chance of leaving as a gas. Temperature stability of lakes and ponds is directly attributed to water?s evaporative cooling mechanism. It also prevents the overheating of terrestrial organisms as well. Sweating is the most common form of evaporative cooling. Our bodies perspire, releasing water (sweat) onto our skin, allowing it to absorb excess body heat. As sweat absorbs heat, it evaporates, leaving the skin surface and our bodies cool.

Water as a Versatile Solvent

A solution is properly defined as the homogeneous mixture of a solute and, its dissolving agent, the solvent. For centuries, scientists have searched for the perfect solvent, but none was better than water. Water?s chemical properties provide insight to this dissolving ability. It is mainly attributed to its polar, asymmetric structure. Because it has both positively and negatively charged regions, it is excellent at separating solids with similar ionic/polar properties. Take table salt (NaCl) for instance. NaCl is an ionic compound in which sodium has given up entirely its outer shell electron to chlorine, resulting in a ionic imbalance; sodium and chlorine are positively and negatively charged respectively. Tiny water molecules are able to use this to their advantage. Positive portions of the water molecule, hydrogen, are attracted to chlorine as is negatively charged oxygen to the positively charged sodium. This is what allows salt to dissolve in water. Water?s dissolving abilities are not limited to only ionic compounds, but span through all solutes that contain polar (charged) regions. This is why water is such a versatile solvent. (Ch 2 & 3, Chemistry and Water)

Biological Impact:

Water is the solvent of life. An enormous diversity of solutes are dissolved in the water of biological fluids such as blood and the liquid within all cells.

Freezing and Expansion of Water

As obvious as it may seem, ice floats. But the reason behind solid water?s ability to float is another one of water?s unusual properties. Water, unlike other liquids, is less dense as a solid than as a liquid. While other materials contract when they solidify, water expands. But like all other properties of water, this unusual property is, again, a result of hydrogen bonding. At temperatures above 4? C, water behaves as all liquids do, expanding as it warms, contracting as it cools. Water begins to freeze when its molecules are no longer moving vigorously enough to break hydrogen bonds to neighboring molecules. As the temperature approaches 0? C, water becomes locked into a crystalline lattice, each water molecule bonded to a maximum of four partners. In this structure, the hydrogen bonds distance the molecules so that ice is 10% less dense than liquid water.

Biological Impact:

The expansion of water as it solidifies is especially important to the fitness of the environment. Ice?s natural tendency to float prevents the freezing of lakes, ponds and even oceans during winter, allowing organisms within these ecosystems to survive. Further, because ice floats, it is able to prevent the loss of heat from bodies of water, providing insulation for aquatic organisms. The freezing of water and melting of ice also ease the transition between seasons for organisms. When water solidifies into ice or snow, the heat released warms the surrounding air, helping to temper the autumn. Similarly, during the spring, melting ice absorbs heat, tempering the transition into the warmer season.

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