The chemical nature and biological significance of the water molecule
THE CHEMICAL NATURE AND BIOLOGICAL SIGNIFICANCE OF THE WATER MOLECULE
Water is a vital resource for all living things. It is believed that life originated in water. The bodies of living organisms are mainly composed of water, and all living things need water to survive. The many unique properties of water cause it to have a tremendous impact on our physical environment as well. The water molecule is made up of two hydrogen atoms and one oxygen atom. The oxygen atom shares 2 electrons with each hydrogen atom. Both molecules thereby complete their other shells, the hydrogen atom with 2 electrons and the oxygen atom with 8.
This diagram shows how the hydrogen and two oxygen atoms combine together to form a water molecule.
An important feature of water is its polar nature. Water molecules are dipolar. That is they form at an angle and as a consequence the positively charged hydrogen ions are on one side of the molecule and the negatively charged oxygen ions are on the other side. This polarization leads water molecules to be attracted to each other by a phenomenon called hydrogen bonding. Hydrogen bonding causes water molecules in the atmosphere to combine and fall as raindrops. Hydrogen bonds are rather weak (20 kJ/mol) but significant and in large numbers, provide strength. The oxygen atom of water is strongly electronegative and tends to draw electrons away from the hydrogen atoms. The oxygen has a partial negative charge and the two hydrogen atoms each have a partial positive charge. The positive charges of the hydrogens are electrostatically attracted to the negatively charged oxygens of two neighbouring water molecules leading to hydrogen bonding between water molecules.
This diagram shows show the dipolar nature of the water molecule.
Water is called a universal solvent because of its ability to dissolve many other substances, which is mainly due to its dipolar nature. When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute. The partially negative dipoles of the water are attracted to positively charged components of the solute, and the partially positive dipoles are attracted to the negatively charged components on the solute.
In general, ionic and polar substances such as acids, alcohols, ...
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Water is called a universal solvent because of its ability to dissolve many other substances, which is mainly due to its dipolar nature. When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute. The partially negative dipoles of the water are attracted to positively charged components of the solute, and the partially positive dipoles are attracted to the negatively charged components on the solute.
In general, ionic and polar substances such as acids, alcohols, and salts are easily soluble in water but non-polar substances such as fats and oils are not. An example of an ionic solute is table salt; the sodium chloride, NaCl, separates into Na+ cations and Cl- anions, each being surrounded by water molecules. The ions are then easily transported away from their crystalline lattice into solution. An example of a non-ionic solute is table sugar. The water forces hydrogen bonds to the dipolar regions of the sugar molecule and allows them to be carried away into solution. These solvent properties of water are vital in biology, because many biochemical reactions take place only in solution. For example, reactions in the cytoplasm and blood.
Water has the highest heat of vaporization of any liquid (540 calories/gram). This involves the rupturing of H¯ bonds to form water vapour and means that large amounts of heat energy are needed to evaporate even small amounts of water. The release of this energy is the driving force behind the world's weather systems. In addition, this property helps to keep the body temperature of living organisms constant, since a large amount of heat can be dissipated by vaporizing a small amount of water. This process generally requires about 40kJ/mol of energy and is great for evaporative cooling. High thermal conductivity, high heat capacity and high melting point also contribute to the thermal buffering capacity of water.
The ability of water to stabilize temperature depends on its relatively high specific heat. The specific heat of a substance is defined at the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1ºC. The specific heat of water is 1.00 cal/g ºC. Compared with most other substances, water has an unusually high specific heat. For example, ethyl alcohol, the type in alcoholic beverages, has a specific heat of 0.6 cal/g ºC.
The relationship between heat and temperature change is usually expressed in the form shown below where C is the specific heat.
Because of the high specific heat of water relative to other materials, water will change its temperature less when it absorbs or loses a given amount of heat. Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat. Water resists changing its temperature; but when it does change its temperature, it absorbs or loses a relatively large quantity of heat for each degree of change.
We can trace water's high specific heat, like many of its other properties, to hydrogen bonding. Heat must be absorbed in order to break hydrogen bonds, and heat is released when hydrogen bonds form. A calorie of heat causes a relatively small
change in the temperature because must of the heat energy is used to disrupt hydrogen bonds before the water molecules can begin moving faster. When the temperature of water drops slightly, many additional hydrogen bonds form, releasing a considerable amount of energy in the form of heat.
The specific heat capacity of water is very relevant to life on Earth today. By warming up by only a few degrees, a large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer. At night and during winter, the gradual cooling of water can warm the air. This is the reason coastal areas generally have milder climates than inland regions. The high specific heat of water also makes ocean temperatures quite stable, creating a favourable environment for marine life. Thus, because of its high specific heat, the water that covers most of Earth keeps temperature fluctuations within limits that permit life. Also, because organisms are made primarily of water, they are more able to resist changes in their own temperatures than if they were made of a liquid with a lower specific heat.
Another importance of water's specific heat capacity is the fact that its hydrogen bonds keep the molecules far enough apart to make ice about 10% less dense than liquid water at 4ºC. When ice absorbs enough heat for its temperature to increase to above 0ºC, hydrogen bonds between molecules are disrupted. As the crystal collapses, the ice melts, and molecules are free to slip closer together. Water reaches it greatest density at 4ºC and then begins to expand as the molecules move faster. The ability of ice to float, because of the expansion of water as it solidifies is an important factor in the fitness of the environment. If ice sank, then eventually all ponds, lakes, and even the oceans would freeze solid, making life as we know it impossible on Earth.
The density of water is 1g/millilitre. It is a very unique property because unlike any other substance it is least dense when it is in a solid state (ice) and most dense just before it freezes at 39.2ºC. This unique property of water causes the 'turnover' and mixing of nutrients and oxygen in temperate lakes and ponds. Water that is warmer or cooler than 4ºC is less dense, which means that as water cools and gets closer to 4ºC the molecules move closer together and take up less space. As water gets colder than 4ºC, the molecules move farther apart and water becomes lighter and less dense. As ice at 0ºC, water expands in volume by about 10% and is at its least dense. This causes it to be able to float.
Water has extremely high surface tension. Surface tension can be defined as the force per unit length pulling perpendicularly to a line in the plane of the surface. Because of its high surface tension, water can support fairly large objects placed carefully on its surface. Surface tension is due to the cohesion of its molecules, which is intermolecular attraction between like molecules in the liquid state, relative to the adhesion of its molecules, which is the attraction between water and other molecules. Surface tension can be greatly affected by certain solutes, such as fatty acids and lipids because they may become concentrated at interfaces. These molecules usually have both polar and non-polar regions. This feature is important when water is carried through xylem up stems in plants; the strong intermolecular attractions hold the water column together, and prevent tension caused by transpiration pull. Other liquids with lower surface tension would have a higher tendency to 'rip', forming vacuum or air pockets and leaving the xylem vessel unusable.
