11.0.25.01
Environmental Qualities of Water
The Earth has been called the Water Planet. Water is one of the most abundant compounds on this planet and it has unique properties, which gives water great environmental significance. The Earth is the only planet in our solar system with an environment containing water in its three physical forms; solid, liquid and gaseous. The unique properties of water and the significance to the environment are several. Consider the fact that a temperature range of only 100 degrees centigrade (see Appendix C, page A-3 for Metric Unit Conversion Tables) separates the three physical forms of water, i.e., ice (0o C = 32o F), liquid (between 0o and 100o C) and steam (100o C = 212o F). Biological life has evolved to be able to live throughout this range. A listing of the physical properties of water and the environmental significance of these factors are:
1.The specific heat is a measure of the amount of heat that must be absorbed by a substance to increase its temperature. All substances absorb heat at different rates. The amount of energy required raising a gram of a substance one-degree centigrade at room temperature and standard pressure (a constant pressure at sea level) is called the specific heat expressed as a ratio to the specific heat of water which is 1. Thus the specific heat of water is 1, which means it takes 1 calorie of energy to raise 1 gram of water 1 degree centigrade at room temperature and standard pressure. When you compare the value of water to oxygen with a specific heat of 0.0156 you can see that it takes a great deal of energy to heat water. Likewise water must lose heat to cool and compared to oxygen it must lose a lot of heat energy to reduce its temperature one-degree. Thus water is slow to boil or cool and must absorb or convect a great deal of heat to gain or lose a degree C. In the global environment water is thus a good thermal buffer. A great deal of the suns energy must be absorbed by water to heat it and a great deal must be lost to cool it. The Oceans are the major source of energy trapping from the sun because of this fact. The absorption of heat by the oceans from the sun serve as the major source of energy storage on earth. The slowness with which the ocean (water) heats or cools, is one of the reasons maritime climates tend to be less extreme than continental climates.
Because water has such a high specific heat it makes a great thermal buffer. It is well known that large bodies of water will cool the atmosphere in summer and warm it in winter. So-called maritime climates tend to be moderate rather than extreme because the water gets the coldest in March and warmest in September. The air temperature tends to warm and cool more quickly and thus a body of nearby water will moderate the atmospheric temperature.
2. It might be expected that if a 100 calories of energy will raise a gram of water to the vapor state that one more calorie would cause the water to convert to a gaseous state. Because of the unique bonding of hydrogen to oxygen to hold water together in the liquid phase it takes 539 calories per gram to break the bond and allow the water to enter the gaseous state (water vapor). This is known as the heat of vaporization. Kerosene only requires 67 calories per gram to vaporize.
Because water has such a high heat of vaporization it is able to take a huge amounts of heat over the surface of the Earth. When this water vapor condenses it releases the 539 calories and warms the surrounding atmosphere. A swamp cooler works effectively in a dry climate because for every gram of water that evaporates 539 calories of heat are absorbed, thus cooling the surrounding atmosphere as heat is withdrawn from the atmosphere to evaporate the water trickling down the cooler pads.
3. Heat of fusion. 80 calories per gram are needed to melt ice from a solid state to a liquid state. Additionally until the last bit of ice is melted, the temperature will not change. Thus ice must absorb huge quantities of heat before it will melt.
4. Maximum density. Water becomes increasingly dense as it cools such that it reaches its maximum density at 4o C while it is still liquid. As the water gets colder than 4o C it becomes less dense. Thus water freezes from the top down. If water were at maximum density at the freezing point the freezing water would sink to the bottom of the ocean or lake and freeze from the bottom up, likely never to thaw again. This maximum density at 4o C is what causes radiators and engine blocks to crack upon freezing because the water as it freezes it expands. In nature this alternating expansion and contraction in the cracks of rocks causes the breaking of (exfoliating) of rocks in the mountains. The use of antifreeze, even salt, will depress the freezing point to less than 0oC depending how much antifreeze is added to the water.
5. High dielectric constant. A water molecule is polar, which means at one end of the molecule, the molecule is positive and at the other it is negative. This characteristic reduces the attraction of atoms one to the other when in water, making water an excellent solvent.
6. High viscosity. The high viscosity index of water creates drag on things moving through it. This characteristic also gives it a high surface tension, which will through greater buoyancy support greater loads than most liquids.
7. Gas Solubility. In water metabolically important gases are inversely proportionally soluble to the atmospheric concentration of these gases. The fact that 100 ml of water can dissolve 0.34gm of carbon dioxide but a mere 0.007gm of oxygen and 0.003gm of nitrogen has important significance to the whole debate about atmospheric gas exchange between the oceans and the atmosphere. The ocean contains much more carbon dioxide per unit volume than does the atmosphere.