8(a) physical properties of water
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1/1/10 5:19 AM8(a) Physical Properties of Water
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CHAPTER 8: Introduction to the Hydrosphere
(a). Physical Properties of Water
We live on a planet that is dominated by water. More than 70% of the Earth'ssurface is covered with this simple molecule. Scientists estimate that thehydrosphere contains about 1.36 billion cubic kilometers of this substancemostly in the form of a liquid (water) that occupies topographic depressions onthe Earth. The second most common form of the water molecule on our planet isice. If all our planet's ice melted, sea-level would rise by about 70 meters.
Water is also essential for life. Water is the major constituent of almost all lifeforms. Most animals and plants contain more than 60% water by volume. Withoutwater life would probably never have developed on our planet.
Water has a very simple atomic structure. This structure consists of two hydrogenatoms bonded to one oxygen atom (Figure 8a-1). The nature of the atomicstructure of water causes its molecules to have unique electrochemical properties.The hydrogen side of the water molecule has a slight positive charge (see Figure8a-1). On the other side of the molecule a negative charge exists. This molecularpolarity causes water to be a powerful solvent and is responsible for its strongsurface tension (for more information on these two properties see the discussionbelow).
Figure 8a-1: The atomic structure of a water (or dihydrogenmonoxide) molecule consists of two hydrogen (H) atomsjoined to one oxygen (O) atom. The unique way in whichthe hydrogen atoms are attached to the oxygen atom causesone side of the molecule to have a negative charge and thearea in the opposite direction to have a positive charge. Theresulting polarity of charge causes molecules of water to beattracted to each other forming strong molecular bonds.
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When the water molecule makes a physical phase change its molecules arrangethemselves in distinctly different patterns (Figure 8a-2). The moleculararrangement taken by ice (the solid form of the water molecule) leads to anincrease in volume and a decrease in density. Expansion of the water molecule atfreezing allows ice to float on top of liquid water.
Figure 8a-2: The three diagrams above illustrate the distinctarrangement patterns of water molecules as they change theirphysical state from ice to water to gas. Frozen watermolecules arrange themselves in a particular highlyorganized rigid geometric pattern that causes the mass ofwater to expand and to decrease in density. The diagramabove shows a slice through a mass of ice that is onemolecule wide. In the liquid phase, water molecules arrangethemselves into small groups of joined particles. The factthat these arrangements are small allows liquid water tomove and flow. Water molecules in the form of a gas arehighly charged with energy. This high energy state causesthe molecules to be always moving reducing the likelihoodof bonds between individual molecules from forming.
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of bonds between individual molecules from forming.
Water has several other unique physical properties. These properties are:
Water has a high specific heat. Specific heat is the amount of energyrequired to change the temperature of a substance. Because water has a highspecific heat, it can absorb large amounts of heat energy before it begins toget hot. It also means that water releases heat energy slowly when situationscause it to cool. Water's high specific heat allows for the moderation of theEarth's climate and helps organisms regulate their body temperature moreeffectively.
Water in a pure state has a neutral pH. As a result, pure water is neitheracidic nor basic. Water changes its pH when substances are dissolved in it.Rain has a naturally acidic pH of about 5.6 because it contains naturalderived carbon dioxide and sulfur dioxide.
Water conducts heat more easily than any liquid except mercury. This factcauses large bodies of liquid water like lakes and oceans to have essentiallya uniform vertical temperature profile.
Water molecules exist in liquid form over an important range of temperaturefrom 0 - 100° Celsius. This range allows water molecules to exist as aliquid in most places on our planet.
Water is a universal solvent. It is able to dissolve a large number ofdifferent chemical compounds. This feature also enables water to carrysolvent nutrients in runoff, infiltration, groundwater flow, and livingorganisms.
Water has a high surface tension (Figures 8a-3 and 8a-4). In other words,water is adhesive and elastic, and tends to aggregate in drops rather thanspread out over a surface as a thin film. This phenomenon also causes waterto stick to the sides of vertical structures despite gravity's downward pull.Water's high surface tension allows for the formation of water droplets andwaves, allows plants to move water (and dissolved nutrients) from theirroots to their leaves, and the movement of blood through tiny vessels in thebodies of some animals.
Figure 8a-3: The following illustration shows how watermolecules are attracted to each other to create high surfacetension. This property can cause water to exist as an
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tension. This property can cause water to exist as anextensive thin film over solid surfaces. In the exampleabove, the film is two layers of water molecules thick.
Figure 8a-4: The adhesive bonding property of watermolecules allows for the formation of water droplets (Photo© 2004 Edward Tsang).
Water molecules are the only substance on Earth that exist in all threephysical states of matter: solid, liquid, and gas. Incorporated in thechanges of state are massive amounts of heat exchange. This feature playsan important role in the redistribution of heat energy in the Earth'satmosphere. In terms of heat being transferred into the atmosphere,approximately 3/4's of this process is accomplished by the evaporation andcondensation of water.
The freezing of water molecules causes their mass to occupy a largervolume. When water freezes it expands rapidly adding about 9% by volume.Fresh water has a maximum density at around 4° Celsius (see Table 8a-1).Water is the only substance on this planet where the maximum density of itsmass does not occur when it becomes solidified.
Table 8a-1: Density of water molecules at various temperatures.
Temperature (degrees Celsius) Density (grams per cubic centimeter)0 (solid) 0.91500 (liquid) 0.9999
4 1.000020 0.9982
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40 0.992260 0.983280 0.9718
100 (gas) 0.0006
Study Guide Additional Readings Internet Weblinks Citation: Pidwirny, M. (2006). "Physical Properties of Water". Fundamentals of Physical Geography, 2ndEdition. Date Viewed. http://www.physicalgeography.net/fundamentals/8a.html
Created by Dr. Michael Pidwirny, University of British Columbia Okanagan
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Copyright © 1999-2008 Michael Pidwirny
05/07/2009 15:21
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