1/1/10 5:21 AMWater Molecule

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Water Molecule

The Water Molecule and its Properties

The water molecule is formed from two hydrogen atoms and one oxygenatom. The bonding angle of the two hydrogens is almost 105 degrees ratherthan 180 degrees which would make the molecule symmetrical. This causesit to be dipolar, giving it a positive and negative side which accounts for itsunique properties. This allows the formation of hydrogen bonds betweenadjacent molecules. There is a weak intermolecular force of electrostaticattraction between the molecules which is known as van der Waals force.This causes the molecules to act as larger units than the individualmolecules.

Water is a very unusual compound; it is very common and is found in allthree conditional states, solid (as ice), liquid (as water) and gas (as watervapor). Other types of bonding can occur, such as covalent bonding (as seenin the formation of molecular oxygen) or ionic bonding (as seen in theformation of salt or sodium chloride[NaCl]). Hydrogen bonding can break upthe electrical attraction of atoms of solids and dissolves them.

1/1/10 5:21 AMWater Molecule

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In ice crystals the water molecules are widely separated, while in the liquidform they are closer together although less tightly bound. Therefore ice isbulkier and less dense and floats on water. If we compare the freezing andboiling points of water with what one would predict from extrapolating themolecular weights of other molecules, we see that it would be predicted tofreeze at -90 degrees C and boil at -68 degrees C. What a different world wewould have. So much for the validity of extrapolation.

The heat capacity of water is high compared to other common materials.This means that it can absorb or can lose a lot of heat energy withoutchanging its temperature very much. This buffers the environment againstlarge, rapid temperature changes. An example is the more moderate climateof a coastal location compared to one far inland. The diel temperaturechange of the surface waters of the oceans (or lakes, or even a swimmingpool) is small compared to the diel temperature change of the surroundingair. This is due to the high heat capacity of water.

As freshwater goes from ice to liquid or from liquid to gas, it undergoes anobvious change of state. The amount of heat energy that is required tochange its temperature in the same state is referred to as the specific heat.Specific heat can be defined as the amount of heat energy required to raisethe temperature of 1gm 1 degree C. It is expressed in calories. A calorie isdefined as the amount of heat required to raise the temperature of 1gmliquid water 1 degree C. The specific heat of liquid water is 1.0 calories whileit is 0.5 calories for ice. If we look at a graph of temperature versus heatinput, we can follow the change from ice at -100C to water vapor at 150degrees C. In order to get the ice to the melting temperature, it requires100*0.5 or 50 calories. To change from ice at 0 degrees C to water at 0degrees C requires an additional 80 calories (the heat of fusion or melting). Itnow takes 100 calories to heat the water from 0 degrees C to 100 degrees C(100*1). To change from liquid to water vapor at 100 degrees C requires anadditional 540 calories. This is called the heat of evaporation orcondensation. It explains why it seems to take so long to boil water on thestove when it seems about to boil. If we had a constant heat supply under apot at the rate of heating raised to water from, say, 20 degrees C to 100degrees C in 4 minutes, it would take another 6 minutes 45 seconds to boilthe water. Also remember that when water vapor condenses (as in rain), itgives up this energy.

Surface tension of water is high. In fact, water has the highest surfacetension of any common liquid except mercury. ( Here are some othercomparisons.) It is the tendency of water molecules to attract to each otheror cohere to each other at the surface of any water. It can be demonstratedin the formation of a drop of water, of heavier than water objects floating onthe surface or in capillary action in a glass tube.

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