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Water Molecule



The Water Molecule and its Properties


Credit to NASA


The water molecule is formed from two hydrogen atoms and one oxygen atom. The bonding angle of the two hydrogens is almost 105 degrees rather than 180 degrees which would make the molecule symmetrical. This causes it to be dipolar, giving it a positive and negative side which accounts for its unique properties. This allows the formation of hydrogen bonds between adjacent molecules. There is a weak intermolecular force of electrostatic attraction between the molecules which is known as van der Waals force. This causes the molecules to act as larger units than the individual molecules.

Representation of water


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

Dipolar


In ice crystals the water molecules are widely separated, while in the liquid form they are closer together although less tightly bound. Therefore ice is bulkier and less dense and floats on water. If we compare the freezing and boiling points of water with what one would predict from extrapolating the molecular weights of other molecules, we see that it would be predicted to freeze at -90 degrees C and boil at -68 degrees C. What a different world we would 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 without changing its temperature very much. This buffers the environment against large, rapid temperature changes. An example is the more moderate climate of a coastal location compared to one far inland. The diel temperature change of the surface waters of the oceans (or lakes, or even a swimming pool) is small compared to the diel temperature change of the surrounding air. This is due to the high heat capacity of water.

As freshwater goes from ice to liquid or from liquid to gas, it undergoes an obvious change of state. The amount of heat energy that is required to change 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 raise the temperature of 1gm 1 degree C. It is expressed in calories. A calorie is defined as the amount of heat required to raise the temperature of 1gm liquid water 1 degree C. The specific heat of liquid water is 1.0 calories while it is 0.5 calories for ice. If we look at a graph of temperature versus heat input, we can follow the change from ice at -100C to water vapor at 150 degrees C. In order to get the ice to the melting temperature, it requires 100*0.5 or 50 calories. To change from ice at 0 degrees C to water at 0 degrees C requires an additional 80 calories (the heat of fusion or melting). It now 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 an additional 540 calories. This is called the heat of evaporation or condensation. It explains why it seems to take so long to boil water on the stove when it seems about to boil. If we had a constant heat supply under a pot at the rate of heating raised to water from, say, 20 degrees C to 100 degrees C in 4 minutes, it would take another 6 minutes 45 seconds to boil the water. Also remember that when water vapor condenses (as in rain), it gives up this energy.

In an ice structure


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

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