The charges are only very slight, not big enough to count as one whole charge, and are therefore given the Greek symbol, delta. The effect of the uneven charges on the water makes the molecule polar. As one side is slightly positive and the other slightly negative, it is also referred to as a dipolar molecule.
The slightly negative side (oxygen) of one water molecule, is attracted to the slightly positive side (hydrogen) of another water molecule. This force, called a hydrogen bond, is stronger than the dipolar force, but not as strong as the covalent bond.
In a liquid state, one water molecule commonly has two or three hydrogen bonds with other water molecules. This changes because the molecules slide over each other, constantly moving, forming and breaking hydrogen bonds. As water cools, the molecules become more densely compacted and is densest at 4°C. Frozen water or ice is surprisingly less dense than liquid water. This is because a water molecule has the potential to have a maximum of four hydrogen bonds. At 4°C, the water molecules are too compact, and so at 0°C, the molecules need to spread out slightly to make room for all molecules to have four hydrogen bonds.
IMPORTANT PROPERTIES TO LIVING ORGANISMS
Transparency
Water is transparent. This is important because it will allow light to pass through it. This is vital for aquatic plants that need to photosynthesize. Plants, as the beginning of the food chain, are the only living organisms that can transfer the sun’s light energy directly into chemical energy. Without the photosynthesis of aquatic plants, none of the marine animals would be able to survive in a water habitat.
Eye humours are partially made up of water. Although this is also for support purposes, if water was not transparent, then none of the animals would be able to see with light energy. Instead, animals may have to navigate using their common senses (i.e. sound, small, taste, touch) or more specialized senses (i.e. echolocation, infrared, ultraviolet vision).
Cohesion
Cohesion is the attraction found between water molecules, the result of hydrogen bonds. The attraction is greater when the distance between the molecules is smaller. This means that the cohesive forces are stronger in solids than liquids and in liquids than gases. Cohesive forces are particularly useful for small living organisms, such as pond skaters. The attractive forces mean that a body of water is kept inwards from spreading out, producing a skin on the surface, termed surface tension. Pond skaters distribute their weight across a large area, thus are able to skate on top of the surface layer without the skin breaking. Cohesion is important, because without surface tension, these insects would drown.
Adhesion
Adhesion differs to cohesion, as it includes the attraction between water as well as other polar molecules. Like cohesion, adhesion is due to water’s polarity and the fact that hydrogen bonds connect two polar molecules. This adhesive quality makes water an excellent solvent. We can relate this property to living organisms, because water can then dissolve salts and sugars.
Water is able to dissolve any polar substance (whether fully or partially charged) because they surround ions with a hydration shell. In the example of common table salt (sodium chloride), the two ions are separated in water. The sodium, having lost an electron, becomes a cation (positively charged). As a rule, oppositely charged substances attract, and therefore the positive sodium is attracted to the slightly negative oxygen atom of water, and chlorine (the negative anion) is attracted to the slightly positive hydrogen atoms. This means that each ion is then surrounded by water molecules, known as a hydration shell, and it is said to have been dissolved.
This is important to all living organisms’ survival. Without water’s solvent properties, we would not be able to function as well. Our blood might not carry dissolved substances throughout our body as efficiently; plants might not be able to take up minerals from the soil; substances would not be able to pass through membranes if not dissolved in water.
Water’s adhesive property and how it relates to living organisms is also seen in plant’s capillarity. There is adhesion between water and the xylem vessels. The vessels, using adhesion, pulls water upwards. Due to cohesion, the water molecule being pulled up takes another water molecule along with it. The end result is a continuous uptake of water through the xylem vessels. The result is more effective if the xylem vessels were smaller in diameter and if they went vertically upwards.
Thermal properties
Water has a high specific heat capacity (s.h.c.), indicating that it requires a lot of energy to raise the temperature of 1 kg of water by 1°C and vice versa. This is especially important for living organisms that depend on their habitat temperatures remaining relatively constant. Fish, for example, are poikilothermic, meaning their body temperature reflects their surrounding temperature. If there was a sudden change in temperature, their body cells may not function as efficiently, as each type of cell is designed to function at an optimal condition. However, because water has such a high s.h.c., water habitats rarely fluctuate drastically in temperature, thus disturbances in the metabolic reactions are uncommon.
Water also has a high latent heat of vaporization (l.h.). This means that a great deal of energy is required for water to change states (i.e. from liquid to gas). This property makes water an effective coolant. When animals secrete sweat, it signifies overheating in the body. The high l.h. allows sweat to remain on the skin, collect as much of the body’s excess heat as possible before being evaporated and cooling the skin. If the l.h. of water was lower, the body would have an inefficient method of dealing with excess heat. If we were to sweat continuously, only to lose it quickly to the atmosphere, we would lead to dehydration.
The reason why water has such a high s.h.c. and l.h. is because of the hydrogen bonds. Like other substances, water has to gain enough l.h. energy to change states. However, water also has to gain enough energy to break away from the hydrogen bonds holding water together. This required extra energy results in water’s high boiling and melting point.
Support
As explained earlier, water’s cohesiveness has led to water’s density being very high. In other words, the molecules are very closely compacted. They cannot be compacted much further, and therefore water is said to be incompressible. This is useful for support all living organisms. All organisms contain some percentage of water. Water, an incompressible substance, provides support for plants (turgid pressure in the cells hold up and straighten the plant), animals (worms use the fact that like-charges repel, thus they can move by bunching up and pushing out), protection for mammalian fetus (water absorbs shock during the fetus’s growth period) and a medium for life (e.g. the sheer pressure from the weight of whales is taken off from their fragile organs in the weightless world of water).
Freezing
I explained earlier that ice is less dense than water, because the water molecules have to spread out before freezing to make room for each to have four hydrogen bonds. This results in ice floating in water. This is important for two reasons. The first is that living organisms living in aquatic environments do not freeze during the winter. The ice forms a cover layer atop a body of water, insulating the water beneath, so that it does not freeze up completely. The temperature underneath the ice is sufficient for some creatures adapted to live in cold temperatures. The second reason of importance is that it allows animals that do not dwell completely in the sea (i.e. polar bears, penguins, walruses, etc.) with a safe, floating area between vast stretches of water. If not for some of these icebergs, even these water-loving animals can drown from exhaustion.
In conclusion, water is given far less appreciation than it truly deserves. It is essential for all living organisms as a main constituent for photosynthesis and respiration. Its special properties, uncommon in many other molecules, enable our very survival.
