This slight charge gives water its various properties; the first I will discuss is its solvent properties.
Water can act as a solvent. “Things” can dissolve in solvents and therefore “things” can dissolve in water. Substances that dissolve in water sre known as hydrophilic substances Water can dissolve polar or ionic substances, because they contain a charge.
As can be seen from the diagram (right), Ionic substances such as sodium chloride, NaCl, are made up of positive and negative ions. Sodium chloride is held in it's structure by the strong attraction between it's positive sodium ions and negative chloride ions. Normally these ionic attractions require a large amount of energy to break but when put into water the negative oxygen side of the water molecules cluster around the positive sodium ions Na+ and the positive hydrogen atoms cluster around the negative chloride ions Cl-. The attraction between the Na+ and Cl- ions is weakened as the ions are separated. As the H20 molecules have surrounded the Na+ and Cl- Ions, they prevent the ions from joining back together and so keeping the substance in the solution
Water’s property as a solvent is biologically important to life as most biochemical reactions such as respiration occur in solution. A medium, that is water, is required for the transportation, as reaction of certain substances. Blood plasma is mostly water. Blood is needed to transport vital substances around organisms. Water cannot dissolve hydrophobic substances such as fats and oils.
Water also has many thermal properties. It has a high boiling point, 100oC, which is unusual for a compound of such small molecular mass, (Mr H2O = 18). Other molecules of similar size such as Carbon dioxide (Mr CO2 = 44) are gas at room temperature where as water is a liquid. The water molecules have a weak, partial negative charge at one region of the molecule (the oxygen atom in water) and a partial positive charge elsewhere (the hydrogen atoms in water).
Thus when water molecules are close together, their positive and negative regions are attracted to the oppositely charged regions of nearby molecules. The force of attraction, shown on the diagram as a dotted line, is a hydrogen bond. Each water molecule is hydrogen bonded to four others. Individual bonds are weak but the sheer number of them means that the total force keeping the molecules together is considerable.
As water has millions of hydrogen bonds, it causes it to have a high heat capacity. This means that water takes substantial heat to raise the temperature of water significantly but once warm, it cools slowly. This is essential to life where internal body temperature has to be maintained at a constant temperature and fluctuations can result in a breakdown of essential processes. Large bodies of water will remain at an almost constant temperature with only very gradual changes, which makes temperature regulations for organisms far more straightforward.
Because of the large number of bonds holding water molecules together, it takes 2 kJ per gram of water, which is a considerable amount of energy to separate the bonds and turn the liquid to vapour. Water is therefore described as having a high latent heat of evaporation. Animals use this property of water by using excess body heat to evaporate water from their surfaces, resulting in them transferring a lot of energy into the environment but only losing a little water. Sweating and panting are based on this principle.
Water’s freezing property is also quite unique. The density of water when frozen (ice) at 0oC is less than liquid water. Most substances when turning from liquids to a solid usually become denser, as the energy in them is reduced, so they are held closer together, but this is not the case with water. As water cools, its density does increase. Hydrogen bonds between the water molecules take on a more latticed formation as ice. Yet ice floats on the surface of water, which means it’s, density must be lower than that of water. Water is at it's most dense at 4oC which is when its bonds are closest together. When water freezes the lattice arrangement of its structure move apart slightly and it floats on the surface. This means that the layer of ice insulates the water below which stays at 4oC and aquatic life can continue.
The last important property of water is that it has a high surface tension and cohesion. Surface tension can be seen in test tubes where we see a curved meniscus. This is due to hydrogen bonds causing a film of water to curve in the glass tube. Surface tension is important for some insects such as water skaters and mosquito larvae that live on surface film. They are able to survive because the film allows them to be held above the water. Mosquito larvae hang upside down with the air passage held above the water by surface tension. Its ability to survive depends upon the surface tension. Without it, the larvae would sink and die.
Cohesion is when things stick together, and are attracted to each other. Water molecules are attracted to each other by hydrogen bonds. If one molecule is pulled the neighbouring molecules as a result are also dragged along. Eg, if you apply pressure in a straw, all the water is pulled up. This is an important property as it enables water to be transported upwards, such as in plants. The ability of a plant to dissolve nutrients in roots and move them along the plant depends on water’s property of cohesion.
