The Hydrogen Bonds Between The Water Molecules example essay topic

Water is vital for life, with out water life on earth would cease to exist as every living organism must have water in order to survive. The total amount of water on Earth is fixed, (75% of the earth is covered in water) and most is recycled and re-used. The largest reservoirs are oceans and open seas. Very little is actually contained within living organisms, although water is a major constitute of most life forms. Water is a major component of cells, typically forming between 70 and 95% of the mass of the cell. Therefore humans are made from approximately 80% water by mass as opposed to some soft bodied, aquatic creatures such as jellyfish are made of up to 96% water.

Water also provides an environment for organisms to live in. this is why when space probes are dispatched to other planets in search of life they look for the presence of water to determine if life is possible there. Water itself is a simple chemical compound, each molecule composed of 2 hydrogen atoms and one oxygen atom: H O. The hydrogen and oxygen atoms are bonded covalently. Water is not a linear molecule; the two hydrogen atoms form a bond with the oxygen at the angle of 104.5. Covalent bonds are formed by sharing electrons in the outer orbits of the electron cloud. However, in water the large numbers of protons in the oxygen nucleus have a stronger attraction for these shared electrons than the comparatively tiny hydrogen nuclei. This pulls the electrons slightly closer to the oxygen nucleus and away from the hydrogen so that the oxygen develops a slight negative charge and the hydrogen a slight positive charge.

This makes the molecules strongly dipolar. This charge means that when water molecules are close together the positively charged hydrogen atoms are attracted to the negatively charged oxygen atoms of another water molecule thus forming a weak hydrogen bond. These bonds, though individually weak hold the molecules together with a considerable force because of the sheer number of them. The chemistry of water leads to some exceptionally important, physical properties.

At the temperatures and pressures found on the surface of the Earth, water can be found in all three physical states: solid, liquid and gas. On the basis of its molecular weight (18) water should be a gas at room temperature, however, due to the interaction of water molecules it is a liquid. Pure water freezes at 0 degrees Celsius, because of molecular association its maximum density is reached at 4 degrees C. This permits solid water (ice) to float on liquid water. This molecular association also accounts for the strong surface tension and viscosity exhibited by liquid water, which permits certain animals to move about on its surface and aids water to penetrate tiny spaces in the soil and elsewhere.

Water can dissolve polar or ionic substances and can keep them in solution because of its own polar properties. Substances that dissolve in water are known as 'hydrophilic's substances. Water will dissolve ionic substances that would normally require a large amount of energy to break such as sodium chloride: NaCl. Sodium chloride is made up of positive and negative ions held in its structure by the strong attraction between its positive sodium ions and negative chloride ions.

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. Water also separates covalently bonded molecules such as glucose and sucrose because the polar hydroxyl groups (-OH) in its structure forms hydrogen bonds with the water separating the molecules from each other. Water's properties as a solvent are vital to life as most biochemical reactions such as respiration occur in solution. This is why cell cytoplasm contains about 90% water. Water cannot dissolve hydrophobic substances such as fats and oils, these are used by organisms as cell membranes to separate cells and also as waterproofing as they prevent water from entering the organism if it is covered in a hydrophobic substance.

Water also has many thermal properties as a temperature stabiliser. Water has a boiling point of 100 oC and a melting point of 0 oC, this is unusual for a molecule of its size (water's RMM is 18) because other molecules of a similar size such as CO 2 - RMM 44 and ammonia NH 4 - RMM 18, are all gaseous at room temperature whereas water is a liquid. This is because of the hydrogen bonds which hold the water molecules in a liquid state. Water also has a high specific heat capacity, the result of this is that it takes 4.2 Joules of energy to raise one gram of water by 1 oC. This means that it takes a lot of heat energy 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 also has a high latent heat of fusion from solid to liquid.

It requires 300 J per gram of ice to melt it to water. This means that water stays liquid. This is vital in the case of cytoplasm in cells which is made of a high percentage of water because once frozen the cell would be irreparably damaged. The freezing point of water is also lowered by solutes because the soluble molecules disrupt the hydrogen bonds making the water freeze at a lower temperature and it easier to melt ice.

As there are many solutes in cytoplasm the water will not freeze until well below 0 oC and the cells are protected until the temperature gets extremely low. As water cools its density increases and the hydrogen bonds between the water molecules take on a more latticed formation as ice. Yet ice floats on the surface of water which means its density must be lower than that of water. Water is at its most dense at 4 oC 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 4 oC and aquatic life can continue.

Water is the medium for many reactions, especially those which occur in cells. Water is key in condensation reactions where water is removed from molecules to bond them together, this occurs with many sugars and carbohydrates. Water is also used to split up molecules by adding water. This is called hydrolysis and is essential to animals and plants because it allows them to utilise stored foods which are in long chains by breaking off smaller molecules.

Respiration produces water as a by-product. A large amount of energy is produced by oxidising hydrogen, which contains so much energy it is an explosive gas, into water. Photosynthesis uses water as a source of hydrogen atoms which are needed to produce glucose which is then stored in the plant cells as starch or used for respiration. Without water these two essential reactions would not occur and life would not be able to continue on earth.

An important property of water is also its transparency, it allows sunlight to pass through it so aquatic plants can photosynthesis and on a larger scale, allow life on earth to begin, since life started in the oceans with small organisms which relied on sunlight for the reaction photosynthesis to take place. Water molecules are highly cohesive because of the hydrogen bonds between the molecules. Water forms spherical droplets which have the maximum inner area and least surface area when in contact with a hydrophobic material. The cohesive properties of water allows plants to pull up water through xylem vessels from the roots to the leaves, this is called the transpiration stream. It also means that the water molecules where the water meets the air will be tightly held together and the water molecules below them to form an elastic film known as surface tension. Small creatures can get stuck in the surface water because they cannot break the water surface tension, creatures like water skaters can move across the surface of the water without sinking as they have hydrophobic feet which stops them from breaking the surface tension.

Because of water's strong hydrostatic forces water is incompressible. This provides support for soft bodied creatures such as worms, slugs and jellyfish which therefore do not require a supporting skeletal system. Water allows cells filled with water to become turgid and due to its incompressibility plants can support themselves. In conclusion, water's unique properties make it perhaps the most biologically important substance on the planet Water is an unusual substance, mostly due to its hydrogen bonds, its properties allow it to act as a solvent, a reactant, as a molecule with a cohesive properties, as an environment and as a temperature stabilizer. No other substance shares similar properties to water and in the way that one single molecule can possess such varied and essential characteristics.