Pure water is an odourless, tasteless and colourless liquid. This is an important point as this allows plants to photosynthesise deep under water. If this was not possibly then life on Earth might have failed to exist, as life is believe to have originated in an aqueous environment. The fact that it is colourless means that there is a high transmission of visible light and this means that light may penetrate deeply into living tissues.
Water has a freezing point of 0°C (32°F) and a boiling point of 100°C (212°F) under standard atmospheric pressure. This makes it very different as water has a relative molecular mass of 18 but similar sized molecules such as methane which has a relative molecular mass equallying 16, ammonia (r.m.m = 17) or carbon dioxide (44) are all gases. The molecules in gases can move around freely and independently of each other as they are widely spread. This is different from liquids where the molecules are closer together. Because water is a liquid at room temperature it is suitable for the chemistry of life e.g. most enzymes, especially those in animals have an optimum temperature of between 30°C and 50°C. Water exists as a liquid under the conditions on Earths surface because of the unique hydrogen bonding. These bonds hold the molecules close together. This is the important property that living things depend on. The shape of the water molecule is triangular rather than linear as the angles through the nuclei of the hydrogen and oxygen is approximately 105°. Although water is electrically neutral the negatively charged electrons are drawn away from the positively charged hydrogen nucleus by the hydrogen–oxygen bonds from the oxygen nucleus. This means because of the angle size there is a net charge of positive on the hydrogen atoms and a net charge of negative on the oxygen atom. This is why a water molecule is called a polar molecule because it has an unequal distribution of electrical charge. Weak hydrogen bonds are formed as a result of this charge separation with other water molecules. Hydrogen bonds are also formed with any charged particles dissolved in water, and with charged surfaces. This gives it some unusual properties.
Waters achieves its maximum density (when it’s molecules are most densely packed) at the temperature 4°C (39°F) when it is still a liquid. Due to the more open arrangement of the water molecules in the ice lattice it expands upon freezing. This is unusual as most liquids contact when they cool and so reach their maximum density at their freezing point. So when water freezes the ice is less dense than the water and so it floats on the surface. This prevents the bulk of the water (sea, pond or lake) freezing as the ice on the surface insulates the water below it. This is important because it means that the life living below the surface of the water can survive.
Another important property that water has is that it has very high surface tension, higher than any other substance except for mercury. The reason for this is that at the surface of the water the hydrogen bonds are orientated to point inward toward the other water molecules. Although water has a high surface tension, it surprisingly has a low viscosity, as the water molecules are able to slide past each other quite easily despite the way in which they face. The fact that water has a high surface tension means that water forms droplets and runs off e.g. on a plant leaves the water forms droplets and run off but if this didn’t happen the plant could drown????? Some animals take advantage of this and use the high surface tension to land on and to move about on, for example, a water boatman uses the surface of a pond to skate around on. Water low viscosity allows water to flow though narrow capillaries. Water doesn’t break under tension; it adheres strongly to most surfaces and won’t break the water column when it is drawn though narrow tubes. This is all down to the fact that water has extremely strong adhesive and cohesive properties. This also makes capillarity possible along with low viscosity. Due to these properties osmosis is able to happen, as this needs water to pass though very small spaces in cell walls form an area of high concentration to an area of low concentration. As the water column does not break or pull part it allows water to be lifted by forces applied at the top that means tall trees are able to move water up the xylem. Water is one of the best know ionising agents. It is known as the universal solvent as most substances will dissolve in water. Water reacts with metal oxides to form acids. It combines with certain salts to form hydrates. Also many important chemical reactions rely on water to act as a catalyst.
A great deal of heat energy is required to raise the temperature of water and this is because water has a very high heat capacity higher than any other know substance. This is because a lot of energy is needed to break the hydrogen bond, which hold the molecules together. This means that water takes a relatively slow time to heat up and cool down. This is significant as aquatic environment are slow to change temperature giving the living organisms a chance to adapt. Also bulky organisms have a stable temperature even when the external temperature changes. The evaporation of water needs a lot of heat resulting in evaporation of sweat from the skin or transpiration from leaves cooling having a cooling effect.
I conclude that although water seems like a very simple substance it has some amazing and unique properties and while researching this topic I found that right down to the atoms it’s made up of and the bonds that hold it together water is in fact a very complex substance. This is the reason why it is the substance that all life on Earth depends on, whether it is a tall tree taking advantage of the fact that water has high tensile strength so it can draw water up though the xylem or the fact that water can pass though narrow cell walls so osmosis can occur. I think that many people take this remarkable substance for granted.
