The ability to form hydrogen bonds and its dipole nature makes water a very useful substance. For example, it is an excellent solvent meaning it can easily dissolve lots of other substances. In fact, water can dissolve more substances than any other liquid. This happens because when a molecule of a another substance mixes with water molecules (providing it is hydrophilic), the poles on either end of the molecule attract the oppositely charged ions of the molecule to be dissolved. As they are being pulled in opposite directions the molecules break apart into their ions which then form new hydrogen bonds with the water molecules. This is particularly useful in the blood where plasma is a solution carrying nutrients important for the body’s cells and also waste for removing from the body. However, even though it’s known as the ‘Universal solvent’ water is not able to dissolve hydrophobic substances. These are generally non-polar molecules and examples of hydrophobic liquids include oil and fat.
Another useful property of water is its high surface tension which acts like a thin ‘skin’ on the surface of the liquid. This is because of the cohesive forces acting between individual water molecules within the liquid. At the surface, the molecules only have this force in one direction, down towards the other water molecules. There is no such force between water and the molecules which make up the air as these are non-polar. Therefore, the surface molecules of water are drawn down towards other similar molecules creating a pressure or tension wherever water meets the air. This is why if you slightly overfill a glass or beaker with water, it will not run over but instead form a slight bubble (curved surface) to prevent from spilling. In the opposite fashion, when a narrow container such as a test tube is filled with water, a meniscus will be formed at the surface creating a curved edge of water. More useful examples of water’s surface tension include several species of insect and even reptile being able to effectively walk on water creating a whole new ecosystem on the surface of water masses such as ponds.
Similarly, the cohesion of water molecules pulling together creates ‘capillary action’. Capillary action or capillarity is when a small amount of water is pulled up through a narrow space, even though this sometimes means counteracting the force of gravity as in the example of a pipette or straw. This occurs because the adhesion forces between the water and glass molecules attract the first water molecules to the sides of the tube. The cohesive forces between the water molecules then attract more water upwards and this process continues until the gravitational force is too strong. Because of this property, water can travel along very narrow veins in roots and leaves much more easily than if it did not exist.
Also largely due to the hydrogen bonds in water, it has a relatively high boiling point and specific heat capacity. This is because, even though each individual hydrogen bond is quite weak, there are lots of them which means that it requires quite a lot of energy to break them all before the covalent bonds can even begin to be broken. A high heat capacity means that very large bodies of water such as oceans are able to ‘take in’ and hold lots of heat without actually warming up very much (the same is also true in reverse- it’s difficult to cool oceans down significantly) which acts as an excellent global temperature regulator.
As well as regulating global climates, water also plays a large part in homeostasis- the process of regulating internal body temperatures so that we don’t get sick from things like heat stroke and pneumonia. A common way our bodies can cool us down is by producing sweat on the surface of our skin. Because sweat is mainly water-based, when it evaporates in the heat, it is able to evaporate that heat energy with it leaving us feeling cooler.
A specific property that is unique to water as a naturally occurring substance and also makes life possible is the fact that water in its solid form is less dense than as a liquid meaning ice is able to float. This is because the individual water molecules expand and form an ordered pattern when frozen. They must join with the optimum number of other molecules in order to become a solid and, partly because of the angle at which the hydrogens are joined to the oxygen; this creates an expanded, hexagonal pattern. Ice being able to float means that large bodies of water freeze on the surface first allowing sea-life and marine ecosystems to be able to survive, even when the water is frozen over. The original layer of ice acts as insulation to the rest of the water and prevents it from freezing too far down. As it is widely believed that all life originated from simple undersea organisms, this suggests that no life would be possible if not for life being able to survive under water. In addition, if all the ice caps in the North Pole were to sink, the amount of water that would be displaced would entirely engulf all land.
Another important feature of the water molecule is that it’s an amphiprotic molecule meaning that it can both accept protons and be a proton donor. Therefore, in any amount of pure water or aqueous solution, water molecules can rearrange themselves by losing a hydrogen nucleus to form a hydroxide ion (OH-) and a free proton (H+), of which the proton will then react with another water molecule to form hydronium (H3O). In other words, the dynamic equilibrium equation for the autoionization of water is: 2H2O ←→ H3O+ + OH-. This characteristic makes water a good regulator of pH which is especially useful in cells alongside ongoing reactions.
Importantly, water is also transparent so living organisms are able to see through it. This is another factor making marine ecosystems possible and it also allows vision through the tears (saline solution) in our eyes.
Conclusively, the properties of water, due in large to its molecular structure, is what makes it possible for any form of life to survive. It sustains living organisms and balances systems both to create a suitable environment for life and with the organisms themselves, regulating specific variables like pH, temperature and, obviously, water content.
