Life as we know it would not exist without water. Over 70% of the planet is covered in water. And our bodies are around 70% water. But why is water so important? The structure of a water molecule, and the fact that it is capable of forming hydrogen bonds with multiple other water molecules simultaneously, provides water molecules with some very unique characteristics. These include the ability to act as a universal solvent, the properties of cohesion and adhesion, a high surface tension, a high heat capacity, and changes in density based on temperature. Let’s take a look of each of these in turn. Water is often called the universal solvent, which means that many substances dissolve in it. Water’s solvency has to do with the polarity and hydrogen bonding ability of water molecules. When a salt, such as sodium chloride is placed in water, the negative ends of the water molecules are attracted to the positively-charged sodium ions, and the positive ends of the water molecules are attracted to the negatively-charged chloride ions. This attraction causes the sodium and chloride ions to break up, or dissociate, in water. Why is this important? Since our cells are mostly water these solutions of ions and molecules allow chemical reactions to occur much more frequently, which is an important feature to allow organisms to respond to their environment. Water molecules are also cohesive and adhesive. The hydrogen bonds in water cause water molecules to stick together, a property known as cohesion. Polar bonds also give molecules an adhesive property, or the ability to cling to other polar surfaces. Why is this important? Well, water fills the internal transport systems of plants and animals. Because of these cohesive and adhesive properties water allows for the efficient transport of nutrients and wastes within an organism. Water also has a high level of surface tension. Because water molecules at the surface are more strongly attracted to each other than to the air above, water molecules cling tightly to each other. The surface tension of water explains why it beads up on waxy surfaces, and why water-striding insects are able to walk on the surface of water. Water has a high capacity for heat. What this means, is that the many hydrogen bonds that link water molecules let water absorb a large amount of heat without changing its chemical state, for example, from liquid to gas. This not only stabilizes the temperature of bodies of water, such as lakes and oceans, but also plays an important role in the physiology of an organism. Since life if based on water, and the temperature of water rises and falls slowly, living organisms are better able to maintain their normal internal temperatures, and are protected from rapid temperature changes. And finally, the density of water is based on its temperature. However, unlike many other compounds which contract when they the configuration of the hydrogen bonds in water causes it to expand when it freezes. This expansion causes ice to have a lower density than liquid water, and therefore ice floats on liquid water. If ice were more dense than liquid water, it would sink, and ponds and lakes would freeze solid, making life there impossible. Instead, bodies of water always freeze from the top down. The ice on the surface acts as an insulator to protect the water below from freezing. This protects aquatic organisms so that they can survive the winter.