Water Water Water

Water, as the source of life, is extraordinarily important to us and to all life on earth. We see it, use it and drink it in our daily lives. But why is water essential for life? In other words, what properties of water make it the prime ingredient of life?

Water with its molecular formula, H2O, has as many as 20 properties, making it the essential basis of life. But only 5 properties, which I personally think is quite unique and significant,  are going to be discussed.

1. universal solvent

Scientists often call water "universal solvent" simply because it can dissolve more substances than any other liquids. Since water is a polar molecule, its positive end is attracted to negatively charged ions or the negative sides of other polar molecules, and its negative side is attracted to positively charged ions or the positive sides of other polar molecules. For example: if table salt (NaCl) is added in water, it quickly falls apart. The positively charged sodium ion (Na⁺) will  be surrounded by eager water molecules with the negative sides and the negatively charged chlorine ion (Cl^-) will be surrounded by other water molecules with positive sides. This properties of water allows transport of nutrients vital to lives. 

2. cohesion and adhesion
Hydrogen bonds between water molecules make them stick together, which is called cohesion. This results in high surface tension at the surface of water, which means that it’s actually pretty hard to break the surface of water compared to other liquids. Surface tension is what allows some things to float on water even if they’re denser than water. Adhesion is the the sticky nature of water to other substances rather than to itself. You can see this property when some water drops stays inside of the glass when you try to pour out the water. This is the same as how water clings living things. Most plants have adapted to take the advantage of water's adhesion to help transport water from the roots to the top. In one of the tallest plant: redwood tree, water moves more than 310 feet above the ground from its roots to leaves. 

3. high specific heat capacity
Water absorbs/ releases more heat than other substances for each degree Celcius increase/decrease. This property of water makes it excellent for cooling and transferring heat in thermal and chemical processes. This is also why the temperature at coastal area is relatively constant than that of inland. Thus, organisms living in the ocean, river are provided with a constant environmental temperature, and high water content of plants and animals living on land also helps them to maintain a relatively constant internal temperature. 

4. unique density
We all know that for most compounds, the solid is denser than the liquid, meaning that the solid will sink to the bottom of the container holding the liquid. But this is not for water. The truth is that water actually becomes less dense when it freezes, which allows the solid form: ice, to float on the liquid form: water. This explains why a pond freezes from the surface down, rather than from the bottom up. This property of water is quite important for organisms that live underwater. If frozen water sunk, small bodies of water would be more likely to freeze completely in the winter. But thank to density, a layer of ice instead insulates the underlying water effectively, allowing many aquatic organisms to survive through the winter. 

5. buffer nature
Before I start, it is important to know that water molecules have tendency to ionize, that is they dissociate into hydrogen ions (H+) and hydroxide ions (OH-), which are weak acid and weak base. And we know that buffer happens between a weak acid and a weak base, which it resist pH changes by donating or accepting hydrogen ions (H+) as needed. This is important because, as you’ll learn later, most of the chemical processes that occur in living organisms are highly sensitive to pH, and drastic changes in pH can cause some serious trouble. The dissociated water molecules are what give water its buffering ability. If  an acid is added to solution, some of the free OH^- ions will bind to the newly added H⁺ ions, which will moderate the decrease in pH. Similarly, if a base is added, the base will bind to the free H⁺ ions in solution, which will moderate the increase in pH.