Water, although exhibiting a simple chemical structure, demonstrates astounding properties that have time and time again challenged our chemical theories. These properties are those that make this vital substance necessary for life. Unlike similar compounds which are gasses at room temperature, water exists as a liquid. It acts as an effective insulator, and can be used as a cooling agent in organic systems. Water is often called the "universal solvent" because of its ability to dissolve polar substances. This liquid also has the remarkable properties of cohesion and adhesion, leading to the phenomenon of capillary action. Of course, the natural question that scientists asked after said phenomena were observed was: why? The believed causes and effects of the properties of water will be explored in this article.
Thermal Properties of Water The thermal properties of water are essential for life. Perhaps the most fundamental thermal property of water to life is its large specific heat capacity (4.19J/(gC)). The water molecule has many degrees of freedom, allowing water to absorb a very large amount of thermal energy with only a small increase in temperature. This property contributes to the stability of organisms that contain water, as the specific heat capacity decreases the fluctuations in the organism's temperature, allowing fundamental chemical reactions to proceed more reliably. Water's high heat of vaporization and low heat of fusion allow water to exist as a liquid within the temperature range in which essential chemical reactions can occur. The high heat of vaporization and low heat of fusion are due to the nature of the water molecule. Unlike similar hydrogen compounds which exist as gasses at room temperature (such as H2S), water molecules have the ability to hydrogen-bond to each other, a property due to the high electronegativity of the oxygen atom. The hydrogen nucleus is left more exposed, allowing it to form a strong positive pole which attracts the negatively-charged oxygen atoms of other molecules. As the molecules of water bond strongly to one another, and because of the high specific heat capacity of water (many internal degrees of freedom), the energy required to overcome the attraction between the water molecules is very large, causing the heat of vaporization to be high. As water molecules can store large amounts of thermal energy, much thermal energy must be removed before the molecules form a solid, resulting in a low heat of fusion. The three-dimensional structure of the water molecule results in a unique property: water's maximum density is at about 4C rather than in its solid form. The water molecule has a bond angle of 105 degrees, and the intermolecular hydrogen bonding occurs only between oxygen and hydrogen atoms within the molecules. Therefore the three-dimensional structure of solid water contains much empty space between each molecule, resulting in a lower density than liquid water, as liquid water is not set in a fixed rigid shape.
Solvent Properties Water is often called the "universal solvent" and rightly so, as its remarkable properties allow for great solubility of many polar compounds. This property is essential for life, as it allows water to be used as an effective medium in which chemical reactions can occur. All major chemicals in cells are found in aqueous form (DNA, proteins and carbohydrates for example). When a substance is dissolved in water, its surface area is maximized, as it is broken into its most fundamental units. Ionic compounds become aqueous ions, and polar compounds are separated into individual molecules. Kinetic molecular theory states that an increased surface area results in an increased rate of reaction, allowing biological processes to proceed more efficiently.
When a substance is dissolved in water, the following steps take place:
1. Inter-molecular bonds in the water must be broken.
2. Inter-molecular bonds in the solute must be broken.
3. New intermolecular bonds are formed between water and the particles of solute.
The first two steps are endothermic (requiring energy), while the final is exothermic (releasing energy). In the first step, the hydrogen-oxygen hydrogen bonds must be overcome. The greater the thermal energy of the water molecules, the easier this step is to accomplish, as the energy required to overcome the bond will be partially fulfilled by the thermal energy. Thus the solubility of substances in water increases at higher temperatures. Water's solvent properties allow it to not only function as a reaction medium, but as an effective transport agent. Blood is mostly composed of water, which carries essential nutrients to body cells, and carries waste products back toward the heart.
Transparency Water's transparency allows aquatic photosynthesizing plants access to sunlight. Aquatic photosynthesis allows for a source of dissolved oxygen in bodies of water, allowing life to exist under the surface.
Water's cohesion is due to the strong intermolecular hydrogen bonding that the substance experiences. This cohesion allows water to remain in the liquid phase, as the strong attractive forces prevent individual particles from escaping in large quantities and forming vapour at low temperatures. This is extremely important in plants, where water loss must be minimised. As well, cohesion is necessary in capillary forces. As water molecules are attracted to the sides of their container (adhesion), other water molecules are attracted to those that are adhering to the container, causing capillary forces. These forces are essential in plants, as water must be transported from the soil to the trunk and leaves.