Water can exist in three states: solid (ice), liquid or gas (vapor). Liquid water is probably the medium in which life originated on the Earth and it is in water that life evolved for its first billion years. In this section, will be explored how the structure and interactions of water molecules make water essential to life.
Water has a unique structure and special properties
The water molecule H2O has unique chemical features. As we learned in the preceding sections, water is a polar molecule that can form hydrogen bonds. In addition, the shape of water is a tetrahedron. The four pairs of electrons in the outer shell of oxygen repel one another, producing a tetrahedral shape. These chemical features explain some of the interesting properties of water, such as the ability of ice to float, the melting and freezing temperatures of water, the ability of water to store heat, and the ability of water droplets to form. These properties are described in detail below.
In water’s solid state (ice), individual water molecules are held in place by hydrogen bonds, creating a rigid, crystalline structure in which each water molecule is hydrogen-bonded to four other water molecules. Although the molecules are held firmly in place, they are not as tightly packed as they are in liquid water. In other words, solid water is less dense than liquid water which is why ice floats in water. If ice were to sink in water, as almost all other solids do in their corresponding liquids, ponds and lakes would freeze from the bottom up, becoming solid blocks of ice in winter and killing most of the organisms living in them. Once the whole pond had frozen, its temperature could drop well below the freezing point of water. But, because ice floats, it forms a protective insulating layer on the top of the pond, reducing heat flow to the cold air above. Thus, fish, plants and other organisms in the pond are not subjected to temperatures lower than 0°C, the freezing point of pure water. The recent discovery of liquid water below the polar ice on Mars has created speculation that life could exist in that environment.
Melting and freezing
Water is a moderator of temperature changes. Compared with other nonmetallic substances of the same size, molecular ice requires a great deal of heat energy to melt. In the opposite process — freezing — a great deal of energy is lost when water is transformed from liquid to solid.
Water contributes to the surprising constancy of the temperature found in the oceans and other large bodies of water throughout the year. The temperature changes of coastal land masses are also moderated by large bodies of water. Indeed, water helps minimize variations in atmospheric temperature across the planet. This moderating ability is a result of the high heat capacity of liquid water. The specific heat of a substance is the amount of heat energy required to raise the temperature of 1 gram of that substance by 1°C. Raising the temperature of liquid water takes a relatively large amount of heat because much of the heat energy is used to break the hydrogen bonds that hold the liquid together. Compared with other small molecules that are liquids, water has a high specific heat.
Evaporation and cooling
Water has a high heat of vaporization, which means that a lot of heat is required to change water from its liquid to its gaseous state (the process of evaporation). Once again, much of the heat energy is used to break hydrogen bonds. This heat must be absorbed from the environment in contact with the water. Evaporation, thus, has a cooling effect on the environment — whether a leaf, a forest or an entire land mass. This effect explains why sweating cools the human body: as sweat evaporates off the skin, it uses up some of the adjacent body heat.
Cohesion and surface tension
In liquid water, individual water molecules are free to move about. The hydrogen bonds between the molecules continually form and break. In other words, liquid water has a dynamic structure. On average, every water molecule forms 3.4 hydrogen bonds with other water molecules. This number represents fewer bonds than exist in ice but it is still a high number. These hydrogen bonds explain the cohesive strength of liquid water. This cohesive strength permits narrow columns of water to stretch from the roots to the leaves of trees more than 100 meters high. When water evaporates from the leaves, the entire column moves upward in response to the pull of the molecules at the top. Water also has a high surface tension which means that the surface of liquid water exposed to the air is difficult to puncture. The water molecules in this surface layer are hydrogen-bonded to other water molecules below. The surface tension of water permits a container to be filled slightly above its rim without overflowing and it permits small animals to walk on the surface of water.
Water is the solvent of life
A living organism is over 70 percent water by weight, excluding minerals such as bones. Many substances undergo reactions in this watery environment. Others do not and, thus, form biological structures (such as bones). A solution is produced when a substance (the solute) is dissolved in a liquid (the solvent) such as water (forming an aqueous solution). Many of the important molecules in biological systems are polar and, therefore, are soluble in water.
Reactions that take place in an aqueous solution can be studied in two ways:
– Qualitative analysis deals with substances dissolved in water and the chemical reactions that occur there.
– Quantitative analysis measures concentrations or the amount of a substance in a given amount of solution.
Fundamental to quantitative thinking in chemistry and biology is the mole concept. A mole is the amount of an ion or compound (in grams) whose weight is numerically equal to its molecular weight. Thus, a mole of table sugar (C12H22O11) weighs 342 grams.