AP Biology: The Properties of Water
AP Biology: The Properties of Water
Water has many unique properties that make life possible on Earth. One property is cohesion. The cohesion property is properly defined as the binding of water molecules by hydrogen bonds. Water has this property as a result of the chemical bonding between water. Cohesion of the strong hydrogen bonds allows the water molecules to stick together, almost as a unit of one. A force exerted on one of the molecules will be exerted on all of the adjacent molecules as a result of cohesion. Cohesion, often with the cooperation of adhesion, the clinging of one substance to another, adds to the function and ability of water to overcome strong natural forces, such as gravity. When water is in its liquid state of matter, the hydrogen bonds are very frail and weak, about one-twentieth as strong as covalent bonds. The bonds are made, broken, and remade very quickly.
Each hydrogen bond lasts only a few trillionths of a second, but the constant synthesis of new bonds with a succession of partners acquires equilibrium. Therefore, a significant percentage of all the water molecules are bonded to their neighbors, making water a more orderly structured liquid than most other known liquids. A property related to cohesion is surface tension, a measure of how difficult it is to stretch or break the surface of a liquid. Water is known to have a greater surface tension than most other liquids.
An ordered arrangement of hydrogen-bonded water molecules is present at the boundary between water and air. As a result water behaves as though it is coated with an invisible film along the surface. An example how the cohesion of water affects the functioning of living organisms is present in plants. Evaporation from the leaves in plants pulls water up from the roots. Cohesion due to hydrogen bonding helps hold the column of water molecules together within the xylem vessels located in the stem or trunk of a plant. Adhesion helps the process by resisting the pull of gravity against the upward motion of the molecules.
Another property of water is its solubility. Water is known as the universal solvent of life. Many substances can be combined with water to form a solution, a homogeneous mixture between two substances. Water, in solutions, is known as the solvent. The substance or substances being dissolved is known as the solute. An aqueous solution is when a homogenous mixture where water is the solvent is present. As found by the medieval alchemists, water is the most soluble liquid. Although water is technically not universal, it is very versatile solvent. Water’s solubility is a result of its polarity. Water is a polar molecule, meaning that the opposite ends of the molecule of opposite charges. In a water molecule, the polar covalent bonds allow the oxygen region of the molecule to have a partial negative charge and the hydrogen regions to have a partial positive charge. When ionic crystals are placed in water, they are ionized. The partially negative ion from the crystal bond to the hydrogen ions in water.
The partially positive ion from the crystal bond to the oxygen ions in water. The sphere of water molecules around each dissolved ion is called a hydration shell. Water eventually dissolves all the ions. As a result, there is a solution containing two solutes from the salt homogeneously mixed with water, the versatile solvent. Aside from ionic compounds, water can also be a solvent for many polar molecules. An effect of the versatile solubility can be demonstrated in the functioning of many liquid substances of living organisms, such as blood, the sap of plants, and the liquid contained in cells. Water’s solubility allows for these liquids to have a universal concentration throughout the entire liquid, making the distribution of the ions or molecules in the solution equal.
Another property of water is its high specific heat. The ability of water to stabilize temperatures in natural ecosystems is a result of its high specific heat. Specific heat is defined as the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by one degree Celsius. Water’s specific heat is defined as one calorie per gram per degree Celsius. This information comes from the definition of a calorie, the amount of heat that causes one gram of water to change its temperature by one degree Celsius. Because of water’s high specific heat, water’s temperature will change less when it absorbs or loses a certain amount of heat. Water resists changing its temperature, and when it happens to change it, it absorbs or loses a large quantity of heat for each change in temperature. Water’s specific heat is a direct result of hydrogen bonding. Large amounts of heat must be absorbed in order to break the hydrogen bonds, and large amounts of heat are released when hydrogen bonds form.
One calorie doesn’t cause a large change in the temperature primarily because most of the heat energy is used to disrupt the hydrogen bonds before the water molecules can start moving faster. When the temperature drops slightly, many hydrogen bonds form, releasing a large amount of heat energy. Water’s high specific heat is directly related to life on Earth through climate. Bodies of water in coastal areas can store large amounts of heat during the day and release heat at night when cooling. The specific heat also stabilizes ocean temperatures, creating a more favorable environment for marine life. Therefore, as a result of water’s high specific heat, the water on Earth keeps temperature changes on land and in water within life-permitting limits. Animals are also mostly made of water, allowing them to resist changes in their own temperatures. Water is so abundant and present in everyday life that it’s easy to neglect the fact that it is an exceptional substance with many extraordinary qualities. Following the theme of emergent properties, water’s unique behavior can be traced to the structure and interactions of its molecules.