![]() The polarity of the water molecule makes it an effective solvent and is important in its many roles in living systems. These spheres of hydration are also referred to as hydration shells. A negatively charged chloride ion is surrounded by the partially positive charges of hydrogen atoms in water molecules. A positively charged sodium ion is surrounded by the partially negative charges of oxygen atoms in water molecules. In the case of table salt (NaCl) mixed in water, the sodium and chloride ions separate, or dissociate, in the water, and spheres of hydration are formed around the ions. This is referred to as a sphere of hydration and serves to keep the particles separated or dispersed in the water. The charged particles will form hydrogen bonds with a surrounding layer of water molecules. Water is, therefore, what is referred to as a solvent-a substance capable of dissolving another substance. (credit a: modification of work by Jane Whitney credit b: modification of work by Carlos Ponte) Water Is an Excellent Solventīecause water is polar, with slight positive and negative charges, ionic compounds and polar molecules can readily dissolve in it. Ice’s lower density enables it to (b) float on water. Figure 2.8 (a) The lattice structure of ice makes it less dense than the freely flowing molecules of liquid water. If this did not happen, plants and animals living in water would freeze in a block of ice and could not move freely, making life in cold temperatures difficult or impossible. In lakes, ponds, and oceans, ice will form on the surface of the water, creating an insulating barrier to protect the animal and plant life beneath from freezing in the water. This means that ice floats on the surface of a body of water (Figure 2.8 b). When frozen, ice is less dense than liquid water (the molecules are farther apart). These bonds remain intact and begin to form a rigid, lattice-like structure (e.g., ice) (Figure 2.8 a). Evaporation of sweat, which is 90 percent water, allows for cooling of an organism, because breaking hydrogen bonds requires an input of energy and takes heat away from the body.Ĭonversely, as molecular motion decreases and temperatures drop, less energy is present to break the hydrogen bonds between water molecules. This process results in the release of individual water molecules at the surface of the liquid (such as a body of water, the leaves of a plant, or the skin of an organism) in a process called evaporation. As energy input continues, the balance between hydrogen-bond formation and destruction swings toward the destruction side. ![]() This means that water moderates temperature changes within organisms and in their environments. Because these bonds can be created and disrupted rapidly, water absorbs an increase in energy and temperature changes only minimally. Increased energy disrupts the hydrogen bonds between water molecules. Water absorbs a great deal of energy before its temperature rises. As the motion increases, energy is higher and thus temperature is higher. Temperature is a measure of the motion (kinetic energy) of molecules. The hydrogen bonds in water allow it to absorb and release heat energy more slowly than many other substances. Figure 2.7 As this macroscopic image of oil and water shows, oil is a nonpolar compound and, hence, will not dissolve in water. These nonpolar compounds are hydrophobic (“water-fearing”) and will not dissolve in water. Hydrogen bonds are not readily formed with nonpolar substances like oils and fats. When a substance readily forms hydrogen bonds with water, it can dissolve in water and is referred to as hydrophilic (“water-loving”). Water also attracts other polar molecules (such as sugars), forming hydrogen bonds. Each water molecule attracts other water molecules because of the positive and negative charges in the different parts of the molecule. Because of these charges, the slightly positive hydrogen atoms repel each other and form the unique shape. There is no overall charge to a water molecule, but there is a slight positive charge on each hydrogen atom and a slight negative charge on the oxygen atom. The shared electrons spend more time associated with the oxygen atom than they do with hydrogen atoms. The hydrogen and oxygen atoms within water molecules form polar covalent bonds. Approximately 60–70 percent of your body is made up of water. Water is one of the more abundant molecules in living cells and the one most critical to life as we know it. Do you ever wonder why scientists spend time looking for water on other planets? It is because water is essential to life even minute traces of it on another planet can indicate that life could or did exist on that planet.
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