A molecule of water is composed of two atoms of hydrogen and one atom of oxygen. The one and only electron ring around the nucleus of each hydrogen atom has only one electron. The negative charge of the electron is balanced by the positive charge of one proton in the hydrogen nucleus. The electron ring of hydrogen would actually prefer to possess two electrons to create a stable configuration. Oxygen, on the other hand, has two electron rings with an inner ring having 2 electrons, which is cool because that is a stable configuration. The outer ring, on the other hand, has 6 electrons but it would like to have 2 more because in the second electron ring, 8 electrons is the stable configuration. To balance the negative charge of 8 (2+6) electrons, the oxygen nucleus has 8 protons. Hydrogen and oxygen would like to have stable electron configurations but do not as individual atoms. They can get out of this predicament if they agree to share electrons (a sort of an energy "treaty"). So, oxygen shares one of its outer electrons with each of two hydrogen atoms, and each of the two hydrogen atoms shares it's one and only electron with oxygen. This is called a covalent bond. Each hydrogen atom thinks it has two electrons, and the oxygen atom thinks that it has 8 outer electrons. Everybody's happy, no?
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Studies have shown that clustering of water molecules occurs in solutions because of so-called hydrogen bonds (weak interaction), which are about 10% of the covalent water bond strength. This is not inconsiderable and energy is required to break the bonds, or is yielded by the formation of hydrogen bonds. Such bonds are not permanent and there is constant breaking and reforming of bonds, which are estimated to last a few trillionths of a second. Nonetheless, a high proportion of water molecules are bonded at any instant in a solution. But this structure leads to the other important properties of water. We will consider, for the purposes of this course, only six of these important properties:
As mentioned above, these properties have importance to physical and biological processes on Earth. Effectively, large amounts of water buffer Earth surface environmental changes, meaning that changes in Earth-surface temperature, for example, are relatively minor. Thus, the high heat capacity of water promotes continuity of life on Earth because water cools/ warms slowly relative to land, aiding in heat retention and transport, minimizing extremes in temperature, and helping to maintain uniform body temperatures in organisms. However, there are other effects of water properties as well. Its low viscosity allows rapid flow to equalize pressure differences. Its high surface tension allows wind energy transmission to sea surface promoting downward mixing of oxygen in large water bodies such as the ocean. In addition, this high surface tension helps individual cells in organisms hold their shape and controls drop behavior (have you seen "An Ant's Life"?). Also, the high latent heat of evaporation is very important in heat/water transfer within atmosphere and is a significant component of transfer of heat from low latitudes, where solar energy influx is more intense to high latitudes that experience solar energy deficits. Take a few minutes to learn why water is the most fascinating and important substance in the universe. Page 3These student materials complement the Water Science and Society Instructor Materials. If you would like your students to have access to the student materials, we suggest you either point them at the Student Version which omits the framing pages with information designed for faculty (and this box). Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials.
Water does not give up or take up heat very easily. Therefore, it is said to have a high heat capacity. In Colorado, it is common to have a difference of 20˚ C between day and night temperatures. At the same time, the temperature of a lake would hardly change at all. This property originates because energy is absorbed by water as molecules are broken apart or is released by molecules of water associating as clusters. Take a few minutes to watch the video below to help you understand heat capacity. « Previous Page Next Page » Water has an amazing ability to adhere (stick) to itself and to other substances. Hydrogen Bonds Hydrogen bonds form when hydrogen atoms covalently bonded to nitrogen (N), oxygen (O), or fluorine (F) in the form of covalent compounds such as ammonia (NH3), water (H2O) and hydrogen fluoride gas (HF). In these molecules, the hydrogen atoms do not pull as strongly on the shared electrons as the N, O, or F atoms. Therefore, the molecules are polar; the hydrogen atoms become positively charged and are able to form hydrogen bonds to negative ions or negatively charged parts of other molecules (such as the N, O, and F atoms that become negatively charged in these compounds). Hydrogen bonds are not true bonds like covalent bonds or ionic bonds. Hydrogen bonds are attractions of electrostatic force caused by the difference in charge between slightly positive hydrogen ions and other, slightly negative ions. These attractions are much weaker than true ionic or covalent bonds, but they are strong enough to result in some interesting properties.
A molecule of water has two hydrogen atoms. Both of these atoms can form a hydrogen bond with oxygen atoms of different water molecules. Every water molecule can be hydrogen bonded with up to three other water molecules (See Fig. 3-7). However, because hydrogen bonds are weaker than covalent bonds, in liquid water they form, break, and reform easily. Thus, the exact number of hydrogen bonds formed per molecule varies. Cohesion Molecules of pure substances are attracted to themselves. This sticking together of like substances is called cohesion. Depending on how attracted molecules of the same substance are to one another, the substance will be more or less cohesive. Hydrogen bonds cause water to be exceptionally attracted to each other. Therefore, water is very cohesive. We see evidence of water’s cohesiveness every day – in water drops and in streams of water. Our experience with water, however usually involves water touching something else or being acted upon by gravity. To really get a sense of water’s cohesiveness, scientists looked at the behavior of water in space (see Fig. 3-8). In space, water is able to form perfectly round spheres because the attraction of water to itself pulls the water into the shape with the least amount of surface area compared to the volume – a sphere. Fig. 3-8: Water drops in space. (A)European Space Agency astronaut Pedro Duque of Spain watches a water bubble float between him and the camera, showing his image refracted, on the International Space Station. (B) A large water sphere made on a 5 cm diameter wire loop by U.S. astronaut Dr. Pettit. Adhesion Adhesion is similar to cohesion, but it involves unlike (i.e. different) substances sticking together. Water is very adhesive; it sticks well to a variety of different substances. Water sticks to other things for the same reason it sticks to itself – because it is polar so it is attracted to substances that have charges. Water adheres to many things— it sticks to plants, it sticks to dishes, and it sticks to your eyebrows when you sweat. In each of these cases water adheres to or wets something because of adhesion. This is why your hair stays wet after you shower. Molecules of water are actually sticking to your hair (Fig. 3-9). Adhesion also explains why soil is able to hold water (and form mud). Fig. 3-9: Child with wet hair (a) and enlarged photo of individual drops of water on wet hair (b).
Investigate the cohesive and adhesive properties of water. Surface Tension The cohesion of water creates surface tension where air and water meet. You observed this in Activity 2 when you looked at the ability of water to pile on top of a penny without spilling over (see Fig. 3-11). The hydrogen bonds between water molecules at the surface are analogous to the to members of a red rover team holding hands. When playing red rover, team members line up to form a chain to try and prevent someone from running through their joined hands (Fig. 3-12). The linked hands represent the hydrogen bonds between water molecules that can prevent an object from breaking through. Of course, a faster or heavier person can more easily break through the hand bonds during a game of red rover. Similarly a heavy object, or one that isn’t carefully placed on the surface of the water, can break the surface tension. Remember, for example, how the paper clip needed to be placed carefully on the water’s surface in order for it to float (Activity 2).
Because of hydrogen bonding, water can actually support objects that are more dense than it is. Water molecules stick to one another on the surface, which prevents the objects resting on the surface from sinking. This is why water striders and other insects can “walk” on water! It is also what allowed you to float a paper clip on water and the reason why a belly flop off the high dive into a pool of water is painful. See Fig. 3-14.
In fact, because liquid water is so good at sticking to itself and to other substances, it can rise up a surface against the force of gravity! We call this climbing tendency of water capillarity (also called capillary action). You saw capillarity in Activity 2 when you placed glass tubing in water. Capillarity starts when the water molecules nearest the wall of the tube are attracted to the tube more strongly than to other water molecules. The water molecules nearest the glass wall of the tube rise up the side (adhesion), dragging other water molecules with them (cohesion). Water level in the tube rises until the downward force of gravity becomes equal to than the adhesion and cohesion of water. |
Hydrogen and oxygen atoms are held together within an individual water molecule by bonds
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