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This reading introduces students to the chemical and physical properties of water - something you MUST understand in order to get a grip on life in water.
Reading 1: Water
1. Be able to describe the molecular characteristics of water
2. Be able to explain what makes water a polar molecule
3. Be able to comment on the significance of the polar nature of water
4. Be able to list and describe physical characteristics of water
5. Be able to describe the density anomaly of water
You have to learn about water if you want to be a marine biologist. Without being dramatic, it is safe to say that water is the most important and biologically valuable substance on Earth. Water is the stuff that makes aquatic environments different than terrestrial habitats, and most people know amazingly little about water and typically take it completely for granted until they don’t have it. For one thing, water is the only substance on Earth that exists naturally and simultaneously in its solid, liquid, and gaseous states. It’s unique physical and chemical properties make it a vital substance at all levels of biological organization, from molecules to the global ecosystem.
Characteristics of Water
Water: A Polar Molecule
Just about everyone knows that H20 is the chemical formula for water. Most people, however, don’t know what water is like at the molecular level and why this matters. Elemental oxygen has an atomic number of 8. Two of its electrons are in an inner electron orbital, but the other six electrons are in outer orbitals. A Lewis dot diagram for oxygen shows two orbitals populated with two electrons and two orbitals containing a single electron each (Fig. 1-1). The orbitals with one electron need one more electron each in order to be full and stable.
Figure 1-1. Lewis dot diagram for oxygen. Dots represent electrons. (Image: ARH)
Two hydrogen atoms form covalent bonds with one atom of oxygen to form water. The protons in the hydrogen atoms of H2O repel each other, and you might assume and that water forms a linear molecule with oxygen in the middle and hydrogen atoms sticking out both ends, but water is not linear. The hydrogen atoms are actually on the same side of the oxygen atom. Here’s why. There are pairs of electrons in oxygen’s other two orbitals. These are called lone pair electrons. The lone pair electrons are slightly more repulsive to each other than the protons of the hydrogen atoms are to each other. This causes the hydrogen atoms to be displaced to one side of the oxygen atom, and the two lone pair electrons remain on the other side (Fig 1-2). Because two of oxygen’s electron orbitals are full it forms bonds with up to only two other atoms, but if you include the lone pair electrons, water has a tetrahedral form. The bond angles in a uniform tetrahedral are all 109.5o, but water is an unequal tetrahedral. The bond angle between the hydrogen atoms in water is only 104.45o. This is because the lone pair electrons repel each other more strongly than the hydrogen atoms do, forcing the hydrogen nuclei closer together than they would otherwise be.
Figure 1-2. Tetrahedral nature, bond angle, and partial charges of water. O = oxygen atom; H = hydrogen atom; 2e- = lone pair electrons; δ+ = partial positive charge; δ- = partial negative charge. (Image: ARH)
The unequal distribution of electrons in water produces partial positive and partial negative charges (Fig. 1-2). A partial positive charge is associated with each hydrogen nucleus, and a partial negative charge is associated with each of the lone pair electrons. The existence of partial charges around a molecule produces a dipole moment. In water this imbalance is constant so each water molecule experiences a permanent dipole moment.
The permanent dipole moment makes water a polar molecule. One of the most significant things a polar molecule can do is form a hydrogen bond with another polar molecule. A hydrogen bond forms when a partial positive charge on one molecule comes into close proximity to a partial negative charge on another polar molecule. These opposite partial electrical charges attract each other and form a hydrogen bond. Because hydrogen bonds are weak, they readily form, break, and reform. A water molecule can form up to four hydrogen bonds at a time (Fig. 1-3). The number of hydrogen bonds existing between water molecules and between water molecules and other things is a function of temperature. The number and duration of hydrogen bonds between water molecules determines whether water is in its solid, liquid, or gaseous form. Water molecules are constantly reoriented in relation to each other as hydrogen bonds break and reform. Hydrogen bonds form and break 1011 to 1012 times per second in liquid water at 0oC, and 105 to 106 times per second in ice at 0oC.
Figure 1-3. Hydrogen bonding between water molecule. Dashed lines indicate hydrogen bonds. (Image: ARH)
Adhesive and Cohesive Nature of Water
Water forms hydrogen bonds with itself and with any other polar substance. This characteristic makes water makes a universal solvent. This means that any charged atom or polar molecule can dissolve in water. Substances that can dissolve in water include sugars, salts, ions, etc. Molecules lacking partial charges, also known as non-polar molecules, cannot readily dissolve in water. The ability of water to form hydrogen bonds with other polar molecules gives it an adhesive nature. A meniscus along the inner wall of a glass tube is an evidence of this characteristic (Fig. 1-6). The adhesive nature of water combined with its cohesiveness also produces capillary action.
Figure 1-4. The adhesive nature of water produces a meniscus because water adheres to the inner wall of the glass cylinder. (Image: ARH)
Water is also cohesive. This characteristic allows capillary action to occur, and it also produces the phenomenon called surface tension. A water molecule in the middle of a mass of water can form bonds in any direction, but water molecules at the surface can form bonds only laterally along the surface and with molecules below them (Fig. 1-5). Remember that water is tetrahedral. When molecules at the surface bond with their neighboring water molecules the natural tendency of these bonds is to be bent, but bonds between surface water molecules are put under stress since they also bond to molecules below them. The molecules below the surface pull down on the water molecules above them. This downward stress makes bonds between surface water molecules flatter than normal. This is what produces surface tension, and surface tension exists in bodies of water ranging from small droplets in the air to the entire ocean.
Figure 1-5. Bonding directions of water at the surface and in the middle of a body of water. The darker upper layer is where horizontal hydrogen bonds are under physical stress and create surface tension. (Image: ARH)
Surface tension can be a significant factor in aquatic systems. Plankton, organic matter, and other substances can be trapped in or on it. In the lab even dense objects can be supported by surface tension (Fig. 1-6). Surface tension increases in cold water and when water contains dissolved salts. Surface tension is weaker in warm water and when dissolved organics, fats, oils, and even floating plants are present. A few organisms, such as water striders, live most of their life on the surface tension of water. The thin microhabitat in and on the surface tension is called the neuston.
Figure 1-6. Dense objects supported by surface tension: a scalpel blade (left), and a pin (right). (Images: ARH)
Specific Heat of Water
Specific heat is the amount of energy in calories needed to raise the temperature of 1.0 g of a substance 1oC. Water has an extremely high specific heat of 1.0, the highest of any common substance on Earth. This means that water has to absorb a lot of energy to increase its temperature even a little bit. This also means that water must lose a lot of energy before it cools even a little bit. This tendency to be able to gain or lose large amounts of energy without much heat flux is called thermal inertia, and makes water a vital component of living things. For example, the most abundant substance in our bodies is water, so our bodies resist thermal fluctuations and increases our ability to maintain a constant internal core temperature even when atmospheric temperature fluctuates widely. What this means for aquatic organisms is that since they live in water their environmental temperature is extremely stable compared to that of air. This also means that the ocean can take up a massive amount of energy in the tropics and be moved in surface currents hundreds to thousands of miles to higher latitudes, thus moderating the Earth’s climate at low and high latitudes.
In addition, water exhibits a latent heat of melting, fusion, evaporation, and sublimation. The latent heat of melting is 79.72 cal g-1 of ice at standard temperature and pressure (STP). This means that in order for ice at 0oC to melt into liquid water at 0oC, each gram of ice has to take up 79.72 cal of energy. The latent heat of fusion, the reverse process to melting, means that to change liquid water at 0oC into ice at 0oC each gram of water has to lose 79.72 cal. The latent heat of evaporation is much higher, 540 cal g-1. This is the amount of energy that must be absorbed by liquid water at STP to convert it into water vapor. Of course 540 cal is also released when each gram of water condenses. Lastly, the latent heat of sublimation is 679 cal g-1 at STP. This is the amount of energy each gram of ice has to absorb in order to release molecules of water directly as water vapor. The latent heat of melting, fusion, evaporation, and sublimation show us that water has to gain or release large amounts of energy in order to undergo phase changes.
Density Anomaly of Water
Density is a function of an object’s mass divided by its volume. Pure water has a density of 999.84 kg m-3 at STP and maximum density of 1000 kg m-3 at 3.98oC. By comparison, air has a density of 1.29 kg m-3 at STP, so water is about 775 times denser than air. As a result, water provides objects far greater buoyancy than air. Density is therefore a significant factor for aquatic organisms. Water density is affected by temperature and salinity.
Figure 1-7. The temperature - density relationship for pure water. Maximum density of 1000 kg m-3 occurs at 3.98oC. (Image: ARH)
Temperature affects water density as indicated in Fig. 1-7. The density of pure liquid water increases from about 958 kg m-3 at 100oC to a maximum density of 1000 kg m-3 at 3.98oC. Its density then declines slightly until it reaches 0oC. When water loses enough additional heat to reach the latent heat of fusion it undergoes a phase change into its crystalline form – ice. When this happens its density drops precipitously to 917 kg m-3. When water is a liquid, even at 0oC, it forms and breaks hydrogen bonds so rapidly that one water molecule may be bonded to one, two, three, or even four other water molecules at a time, but the duration of those bonds is so fleeting that a crystalline structure is not produced. When water freezes, however, enough energy is lost from the water that when bonds are formed between water molecules those bonds are retained long enough to form a 3D crystalline structure. The crystalline structure of ice is made up of interconnected rings of six water molecules each (Fig. 1-8). The tetrahedral bonding between water molecules in ice keeps those molecules farther away from each other on average than they are in the liquid water. The resulting 3D lattice crystalline structure of ice includes spaces within and between 6-member rings. These spaces make ice less dense than water. The density difference between liquid water and ice is called the density anomaly of water. This explains why ice floats in water.
Figure 1-8. Three-dimensional structure of ice. Spaces within the 6-member rings and between 6-member rings make ice less dense than liquid water. Blue balls are water molecules. Black dashed lines are hydrogen bonds between water molecules in the foreground. Gray dashed lines are hydrogen bonds between water molecules in the background. Red dashed lines are hydrogen bonds between water molecules in the foreground and the background. (Image: ARH)
Water is the only commonly occurring substance on Earth that is less dense as a solid than as a liquid. This single characteristic of water allows life, as we know it to exist on Earth. If water were like just about everything else, denser as a solid than a liquid, ice would sink to the bottom of lakes and the ocean whenever it forms. The ocean would become super-cooled and then filled with ice, and it would never melt. Eventually all bodies of water of Earth would become solid masses of ice except perhaps for a thin layer of water at the surface melted by solar radiation. But because ice floats, ice actually forms an insulating layer at the top of bodies of water, allowing water underneath to remain liquid even when the air above ice reaches temperatures far below freezing, allowing organisms below the ice to survive.
The density of water is also affected by salinity. The temperature-density curve for water is non-linear (Fig. 1-7). This is not the case for salinity (Fig. 1-9). Temperature alone can increase water density to 1000 kg m-3, but by adding salt or other solutes, water density be increased well above that. Water can dissolve up to about 38 g of salt per 100 g of water at STP, but seawater normally contains only about 3.3-3.7 g of salt per 100g of water.
Figure 1-9. The salinity - density relationship for water at 15oC. (Image: ARH)
The importance of water density cannot be overstated, since density differences are what keep masses of water of different densities from mixing with each other. Figure 1-10 shows the relationship between temperature differences and the relative thermal resistance to mixing resulting from those temperature-density differences. These data show that a difference of even a few degrees is enough to provide a significant resistance to mixing, as evidenced by the large relative thermal resistance that exists in the thermocline shown in Fig. 1-10. These density differences are enough to keep warm surface waters of the epipelagic zone from mixing with deeper, colder water, and are enough to keep warmer inshore waters from mixing with colder offshore waters. At the same time, density driven by temperature and salinity drive the world’s most important deep ocean current – the global conveyor belt. This current plays a major role in regulating Earth’s climate.
Figure 1-10. Typical mid-summer temperature profile and relative thermal resistance values in a small temperate lake. Relative thermal resistance indicates the resistance of successive layers of water to mix with each other based on resistance of freshwater at 4oC to mix with freshwater at 5oC. (Image: ARH)
The chemical and physical characteristics of water determine how it bonds, what dissolves in it, how dense it is, and how resistant neighboring water masses are to mix with each other. The resistance to mixing between successive layers of water limits the movement of nutrients, dissolved oxygen, plankton, and other things suspended or dissolved in the water.
In summary, by gaining a better understanding of the molecular and chemical nature of water, you can better imagine what life is like for aquatic creatures. You can also gain a greater appreciation for how the ocean helps moderate and maintain living conditions on Earth.
1) Explain what makes water a tetrahedral molecule.
2) Explain why water is a polar molecule and why this matters.
3) Explain the density anomaly of water and why this matters.
4) Define the term “relative thermal resistance” and how this limits water mixing, and then comment on why this matters.