A. Effects of Water's Polarity (Activity 3A)
1. The polarity of water molecules results in hydrogen bonding.
a. In a water molecule two hydrogen atoms form polar covalent bonds with an oxygen atom. (Fig. 3.1)
1. The region around oxygen has a partial negative charge.
2. The region near the two hydrogen atoms has a partial positive charge.
b. A water molecule is a polar molecule with opposite ends of the molecule having opposite charges.
c. The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.
2. Organisms depend on the cohesion of water molecules.
a. Cohesion is water molecules sticking to other water molecules.
1. The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.
2. They form, break, and reform with great frequency.
3. At any instant, a substantial percentage of all water molecules are bonded to their neighbors so that they hold the water together, a phenomenon called cohesion.
b. Cohesion among water molecules plays a key role in the transport of water against gravity in plants.
1. Water that evaporates from a leaf is replaced by water from vessels in the leaf.
2. Hydrogen bonds cause water molecules leaving the veins to tug on molecules further down.
3. This upward pull is transmitted to the roots. (Activity 3B)
4. Adhesion, clinging of one substance to another, contributes to water transport also, because water adheres to the walls of the vessels.
c. Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. (Fig. 3.3)
1. Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.
2. Some animals can stand, walk, or run on water without breaking the surface.
3. Water moderates temperatures on Earth.
a. Water's high specific heat
1. The specific heat of a substance is the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1oC.
2. Water has a high specific heat compared to other substances.
3. Water resists changes in temperature because it takes a lot of energy to speed up its molecules. (Heat makes molecules move faster.)
4. Water�s high specific heat is due to hydrogen bonding.
a. Heat must be absorbed to break hydrogen bonds.
b. Heating water causes little change in the temperature because much of the energy is used to disrupt hydrogen bonds, not move molecules faster.
5. The impact of water�s high specific heat ranges from the level of the whole environment of Earth to that of individual organisms.
a. A large body of water can absorb a large amount of heat from the sun in daytime and during the summer, while warming only a few degrees.
1. At night and during the winter, the warm water will warm cooler air.
2. Therefore, ocean temperatures and coastal land areas have more stable temperatures than inland areas.
b. The water that dominates the composition of biological organisms stabilizes body temperature.
b. Water's high heat of vaporization
1. The transformation of molecules from a liquid to a gas is called vaporization or evaporation.
a. This occurs when the molecule moves fast enough that it can overcome the attraction of other molecules in the liquid.
b. Even in a low temperature liquid, some molecules are moving fast enough to evaporate.
c. Heating a liquid increases movement and increases the rate of evaporation.
2. Heat of vaporization is the quantity of heat that a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state.
a. Water has a high heat of vaporization.
b. This is because hydrogen bonds must be broken before a water molecule can evaporate from the liquid.
3. As a liquid evaporates, the surface of the liquid that remains behind cools - evaporative cooling.
a. This occurs because the most energetic molecules are the most likely to evaporate, leaving the slower moving molecules behind.
b. Evaporation of water from the leaves of plants or the skin of humans removes excess heat.
4. What happens if you put a can of pop in the freezer?
a. Water is unusual because it is less dense as a solid than as a liquid.
1. Most materials contract as they solidify, but water expands.
b. When water is cooled below 4oC it begins to freeze because its molecules are no longer moving vigorously enough to break their hydrogen bonds.
1. When water reaches 0oC, it becomes locked into a crystalline lattice with each molecule hydrogen-bonded to other molecules. (Fig. 3.5)
c. As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.
d. Ice is about 10% less dense than water at 4oC. Therefore, ice floats on water.
1. If ice sank, eventually all ponds, lakes, and even the ocean would freeze solid.
2. During the summer, only the upper few inches of the ocean would thaw.
3. Instead, the surface layer of ice insulates liquid water below, preventing it from freezing and allowing life to exist under the frozen surface. (Fig. 3.6)
5. Water is the solvent of life.
a. Solutions
1. A liquid that is a completely homogeneous mixture of two or more substances is called a solution.
a. A sugar cube in a glass of water will eventually dissolve to form a uniform mixture of sugar and water.
2. The dissolving agent is the solvent and the substance that is dissolved is the solute.
a. In our example, water is the solvent and sugar the solute.
3. In an aqueous solution, water is the solvent.
4. Water is not a universal solvent, but it is very versatile because of the polarity of water molecules.
5. Water is an effective solvent because it so readily forms hydrogen bonds with charged and polar covalent molecules. (Fig. 3.7)
a. For example, when a crystal of salt (NaCl) is placed in water, the Na+ cations form hydrogen bonds with partial negative oxygen regions of water molecules.
b. The Cl- anions form hydrogen bonds with the partial positive hydrogen regions of water molecules.
6. Each dissolved ion is surrounded by a sphere of water molecules, a hydration shell.
7. Polar molecules are also soluble in water because they can also form hydrogen bonds with water.
b. Hydrophilic and hydrophobic substances
1. Any substance that has an affinity for water is hydrophilic.
a. These substances are dominated by ionic or polar bonds.
2. Substances that have no affinity for water are hydrophobic.
a. These substances are dominated by non-ionic and nonpolar covalent bonds.
b. Because there are no consistent regions with partial or full charges, water molecules cannot form hydrogen bonds with these molecules.
c. Oils, such as vegetable oil, are hydrophobic because the dominant bonds, carbon-carbon and carbon-hydrogen, exhibit equal or near equal sharing of electrons.
d. Hydrophobic molecules are major ingredients of cell membranes.
B. pH
1. An acid is a substance that increases the hydrogen ion concentration in a solution.
a. When hydrochloric acid is added to water, hydrogen ions dissociate from chloride ions: HCl > H+ + Cl-
b. Addition of an acid makes a solution more acidic.
2. Any substance that reduces the hydrogen ion concentration in a solution is a base.
a. Some bases reduce H+ by releasing OH- that combines with H+ to form water: NaOH > Na+ + OH-, then OH- + H+ > H2O
b. Solutions with more OH- than H+ are basic solutions.
3. The H+ concentration is typically expressed via the pH scale. (Fig. 3.9)
a. The pH scale, ranging from 1 to 14, expresses the range of H+ concentrations by employing logarithms: pH = - log [H+]
b. In a neutral solution [H+] = 10-7 M, and the pH = 7.
c. The pH of a neutral solution is 7, acidic solutions have pH values less than 7 and basic solutions have pH values more than 7.
d. Each pH unit represents a tenfold difference in H+ concentrations.
1. A small change in pH actually indicates a substantial change in H+ concentration.
4. Most biological fluids have pH values in the range of 6 to 8.
a. The chemical processes in the cell can be disrupted by changes to the H+ and OH- concentrations away from their normal values near pH 7.
b. To maintain cellular pH values at a constant level, biological fluids have buffers.
b. Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution.
c. Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.