Water

Properties of Water Polar molecule Cohesion- high surface tension Adhesion High specific heat and heat of vaporization Density – greatest at 4oC Universal solvent of life

Polarity of Water In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids Because oxygen is more electronegative, the region around oxygen has a partial negative charge. The region near the two hydrogen atoms has a partial positive charge. A water molecule is a polar molecule with opposite ends of the molecule with opposite charges. 2 corners are orbitals with unshared electrons and a weak negative charge 2 are occupied by hydrogen atoms that have polar covalent bonds

Water has a variety of unusual properties because of attractions between these polar molecules. The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. Each water molecule can form hydrogen bonds with up to four neighbors. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

HYDROGEN BONDS Extraordinary Properties that are a result of hydrogen bonds. Cohesive behavior Resists changes in temperature High heat of vaporization Expands when it freezes Versatile solvent Hold water molecules together Each water molecule can form a maximum of 4 hydrogen bonds The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds. They form, break, and reform with great frequency

Organisms Depend on Cohesion and Adhesion Cohesion and Adhesion is responsible for the transport water from the roots to the top of plants Cohesion- the water molecules stick to each other Adhesion- water sticks to something else, in this example it’s the wall of the vessels in the plant vessels.

Cohesion Among Water Molecules Biology: Life on Earth (Audesirk) Cohesion Among Water Molecules Gives water a high surface tension. With webbed feet bearing specialized scales, the basilisk lizard of South America makes use of surface tension, caused by cohesion, to support its weight as it races across the surface of a pond. In giant redwoods, cohesion holds water molecules together in continuous strands from the roots to the topmost leaves even 300 feet (about 100 meters) above the ground. increasingly acidic (H+ > OH–) increasingly basic (H+ < OH–) http://www.youtube.com/watch?v=45yabrnryXk Chapter 2 7

Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. Water behaves as if covered by an invisible film. Some animals can stand, walk, or run on water without breaking the surface.

Moderates Temperatures on Earth High Specific Heat Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Celsius Scale at Sea Level 100oC Water boils 37oC Human body temperature 23oC Room temperature 0oC Water freezes Moderates Temperatures on Earth  

Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1oC. Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for: Prevention of temperature fluctuations that are outside the range suitable for life. Coastal areas having a mild climate A stable marine environment

Evaporative Cooling High Heat of Vaporization The cooling of a surface occurs when the liquid evaporates This is responsible for: Preventing organisms from overheating Evaporative Cooling

Water is less dense as a solid than as a liquid When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners. 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. Ice is about 10% less dense than water at 4oC.

Density of Water Most dense at 4oC Contracts until 4oC Expands from 4oC to 0oC The density of water: Prevents water from freezing from the bottom up which would kill aquatic organisms. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.

Solvent for Life Solution Aqueous solution Hydrophilic Hydrophobic Solute solvent Aqueous solution Hydrophilic Ionic compounds dissolve in water Polar molecules (generally) are water soluble Hydrophobic Nonpolar compounds

Most biochemical reactions involve solutes dissolved in water. There are two important quantitative proprieties of aqueous solutions. 1. Concentration 2. pH

Concentration of a Solution Molecular mass – sum of the weights of all atoms in a molecule (amu = daltons) Mole – amount of a substance that has a mass in grams numerically equivalent to its molecular mass in amu. Avogadro’s number – 6.02 X 1023 A mole of one substance has the same number of molecules as a mole of any other substance.

Molarity Calculate a one molar solution of sucrose, C12H22O16. The concentration of a material in solution is called its molarity. A one molar solution has one mole of a substance dissolved in one liter of solvent, typically water. Calculate a one molar solution of sucrose, C12H22O16. C = 12 amu H = 1 amu O = 16 amu 12 x12 = 144 1 x 22 = 22 16 x 11 = 176 342 For a 2M solution? For a .05 M solution? For a .2 M solution?

Dissociation of Water Molecules Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other. The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H+). The water molecule that lost a proton is now a hydroxide ion (OH-). The water molecule with the extra proton is a hydronium ion (H3O+). Unnumbered Fig. 3.47 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

This reaction is reversible. A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion: H2O <=> H+ + OH- This reaction is reversible. At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-. In pure water only one water molecule in every 554 million is dissociated. At equilibrium, the concentration of H+ or OH- is 10-7M (25°C) . Only a hydrogen ion is actually transferred The transferred proton binds to an unshared orbital of the second water molecule. The water molecule that loses the proton

Acids and Bases An acid is a substance that increases the hydrogen ion concentration in a solution. Any substance that reduces the hydrogen ion concentration in a solution is a base. Some bases reduce H+ directly by accepting hydrogen ions. Strong acids and bases complete dissociate in water. Weak acids and bases dissociate only partially and reversibly. There is only 1 molecule out of 554,000,000 water molecules that is dissociated.

pH Scale The pH scale in any aqueous solution : [ H+ ] [OH-] = 10-14 Measures the degree of acidity (0 – 14) Most biologic fluids are in the pH range from 6 – 8 Each pH unit represents a tenfold difference (scale is logarithmic) A small change in pH actually indicates a substantial change in H+ and OH- concentrations.

Problem How much greater is the [ H+ ] in a solution with pH 2 than in a solution with pH 6? Answer: pH of 2 = [ H+ ] of 1.0 x 10-2 = 1/100 M pH of 6 = [ H+ ] of 1.0 x 10-6 = 1/1,000,000 M 10,000 times greater

Buffers A substance that eliminates large sudden changes in pH. Buffers help organisms maintain the pH of body fluids within the narrow range necessary for life. Are combinations of H+ acceptors and donors forms in a solution of weak acids or bases Work by accepting H+ from solutions when they are in excess and by donating H+ when they have been depleted.

Acid Precipitation Rain, snow or fog with more strongly acidic than pH of 5.6 West Virginia has recorded 1.5 East Tennessee reported 4.2 in 2000 Occurs when sulfur oxides and nitrogen oxides react with water in the atmosphere Lowers pH of soil which affects mineral solubility – decline of forests Lower pH of lakes and ponds – In the Western Adirondack Mountains, there are lakes with a pH <5 that have no fish.