BSCI105- Chapter 3- Water and Life
BSCI105- Chapter 3- Water and Life Bsci105
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This 6 page Class Notes was uploaded by clcindy.lin on Saturday January 30, 2016. The Class Notes belongs to Bsci105 at University of Maryland taught by Dr. Alewall in Summer 2015. Since its upload, it has received 45 views. For similar materials see Intro to biological sciences in Biological Sciences at University of Maryland.
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Date Created: 01/30/16
BSCI105: Chapter 3: Water and Life 1/28/16 9:06 PM 3.1: polar covalent bonds in water molecules result in hydrogen bonding • Oxygen is more electronegative than hydrogen, so the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen, these are polar covalent bonds. • Polar molecule: that's its overall charge is unevenly distributed. • The slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a nearby molecule. The 2 molecules are thus held together by a hydrogen bond. The hydrogen bond form, break, and re-form with great frequency. Each last only a few trillionth of a second. 3.2: 4 emergent properties of water contribute to Earth’s suitability for life • Cohesion of water molecules: the hydrogen bonds hold the substance together; the sticking togethe r of particles of the same substance. Due to hydrogen bonding contributes to the transport of water and dissolve nutrients against gravity. • Adhesion: the clinging of one substance to another. • Surface tension: a measure of how difficult it is to stretch or break the surface of a liquid. The high surface tension of water, resulting from the collective strength of its hydrogen bonds, allows small insects walk on water. • Moderation of temperature by water o Water moderates air temperature by absorbing heat fro m air that is warmer and releasing the stored heat to air that is cooler. o Temperature and Heat: ▯ Kinetic Energy: the energy of motion ▯ The faster a molecule moves, the greater its kinetic energy. ▯ The Kinetic energy associated with the random movement of atom s or molecules is called thermal energy. ▯ Temperature: is a measure of energy that represents the average kinetic energy of the molecules in a body of matter, regardless of volume, whereas the total thermal energy depends in part on the matter’s volume. ▯ Heat: thermal energy in transfer from one body of matter to another. ▯ Calorie (cal): amount of heat it takes to raise the temperature of 1 g of water by 1 degree Celsius. ▯ Kilocalories (kcal) : is the quantity of heat required the raise the temperature of 1 kilogram (kg) of water by 1 degree Celsius. ▯ Joule (J): one joule equals to 0.239 cal; 1 cal equals 4.184 J. o Water’s High Specific Heat ▯ Specific heat: the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 degree C. ▯ Because of the high specific heat of water relative to other materials, water will change its temperature lass than other liquids when it absorbs or loses a given amount of heat. ▯ Specific heat can be thought of as a measure of how well a subs tance resists changing its temperature when it absorbs or releases heat. o Evaporating Cooling: ▯ Molecules moving fast enough to overcome these attractions can depart the liquid and enter the air as a gas. This transformation from a liquid to a gas is calle d vaporization, or evaporation. The speed of molecular movement caries and that temperature is the average kinetic energy of molecules. ▯ Heat of vaporization : the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the ga seous state. To evaporate 1 g of water at 25 C, about 580 cal. of heat is needed. ▯ Water’s high heat of vaporization is another emergent property resulting from the strength of its hydrogen bonds, which must be broken before the molecules can exit from the liquid in the form of water vapor. ▯ As liquid evaporates, the surface of the liquid that remains behind cools down: evaporative cooling. This occurs because the “hottest” molecules, those with the greatest kinetic energy, are the most likely to leave as g as. Contributes to stability of temperature. • Floating of Ice on Liquid Water: o Water is one of the few substances that are less dense as a solid than as a liquid. o The hydrogen bonds keep the molecules at “arm’s length,” far enough apart to make ice about 10% less dense than liquid water at 4 degree C. As crystal collapses, the ice melts, and molecules are free to slip closer together. • Water: The Solvent of Life o Solution: a liquid that is a completely homogeneous mixture of two or more substances. o Solvent: the dissolving agent of a solution o Solute: the substance that is dissolved o Aqueous solution: one in which the solute is dissolved in water, water is the solvent. o Hydration shell: the sphere of water molecules around each dissolved ion o A compound does not need to be ionic to dissolve in water; many compounds made up of nonionic polar molecules. • Hydrophilic and hydrophobic substances: o Hydrophilic: any substance that has an affinity for water. Substances can be hydrophilic without actually dissolving b ecause some molecules in cells are so large that they do not dissolve. o Hydrophobic: substances that are nonionic and nonpolar (cannot form hydrogen bonds) repel water. (oil) o Solute Concentration in Aqueous Solutions: ▯ To understand such reactions, we must know how many atoms and molecules are involved can calculate the concentration of solutes in an aqueous solution. We use mass to calculate the number of molecules. ▯ Molecular mass: the sum if the masses of all the atoms in a molecule. 23 ▯ Mole (mol): an exact number of objects: 6.02 x 10 (the Avogadro’s number) ▯ Molarity: the number of moles of solute per liter of solution - the unit of concentration most often used for aqueous solutions. 3.3: Acidic and basic conditions affects living organisms • A hydrogen atom participating in a hydrogen bond between 2 water molecules shifts from one molecule to the other. The hydrogen atom leaves its electron behind and what is actually transferred is + + a hydrogen ion (H ), a single proton with a charge of 1 . The water molecule that lose proton is one a - - hydroxide ion (OH ), which has a charge of 1 . The proton binds to the other water molecule, making that molecule a hydronium ion (H O ).3 + • The concentration of H and OH are equal to water, but adding certain kinds of solutes , called acid or bases, disrupts this balance. Scientists use the pH scale to describe how acidic or basic a solution is. • Acids and Bases: + o When acids dissolve in water, they donate additional H to the solution. An acid is a substance that increases the hydrogen ion concentration of a solution. ▯ Ex: HCL ▯ H + Cl (this source of H+ results in an acidic solution – one having more H+ than OH-. o A substance that reduce the hydrogen ion concentration of a solution is called a base. Solutions with a higher co ncentration of OH- than H+ are known as basic solutions. • pH Scale: o In any aqueous solution at 25 degree C, the product of the H+ and OH - concentrations is constant at 10 . A neutral solution at 25 degree C (room temp) [H+] = 10 -7and [OH-] = 10 ,-7 so the product of those two is 10 -14([H+][OH-] = 10 -14) The brackets in the equation indicate molar concentration. o The pH scale runs from 0 to 14. A pH below 7 is acidic. A pH above 7 is basic ▯ A solution with a pH of 6 is 10 times more acidic (has 10 tim es more hydrogen ions) than pure H 2 with a pH of 7 • o pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log[H+] -7 -7 ▯ For a neutral aqueous solution, [H+] is 10 M, giving us: -log 10 = -(-7) = 7 • Buffers: o A slight change in pH can be harmful, because the chemical processes of the cell are very sensitive to the concentrations of hydrogen and hydroxide ions. o The presence of substances called buffers allows biological fluids ( ex: blood) to maintain a relatively constant pH despite the addition of acids or bases. A buffer is a substance that minimizes changes in the concentration of H+ and OH - in a solution. It does so by accepting hydrogen atoms from the solution when they are in excess and donating hydroge n ions from the solution when they are in excess and donating hydrogen ions to the solution when they have been depleted. • Acidification: A Threat to Water Quality o Ocean acidification: when CO2 (carbon dioxide) dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean pH, a process known as ocean acidification. T his hurts many organisms in the ocean especially coral reefs. 1/28/16 9:06 PM 1/28/16 9:06 PM
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