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Chapter 3 Water and Life Learning Objectives How polar covalent bonds in water molecules result in hydrogen bonding Four emergent properties of water contribute to Earth s suitability for life Acidic and basic conditions affect living organisms The Molecule That Supports All of Life Water is the biological medium on Earth Water is the only common substance to exist in the natural environment in all three physical states of matter The structure of the water molecule allows it to interact with other molecules Water s unique emergent properties help make Earth suitable for life Figure 3 1 Figure 3 1a A young whooper swan paddles after its parent Polar covalent bonds in water molecules result in hydrogen bonding In the water molecule the electrons of the polar covalent bonds spend more time near the oxygen than the hydrogen The water molecule is thus a polar molecule the overall charge is unevenly distributed Polarity allows water molecules to form hydrogen bonds with each other Figure 3 2 Hydrogen bond Polar covalent bonds Animation Water Structure Four emergent properties of water contribute to Earth s suitability for life Four of water s properties that facilitate an environment for life are Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent Cohesion of Water Molecules Collectively hydrogen bonds hold water molecules together a phenomenon called cohesion Cohesion helps the transport of water against gravity in plants Adhesion is an attraction between different substances for example between water and plant cell walls Figure 3 3 H2O Adhesion Two types of water conducting cells Direction of water movement H2O Cohesion 300 m H2O Animation Water Transport Surface tension is a measure of how hard it is to break the surface of a liquid Water has an unusually high surface tension due to hydrogen bonding between the molecules at the air water interface and to the water below Moderation of Temperature by Water Water absorbs heat from warmer air and releases stored heat to cooler air Water can absorb or release a large amount of heat with only a slight change in its own temperature Temperature and Heat Kinetic energy is the energy of motion The kinetic energy associated with random motion of atoms 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 Heat is a measure of the total quantity of kinetic energy due to molecular motion in a body of matter Temperature measures the intensity of heat in a body of matter due to the average kinetic energy of molecules Heat is a measure of the total quantity of kinetic energy due to molecular motion in a body of matter Temperature measures the intensity of heat in a body of matter due to the average kinetic energy of molecules A calorie cal is the amount of heat required to raise the temperature of 1 g of water by 1 C The calories on food packages are actually kilocalories kcal where 1 kcal 1 000 cal The joule J is another unit of energy where 1 J 0 239 cal or 1 cal 4 184 J Water s High Specific Heat The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 C The specific heat of water is 1 cal g C Water resists changing its temperature because of its high specific heat Figure 3 5 Santa Barbara 73 Burbank 90 San Bernardino 100 Riverside 96 Santa Ana 84 Palm Springs 106 Los Angeles Airport 75 Pacific Ocean 68 San Diego 72 40 miles 70s F 80s 90s 100s Vaporization Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas Evaporation is transformation of a substance from liquid to gas Evaporative Cooling As a liquid evaporates its remaining surface cools a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water Floating of Ice on Liquid Water Ice floats in liquid water because hydrogen bonds in ice are more ordered making ice less dense than water Water reaches its greatest density at 4 C If ice sank all bodies of water would eventually freeze solid making life impossible on Earth Figure 3 6 Ice Hydrogen bonds are stable Hydrogen bond Liquid water Hydrogen bonds break and re form Water The Solvent of Life A solution is a liquid that is a completely homogeneous mixture of substances A solvent is the dissolving agent of a solution The solute is the substance that is dissolved An aqueous solution is one in which water is the solvent Water is a versatile solvent Na due to its polarity When an ionic compound is dissolved in water each ion is surrounded by a sphere of water molecules called a hydration shell Cl Na Cl Water can also dissolve compounds made of nonionic polar molecules Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions Hydrophilic and Hydrophobic Substances A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds Hydrophobic molecules related to oils are the major ingredients of cell membranes Solute Concentration in Aqueous Solutions Most chemical reactions in organisms involve solutes dissolved in water When carrying out experiments we use mass to calculate the number of solute molecules in an aqueous solution Molecular mass is the sum of all masses of all atoms in a molecule Numbers of molecules are usually measured in moles where 1 mole mol 6 02 1023 molecules Avogadro s number and the unit dalton were defined such that 6 02 1023 daltons 1 g Molarity M is the number of moles of solute per liter of solution What is the weight of 1 mole of sucrose The chemical formula is C12H22O11 A carbon atom weighs 12 daltons hydrogen 1 dalton and oxygen 16 daltons 12x12 144 1x22 22 16x11 176 144 22 176 342 daltons How much sucrose do you need to weigh out to make 1 mole of sucrose What is the weight of 1 mole of sucrose The chemical formula is C12H22O11 A carbon atom weighs 12 daltons hydrogen 1 dalton and oxygen 16 daltons 12x12 144 1x22 22 16x11 176 144 22 176 342 daltons How much sucrose do you need to weigh out to make 1 mole of sucrose 342 grams A mole of ethyl alcohol C2H6O also contains 6 02 1023 molecules but weighs only 46 g because the molecules are smaller Sucrose solution is prepared 342 grams of sucrose 1 00 mole and 1


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UAB BY 123 - Chapter 3: Water and Life

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