Campbell Biology 10th Edition Fall 2015 UCONN BIO 1107 Chapter 3 Water and Life Polar covalent bonds in water molecules result in hydrogen Key Concepts 3 1 bonding 3 2 suitability for life 3 3 Acidic and basic conditions affect living organisms Four emergent properties of water contribute to Earth s 1 The Molecule that Supports All of Life a Life on earth began in water and evolved there for 3 billion b Three quarters of Earth s surface is covered by water c The solid state of water floats on the liquid a rare property emerging from the chemistry of the water molecule 2 Concept 3 1 hydrogen bonding a The water molecule is deceptively simple Polar covalent bonds in water molecules result in i Shaped like a wide V ii Two hydrogen atoms joined to the oxygen atom by single covalent bonds years iii Oxygen is more electronegative than hydrogen so the electrons of the covalent bonds spend more time closer to oxygen a Polar covalent bonds iv The V like shape makes it a polar molecule meaning its overall charge is unevenly distributed b Oxygen region of the molecule has a partial negative charge and each hydrogen has a partial positive charge c Properties of water arise from attractions between oppositely charged atoms of different water molecules i The slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a nearby molecule 1 Thus held together by a hydrogen bond d When water is in its liquid form its hydrogen bonds are very fragile i Each one only about 1 20th as strong as a covalent bond 1 Hydrogen bonds form break and reform with great Four emergent properties of water contribute to frequency 3 Concept 3 2 Earth s sustainability for life i Cohesive behavior ii Ability to moderate temperature iii Expansion upon freezing iv Versatility as a solvent b Cohesion of Water Molecules i Water molecules stay close to each as a result of hydrogen bonding ii The hydrogen bonds hold the substance together a phenomenon called cohesion d Adhesion c Cohesion due to hydrogen bonding contributes to the transport e Surface tension the clinging of one substance to another helps water of water and dissolved nutrients against gravity in plants travel against gravity by way of hydrogen bonds to the molecules of cell walls that counters gravity break the surface of a liquid is related to cohesion i At the interface of air and water water molecules are a measure of how difficult it is to stretch or hydrogen bonded to one another and to the water below 1 This gives water an unusually high surface tension 2 Makes it behave as though it were coated with an invisible film f Moderation of Temperature by Water i Moderates air temperature by absorbing heat from air that is warmer and releasing that heat into air that is cooler ii Effective as a heat bank because it can absorb or release a relatively large amount of heat with only a slight change in its own temperature iii Temperature and Heat 1 Anything that moves has kinetic energy the energy of motion 2 Kinetic energy associated with the random movement of atoms or molecules is called thermal energy a Thermal energy is related to temperature but temperature is a measure of energy that represents the average kinetic energy of the molecules in a body of matter regardless of volume i Total thermal energy depends in part on the matter s volume 3 Thermal energy in transfer from one body of matter to another is defined as heat 4 A calorie cal is the amount of heat it takes to raise the temperature of 1 gram of water by 1 degree Celsius a Also the amount of heat that 1 gram of water releases when it cools by 1 degree Celsius 5 A kilocalorie 1000 calories is the quantity of heat required to raise the temperature of 1 kilogram of water by 1 degree Celsius a One Joule J is equal to 0 239 cal one calorie equals 4 184 Joules iv Water s High Specific Heat 1 The specific heat of a substance is defined as the amount of heat that must be absorbed for 1 gram of substance to change its temperature by 1 degree Celsius 2 The specific heat of water is 1 calorie per gram and per degree Celsius or abbreviated as 1 cal g C 3 Water has an unusually high specific heat compared to ethyl alcohol where it has a specific heat of 0 6 cal g C a So only 0 6 calories are required to raise the temperature of 1 gram of ethyl alcohol by 1 degree Celsius 4 Water s specific heat is ten times greater than that of iron 5 Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat 6 Can be traced to hydrogen bonds as heat must be absorbed in order to break hydrogen bonds a When the temperature of water drops slightly additional hydrogen bonds are formed and release a considerable amount of energy in the form of heat 1 Molecules of liquid tend to stay together because they are attracted to each other 2 Molecules that are moving fast enough to overcome the attraction escape as gas a Called vaporization or evaporation 3 Some evaporation occurs at any temperature 4 Heat of evaporation is the quantity of heat a liquid must absorb for 1 gram of it to be converted from liquid to the gaseous state a On an organismal level water s high heat of vaporization can cause steam burns 5 Evaporative cooling surface of the liquid left behind cools down a Provides a mechanism that prevents terrestrial occurs as a liquid evaporates and the v Evaporative Cooling organisms from overheating vi Floating of Ice on Liquid Water 1 Less dense as a solid than as a liquid 2 Water expands when it solidifies unlike other elements 3 Temperatures above 4 degrees Celsius water acts like any other normal liquid but at 0 degrees Celsius water molecules move too slowly to break the hydrogen bonds a The molecules become locked into a crystalline lattice each molecule hydrogen bonded to four other partners 4 The hydrogen bonds keep the molecules apart which makes the ice about 10 less dense than liquid water 5 Water reaches its greatest density at 4 degree Celsius 6 Floating ice insulates the water below preventing it from freezing and allowing life to exist a Provides a solid habitat for some animals vii Water The Solvent of Life 1 A liquid that is a completely homogenous mixture of two or more substances is called a solution a The dissolving agent is called a solvent 2 An aqueous solution is one in which the solute is dissolved in water water is the solvent 3 Oxygen regions of the water molecules are negatively charged and are