The Molecule That Supports All of Life You and the surface of the Earth are mostly water 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 solid liquid gas The structure of the water molecule allows it to interact with other molecules which leads to Water s unique emergent properties help make Earth suitable for life Concept 3 1 Polar covalent bonds in the 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 and other molecules hydrogen bonding up to 4 hydrogen bonds other water molecules but not all all the time Hydrogen bonding of water molecules Hydrogen bonds between H2O molecules are weak o About 1 20th 5 as strong as covalent bonds H bonds form break and reform rapidly o Depends on the temperature warmer faster At any instant a substantial percentage of all water molecules are bonded to their neighbors creating a high level of structure Concept 3 2 Four emergent properties of water contribute to Earth s suitability for life Four of water s properties that facilitate an environment for life are o Cohesive behavior o Ability to moderate temperature o Expansion upon freezing when in the solid phase o Versatility as a solvent That is because of the polar structure of water and hydrogen bonding between water molecules are central to these properties Cohesion of Water Molecules Hydrogen bonds hold water molecules together is called cohesion Cohesion helps the transport of water against gravity in plants between water molecules Adhesion is an attraction between different substances for example between water molecules and plant cell walls Plants get water from roots and move against gravity 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 Emergent properties Moderation of temperature Water absorbs and releases relatively large amounts of heat with only a slight change in its own temperature Atoms and molecules have kinetic energy energy of motion gas molecules moving around rapidly bumping into eachother o Faster motion higher kinetic energy o Versus potential energy previous lecture Heat a measure of total kinetic energy due to molecular motion Temperature intensity of heat due to average kinetic energy of molecules o As average speed of molecules increases a thermometer reports an increase in Heat passes from warmer objects more KE to cooler ones less KE warmed object temperature ends up with similar KE in both Water has a high specific heat Calorie cal a unit of heat energy 1 degree C o kilocalorie kcal 1000 cal o 1 cal the amount of heat energy needed to raise the temperature of 1 g H2O by Joule J another common energy unit o 1J 0 239cal Specific heat amount of heat absorbed or lost when 1g of a substance changes temperature by 1 degree C up or down o Specific heat of H2O Defined to be 1 cal per gram per degree Celsius 1 cal g degree C High compared to other substances H2O absorbs or releases a relatively large quantity of heat for each degree of temperature change o H2O has a high specific heat relative to many other substances High specific heat of H2O is due to H bonding o Heat absorbed to break H bonds o Heat released when H bonds form o Heat input causes relatively small changes in water temperature because much of the energy disrupts hydrogen bonds Once H bonds are disrupted H2O molecules undergo faster larger motions DIFFUSION Heat of vaporization quantity of heat a liquid must absorb to convert 1 g from liquid to gas o Water has a relatively high heat of vaporization 580 cal of heat to evaporate 1g of water at room or temperature Double that of alcohol or ammonia o Hydrogen bonds H bonds must be broken before a water molecule can evaporate from the liquid Water s high heat of vaporization moderate s climate coast cooler than inland As a liquid evaporates the surface of the liquid that remains behind cools evaporative cooling liquid to atmosphere o The most energetic molecules are the most likely to evaporate leaving behind molecules with lower kinetic energy Emergent properties Insulation of water by floating ice Water is less dense as a solid ice than as a liquid o Most materials contract as they solidify H2O expands o Above 4 degrees C behaves like other liquids Expands when it warms Contracts when it cools o Water begins to freeze 0 degrees C when its molecules are no longer moving vigorously enough to break their hydrogen bonds o When water reaches 0 degrees C water becomes locked into a crystalline lattice with each H2O molecule H bonded to the maximum of four partners o Ice is about 10 less dense than water at 4 degrees C o Therefore ice floats on the cool water below Water is the solvent of life Solution a liquid homogeneous mixture of two or more substances Solvent the dissolving agent Solute the substance that is dissolved o Sugar solute dissolved in water solvent Aqueous solution H2O is the solvent Water is not a universal solvent but it is very versatile because of the polarity of water molecules covalent molecules Water is an effective solvent because it forms hydrogen bonds with charged and polar o Example NaCl in water o Na cations form hydrogen bonds with the partial negative oxygen regions of o Cl anions form hydrogen bonds with the partial positive hydrogen regions of water molecules water molecules o In water ionic bonds are weak Hydration shell each dissolved ion is surrounded by H2O molecules Eventually H2O dissolves all the ions resulting in a solution with 2 solutes Na and Cl Polar molecules are also soluble in H2O because they can all form H bonds with water Large molecules like proteins can dissolve in H2O if they have ionic and polar regions purple blob lysozyme Hydrophilic a substance that has affinity for water water loving o Dominated by ionic or polar bonds o NaCl or simple sugars o Includes substances that do not dissolve because their molecules are too large and too tightly held together cotton Hydrophobic substances that have no affinity for water water hating o Dominated by non ionic and nonpolar covalent bonds o
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