Objectives: 1) Describe the importance of water to living systems 2) Explain how each of water’s properties relate to polarity and H-bonding

Importance of Water More than 70% of our total body weight is water Necessary for photosynthesis H becomes incorporated into organic compounds Oxygen released for us to breathe Solvent for most biochemical reactions Important reactant/product

How Unique is Water?? Water is one of only 3 naturally occurring inorganic liquids (mercury and ammonia) Only chemical compound that exists in all 3 states— solid, liquid, and gas Extremely large liquid range (0 o C o C) Expands, becomes less dense as a solid

Water is a polar molecule Due to differences in electronegativities, water is slightly charged at its poles Oxygen takes on a slight – charge Hydrogens take on a slight + charge ology1111/animations/hydrogenbonds.html ology1111/animations/hydrogenbonds.html

Hydrogen Bonding Water can form H-bonds with up to 4 neighboring water molecules Degree of H-bonding reflects state of matter

Living systems depend on properties of water that result from its polarity and H- bonding Less dense as a solid than a liquid Universal solvent Adhesion and cohesion Capillary action Surface tension High heat of vaporization High specific heat Thermal conductivity

Water is the universal solvent What kinds of substances will dissolve in water? Consider the applications: fat-soluble vs. water soluble vitamins/etc. Form “spheres of hydration” emistry/flash/molvie1.swf emistry/flash/molvie1.swf

Cohesion Water molecules have a strong tendency to stick to one another Cohesion allows water to have a high degree of surface tension Any force is transmitted to the column of water as a whole

Adhesion Ability of water to stick to other substances, esp. charged atoms or molecules Together, cohesion and adhesion allow for capillary action

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Liquid water has H-bonds that form and break constantly at/water/water.swf at/water/water.swf Allows water to have a high specific heat Amount of energy required to raise temp of 1 g 1 o C 1 cal/1 g

Water Vapor As water moves from liquid to gaseous state, H-bonds are broken, allowing water molecules to escape Water has a high heat of vaporization: It takes 540 cal for 1 g of water to move from liquid to gaseous state Allows for evaporative cooling As fast-moving liquid water molecules escape as vapor, they take their heat energy with them

Ice Solid water is less dense than liquid water, allowing it to float

H-bonds lock into lattice structure

Acids and Bases Water molecules have slight tendency to ionize: H 2 O H + + OH - The H + then joins another water molecule resulting in H 3 O + (hydronium) The pH scale is a measure of hydronium concentration expressed in moles/liter

pH scale pH = -log 10 [H + ] Acids are proton donors Increase the # of H + ions Bases are proton acceptors

Acid + Base  Salt Acid = H + + anion Base = OH - + cation H + joins with OH - to get H 2 O Anion can combine with cation to make a salt Ex. HCl + NaOH  H 2 O + NaCl

Buffers Substance(s) that resist pH changes when an acid or base is added Usu. weak acid or base, do not completely ionize Example: Blood in Vertebrates

CO 2 + H 2 0 H 2 CO 3 H + + HCO 3 - Will stay at dynamic equilibrium unless stressed If add excess H + system shifts left and forms carbonic acid If add OH - they combine with H + forming water, system shifts right buffer.htm buffer.htm alchemistry/flash/buffer12.swf alchemistry/flash/buffer12.swf