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Objectives - 5Describe the structure of a water molecule.Explain how water’s polar nature affects its

ability to dissolve substances.Outline the relationship between hydrogen

bonding and the different properties of water.

Identify the roles of solutes and solvents in solutions.

Differentiate between acids and bases.

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PolarityFor the most part, water’s biological

functions stem from its chemical structure.

Remember that in a molecule of water, the two hydrogen atoms covalently bond to one oxygen atom by sharing electrons.

The e- however are not shared equally.

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PolarityThe oxygen atom has a greater ability to

attract electrons to it because it pulls hydrogen’s e-’s toward its nucleus.

The region of the molecule where the oxygen atom is located has a partial negative charge.

The regions where the hydrogen atoms are located have partial positive charges.

So, even though the total charge on a water molecule is neutral, the charge is unevenly distributed across the water molecule.

Polar Compound = compound with an uneven distribution of charge.

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Solubility of WaterThe polar nature of water allows it to

dissolve polar substances.i.e., sugars, ionic compounds, some

proteinsWater does not dissolve nonpolar

substances.i.e., oil

NaCl – soluble in water…Dissolved or dissociated ions are

present in all of the aqueous solutions found in living things.They are necessary in maintaining

normal body functions.

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Hydrogen Bonding

The polar nature of water causes water molecules to be attracted to each other.

This attraction is called a hydrogen bond.

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Hydrogen Bonding, cont’d…The number of hydrogen bonds that exist

depends upon the state of matter that water is in.Solid = ice: all molecules are hydrogen bonded

and do not break.Liquid = waterGas = water vapor

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Hydrogen Bonding, cont’d…Hydrogen bonding accounts for the unique

properties of water. CohesionAdhesionAbsorb large amounts of energy as heatCool surfaces through evaporationThe density of iceDissolve many substances

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Cohesion Water molecules stick to each other as a result

of hydrogen bonding. Cohesion = is the attractive force that holds

molecules of a single substance together.

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AdhesionAdhesion = the attractive force between two

particles of different substances. i.e., water molecules and glass molecules

Capillarity = property where the attraction between molecules results in the rise of the surface of the liquid when in contact with a solid.

The forces of cohesion, adhesion, and capillarity help water rise through narrow tubes against the force of gravity.Water-conducting tubes in the stem of a flower.Capillary tubes

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Temperature ModerationWater has a high heat capacity.Water can absorb or release relatively large

amounts of energy in the form of heat with only a slight change in temperature.

This property of water is related to hydrogen bonding.

To break hydrogen bonds, energy must be absorbed.

Water’s high heat capacity also allows organisms to keep cells at an even temperature despite environment temperature changes.

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Density of IceMost solids are denser than their

liquids.This is not the case for ice / water.

Water expands when it freezes.Ice (solid water) is less dense than liquid

water.

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SOLUTIONSSolution = mixture where one or more substances

are uniformly distributed (mixed) in another substance.Plasma (liquid portion of blood) is a complex solution.

Ions, large molecules, gases – all dissolved in waterKool-aid

Solute = a substance dissolved in the solvent.Solvent = the substance in which the solute is

dissolved.Kool-aid > powder = solute; water = solventNeither the sugar or water are altered chemically.If you boil away the water, the sugar will remain.

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Solutions, cont’d…Concentration (of a solution) = the amount of

solute dissolved in a fixed amount of the solution.

Saturated solution = a solution in which no more solute can be dissolved.

Aqueous solution = water is the solvent.Important to living things.Plants – moist soil (aqueous solution)Body cells exist in aqueous solution (intercellular

fluid) and are also filled with fluid.Most chemical rxns. that happen in the body

occur in aqueous solutions.

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ACIDS AND BASESThe degree of acidity or alkalinity (baseness)

is essential in living systems.IONIZATION of WATER (separation of

water)Water molecules bump into each other as they

move about.Sometimes this results in a chemical change.One water molecule loses a p+ (a hydrogen

nucleus), and the other gains this p+.This reaction happens in two steps.

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Ionization of Water, cont’d…First: one molecule of water pulls apart

another water molecule, (dissociates) into two ions of opposite charge.H2O H+ + OH- The OH- ion is called the hydroxide ion.The free H+ ion can react with another water

molecule shown in this equation: H+ + H2O H3 O+

…the H3O+ is called the hydronium ion.

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ACIDSIn a solution, if the number of hydronium ions

is greater than the number of hydroxide ions…The solution is acidic, it is an acid.Sour tasteIn high concentrations, they are highly

corrosive to some materials.

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BASESA solution is alkaline or basic, when it

contains more hydroxide ions than hydronium ions.Bases have a bitter taste.They feel slippery, because the OH- ions react

with the oils on our skin to form a soap.Commercial soap is the product of a reaction

between a base and a fat.

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pHpH scale = scale for comparing

the relative concentrations of hydronium ions to hydroxide ions within a solution.

The scale ranges from 0 – 14.0 is the most acidic.14 is the most basic (alkaline).7 is neutral (water).

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BUFFERSThe control of pH is important to living things.Enzymes (remember: proteins, RNA molecules)

can function only within a very narrow range of pH.Buffers are chemical substances that regulate pH.Buffers neutralize small amounts of either an acid

or a base added to a solution.Figure 2.14, page 44Stomach acid = 2Urine = 6Intestinal fluid = ~8.1Blood = ~7.4

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WATER AND DIFFUSION

All objects in motion have energy of motion called kinetic energy.

A moving particle of matter will continue to move in a straight line until it collides with another particle.

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BROWNIAN MOTION

Robert Brown – Scottish scientistUsed microscope to observe pollen grains

suspended in waterGrains moved constantly in little jerks

Seemed to be struck by invisible objects He thought this was the result of a life force hidden

within the pollen grains.He repeated experiment with dye particles and

got the same results. Dye particles are not living. Brown had no explanation for the motion, now

called Brownian motion.

We know it’s the random movement of atoms and molecules.

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DIFFUSION

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.Diffusion of water = osmosis

Diffusion results because of the random motion of atoms and molecules (Brownian motion).Diffusion is a slow process

Relies on the random motion of atoms and molecules

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DIFFUSION, CONT’D.

Three key factors dealing with DIFFUSION:

ConcentrationTemperaturePressure

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DIFFUSION, CONT’D…

Concentration:Primary controlling factorThe more concentrated the

substances, the more quickly diffusion occurs.More collisions between the particles of the substances

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DIFFUSION, CONT’D…

Temperature and Pressure:External factors that can also change the

rate of diffusionIncreased temperature causes increased

energy and will cause more rapid particle movement.This will cause an increase in the rate of

diffusion.Increasing pressure also accelerates

increased particle motion… increased rate of diffusion.

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Results of Diffusion…

Diffusion continues until equilibrium is reached.Movement continues; however, there is no overall concentration change.

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Diffusion in Living Systems…

Concentration gradientThe difference in concentration of a substance

across spaceIons and molecules move from an area of

higher concentration to lower concentration.They move with the gradient.

If no other processes interfere, diffusion will continue until there is no longer a concentration gradient.Dynamic Equilibrium

Diffusion is one of the ways that cells move substances in / out of cells.

i.e., oxygen into capillaries from air sacs of the lungs