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Objectives - 5

Objectives - 5. Describe 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.

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Objectives - 5

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  1. Objectives - 5 • Describe 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.

  2. Polarity • For 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.

  3. Polarity • The 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.

  4. Solubility of Water • The polar nature of water allows it to dissolve polar substances. • i.e., sugars, ionic compounds, some proteins • Water 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.

  5. Hydrogen Bonding • The polar nature of water causes water molecules to be attracted to each other. • This attraction is called a hydrogen bond.

  6. 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 = water • Gas = water vapor

  7. Hydrogen Bonding, cont’d… Hydrogen bonding accounts for the unique properties of water. • Cohesion • Adhesion • Absorb large amounts of energy as heat • Cool surfaces through evaporation • The density of ice • Dissolve many substances

  8. 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.

  9. Adhesion • Adhesion = 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

  10. Temperature Moderation • Water 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.

  11. Density of Ice • Most 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.

  12. SOLUTIONS • Solution = 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 water • Kool-aid • Solute = a substance dissolved in the solvent. • Solvent = the substance in which the solute is dissolved. • Kool-aid > powder = solute; water = solvent • Neither the sugar or water are altered chemically. • If you boil away the water, the sugar will remain.

  13. 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.

  14. ACIDS AND BASES • The 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.

  15. 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.

  16. ACIDS • In 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 taste • In high concentrations, they are highly corrosive to some materials.

  17. BASES • A 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.

  18. pH • pH 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).

  19. BUFFERS • The 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 44 • Stomach acid = 2 • Urine = 6 • Intestinal fluid = ~8.1 • Blood = ~7.4

  20. 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.

  21. BROWNIAN MOTION • Robert Brown – Scottish scientist • Used microscope to observe pollen grains suspended in water • Grains 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.

  22. 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

  23. DIFFUSION, CONT’D. Three key factors dealing with DIFFUSION: • Concentration • Temperature • Pressure

  24. DIFFUSION, CONT’D… Concentration: • Primary controlling factor • The more concentrated the substances, the more quickly diffusion occurs. • More collisions between the particles of the substances

  25. DIFFUSION, CONT’D… Temperature and Pressure: • External factors that can also change the rate of diffusion • Increased 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.

  26. Results of Diffusion… • Diffusion continues until equilibrium is reached. • Movement continues; however, there is no overall concentration change.

  27. Diffusion in Living Systems… • Concentration gradient • The difference in concentration of a substance across space • Ions 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

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