jimtrue.com : school : BSC2010 : CH 03: Water & the Fitness of the Environment

Posted by Jim True on February 1, 2004 3:25 AM. Last Updated October 22, 2006 9:23 PM

Disclaimer for all material noted here is at the bottom of this web page.

CH 03: Water & the Fitness of the Environment

Water

• The simple compound, hydrogen oxide, better known as water, is considered indispensable for life as we know it on Earth.
• Life arose in water, and the cells of all life forms possess an internal (and usually external) aqueous environment. Thus, all cells contain water! About 70% of the human body by weight is water.
• Water is a polar molecule with two H's covalently bonded to an O with a 105° angle between them. The molecular formula is H₂O.
• It has a number of important characteristics, some of which are unique to this molecule, and that do not relate to other dihydrides (molecules containing 2 hydrogens):
  • It exhibits a strong molecular attraction to other H₂O molecules (cohesion) as well as other polar surfaces (adhesion).
    • Cohesion allows for surface tension, the difficulty in stretching or breaking a liquid surface. Water has one of the highest surface tensions known. Figures 3.1, p.42, 3.3, p.43
  • It has high specific heat. Water is slow to warm and slow to cool, which provides a much more stable environmental temperature.
    • Air temperature and water temperature are related; takes a while for air temperature to decrease the temperature of water.
    • Heat -- measure of the amount of kinetic energy (motion) of molecules in a system; the molecules are bouncing against each other creating this kinetic energy and creating "heat".
    • Temperature -- measure of heat in a system.
  • Its highest Density is BEFORE the freezing point, therefore, ice floats! figure 3.5, p.45
    • When looking at volumes, # of molecules (mass) in that volume is its density, which is mass per unit of volume. Typically Gas is the least dense, liquid is more dense and solid is most dense.
    • In the case of water, hydrogen bonds will only allow the water molecules to compress just so far, and this will create the crystalline structure that is ice, which creates a density that is less than the density of liquid water, which is what allows ice to float.
    • It is this particular situation which is what allows the oceans to remain liquid. If ice was more dense than liquid water, the ice would sink and the bottom of our oceans would freeze solid until the entire oceans were solid ice.
    • Typically the average depth of the ocean is 1 mile with an average temperature of 37° Fahrenheit. Since ice floats, it provides an insulating layer which allows the temperature of the water to stay constant and allows life to exist under the layer of ice.
  • Excellent Solvent (a substance in which another substance dissolves). Polarity allows it to pull apart most ionic or polar molecules, ie NaCl in H₂O.
    • Solute -- "Dissolvee" -- the substance that dissolves in a solvent. Dissolving in water, means the solute is "water soluble".
    • Solution -- Solute + Solvent.
    • The percentage of the solution tells you how much solute is present, ie 20% aqueous NaCl solution is 20% sodium and chloride atoms and 80% water. figure 3.7, p.45, 3.8, p.46
    • Hydrophilic ("hydro" -- water; "phil-" to love) -- Substances attracted to water by the nature of the bonds they contain, but that DO NOT dissolve when in contact with that water (ie Cotton).
    • Hydrophobic ("hydro" -- water; "phobos" -- to fear) -- substances repelled by water due to the nature of the bonds they contain, but that also do not dissolve in water (ie oil, fats, lipids)
  • Water is the only substance on Earth found in all three states of matter within the normal range of earth temperatures.
  • Water molecules can readily dissociate, that is they can separate into ions (H⁺ and OH^-), and then reform. This happens all the time in water. figure p.47

Water and pH

• pH -- logarithmic scale that measures the concentration of dissociated hydrogen cations in a solution.
• Neutral -- EQUAL number of H⁺ cations and OH^- anions in a solution.
• Acid -- Substance that releases H⁺ ions plus one or more anions. ie HCl + H₂O --> H⁺ + Cl^-
• Base (Alkali) -- Substance that release OH^- ions plus one or more cations. ie NaOH + H₂O --> Na⁺ + OH^-
• Some bases can also afect pH, not by releasing OH^- ions, but by accepting H⁺ ions ie NH₃ + H₂O <--> NH₄⁺
• Strong acids and bases typically dissociate completely in solution, eg HCl and NaOH, whereas weak acids or bases exhibit reversible reactions, that is dissocation and reformation eg NH₃ + H₂CO₃ (carbonic acid) <--> HCO₃^- + H⁺
• The pH scales ranges from 0 to 14, 7 in dead center, being neutral.
• The stronger the acid, the number gets closer to 0; the stronger the base, the number gets closer to 14
• Each increase (or decrease) of 1 on the pH scale is a 10X increase or decrease in acidity or alkalinity figure 3.9, p.48. Good Quiz Question -- What's the difference in acidity between A witha pH of 9 and B with a pH of 5? Count the difference as 10^x, 9 - 5 is 4, or 10⁴ or 10,000 times more acidic.
• Buffer -- a substance which weakens a strong acid or a strong base eg Bufferin.
• Many living organisms have natural buffering systems in order to maintain homeostasis. Hyperventilation (over oxygenated) causes the blood pH to rise, over oxidizing the blood, CO₂ - carbonic acid lowers the pH. Average pH of Blood, 7.4
• While many living systems possess internal pH's near neutral, this is NOT true of all systems nor of all environments.
• Salt -- ionically bonded compounds which in solution produces cations and anions, but NO H⁺ or OH^- ions. eg NaCl + H₂O --> Na⁺ to Cl^-

Disclaimer: These are MY notes taken from classroom lectures while I'm in the classroom. While I'm perfectly happy to share my notes with my classmates and I know I take very good notes, you should still make every effort to attend the class and TAKE YOUR OWN NOTES. I will not transcribe everything the instructor says in the classroom, and I will NEVER post pre-exam reviews. My notes will not replace the value of actually attending class and taking your own class notes.I also cannot attest to their accuracy, other than they are what was provided in the lecture; you should not reference my notes as "expert opionion" by any means, and if you notice an error or omission, please do me the favor of e-mailing me with the correction and I will re-post my notes. End of Disclaimer.