Date post: | 22-May-2015 |
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WATER & pHWATER & pH
Fun Facts
• 50-95% of weight of living things
• Only common substance that exists in solid, liquid and gas forms
• If earth was smooth, depth of water would be 2.5m
• 98% of water is in liquid form– 2% polar, glacial, soil, atmosphere, organisms– Most evaporation is from oceans
Special Properties
• Cohesion/ Adhesion• High surface tension• High specific heat
– required to heat 1g of water 1 degree C
• High boiling point• High heat of vaporization
– Heat required to convert 1 g of water to water vapor – 580 cal
• Ice floats• Versatile Solvent
Hydrogen bonds between water molecules
Water transport in plants
COHESION & ADHESION
Walking on water
HIGH SURFACE TENSION: RESULT OF H-BONDS
Evaporative cooling
The structure of ice
The structure of ice
ICE FLOATS BECAUSE IT IS LESS DENSE
Ice, water, and steam
WATER IS MOST DENSE AT 4 DEGREES C
WaterIce Steam
Ice floats and frozen benzene sinks
Floating ice and the fitness of the environment
A crystal of table salt dissolving in water
WATER DISSOLVES POLAR AND CHARGED MOLECULES
A water-soluble protein
Dissociation
• Dissociation of water:– H2O H+ + OH-
• Dissociation of other things:– Ex: HCl H+ + Cl-
• Acids yield H+ ions
• Bases yield OH- ions
Chemical reaction: hydrogen bond shift
pH
• Aqueous solutions @ 25o C– [H+][OH-] = 10-14
– [ ] = molar concentration
• pH = -log [H+]
• pOH = -log [OH-]
• Ex: neutral solution, [H+] = 10-7
– pH = -log [10-7] = -(-7) = 7
pH
• Each step in pH scale is an order of magnitude (x10)
The effects of acid precipitation on a forest
Pulp mill
Acid rain damage to statuary, 1908 & 1968
Buffers
• Minimize changes in pH– Accepts H+ in excess – donates H+ when depleted
• Weak acid and corresponding base
• Example:
• H2CO3 HCO3- + H+
Rise in pH
Drop in pH
Water Movement
• Water potential – – Potential energy of water molecules
• gravity, pressure, conc. of solutes, etc.
– Movement from a region of greater potential to a region of lower water potential
• Osmotic potential – – Movement across a selectively permeable membrane – Determined by measuring the pressure required to
stop the osmotic movement of water in the solution
Water Movement
Water moves across a selectively permeable membrane
From a region of: To a region of:
Higher water potential Lower water potential
Lower solute conc. Higher solute conc.
Hypotonic soln. (less solute)
Hypertonic soln. (more solute)
Lower osmotic potential Higher osmotic potential
Question 1
Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature?
A. Water’s change in density when it condensesB. Water’s ability to dissolve molecules in the airC. The release of heat by the formation of
hydrogen bondsD. The absorption of heat by the breaking of
hydrogen bondsE. Water’s high surface tension
Question 2
The bonds that are broken when water vaporizes are
A. Ionic bonds
B. Bonds between water molecules
C. Bonds between atoms within individual water molecules
D. Polar covalent bonds
E. Nonpolar covalent bonds
Question 3
Which of the following is an example of a hydrophobic material?
A. PaperB. Table saltC. WaxD. SugarE. Pasta
Question 4
Acid precipitation has lowered the pH of a particular lake to 4.0. What is the hydrogen ion concentration of the lake?
A. 4.0 M
B. 10-10 M
C. 10-4 M
D. 104 M
E. 4%
Question 5
What is the hydroxide ion concentration of the lake described above?
A. 10-7 M
B. 10-4 M
C. 10-10 M
D. 10-14 M
E. 10 M
Summary• Polarity of water results in H-bonding
– Water’s special properties
• Organisms depend on cohesion of water– Pull water up plant vessels– Surface tension
• Water moderates temperatures on earth– High specific heat– Evaporative cooling/ high heat of vaporization
• Ice floats– Lakes and oceans don’t freeze, life goes on under ice
• Water is the solvent of life– Dissolves charged/ polar substances