The Wonder of
Water
Distribution of Water on Earth
Sources of Drinking Water
37%
US:
63%
21%
Virginia:
79%
EPA Public Water System Inventory, FY2000
groundwater
surface water
What Water Do You Drink?
http://www.epa.gov/safewater/dwh/who.html
Water and Heat
• Specific heat: 1 calorie = energy to heat 1 gram of
water by 1o C = 4.184 J/g oC
• Heat of fusion: heat that must be absorbed to cause
melting = 331 J/g for H2O
• Heat of vaporization: heat that must be absorbed to
change liquid into vapor = 2250 J/g for H2O
Electronegativity
• measure of an atom’s attraction for electrons in bonds
O atom is more electronegative than H atom…
…so the “shared” electrons tend to hang out there!
Charge Density Model of Water Molecule
• Since water is bent, this means that one “end” is
more negative, and the other more positive.
• That is, water is polar.
• This in turn makes water molecules bond to
each other in a very special way.
QuickTime™ and a
decompressor
are needed to see this picture.
Structure of Ice
Lots of open spaces!!
(decrease density)
↑
~4 ºC
Natural Phenomenon: Lake Turnover
Water as a Solvent
• solvent = a substance that dissolves other substances
• solute = substance that is dissolved
• solution = a homogenous mixture (if water is the
solvent, it’s called an aqueous solution)
• electrolytes = solutes that conduct electricity
Solute Concentration in Aqueous
Solutions
• percent
• ppm and ppb
• molarity (moles/L)
Example: Calculating ppm
If 1 liter of seawater contains 35 grams of dissolved
NaCl, what is the concentration of NaCl in ppm?
Density of water is 1g/ml, or 1000g/L
(35 g/L)*(1000 mg/1 g)*(1 L/106 mg) = 35,000 mg/106 mg
35,000 mg/106 mg = 35,000 ppm
In moles/liter?
(35 g/L)*(1 mole/58.4 g) = 0.60 M NaCl
Ionic vs. Covalent Compounds
• ions = charged atoms or molecules (e.g. Na+)
• ionic compounds = solid crystalline substances
made of anions and cations (e.g. table salt)
• polar compounds = contain covalent bonds with
electrons that are not shared equally (e.g. water)
Ionic or Covalent?
• Well, a little of both…
• The more electronegativity in the compound, the
more likely the compound will dissociate into
separate ions
• I.e. one atom “borrows” one or more electrons
from the other
• Even water dissociates this way - there is a
percentage of H+ and OH- ions in every glass of
water you drink.
Sodium Chloride Dissolving in
Water
Predicting Charge on Ions
(Hint: Look at valence electrons…)
Na
Na+
Cl
Cl-
Mg
Mg2+
Li
Li+
O
O2-
Al
Al3+
Polyatomic Ions
Formulae for Ionic Compounds
Mg + O
Mg2+ & O2- → MgO
Mg + Cl
Mg2+ & 2(Cl-) → MgCl2
Al + O
2(Al3+) & 3(O2-) → Al2O3
magnesium sulfate Mg2+ & SO42- → MgSO4
calcium phosphate 3(Ca2+) & 2(PO43-) → Ca3(PO4)2