The Wonder of Water

Learning Objectives Describe the distribution of water on Earth and identify sources of drinking water supply Explain the important physical properties of water in terms of its molecular structure and bonding behavior Calculate chemical concentrations of dissolved substances (e.g. percent, ppm, molarity) Distinguish between ionic and covalent compounds Predict the ionic charge on atomic ions Predict the chemical formulas of ionic compounds Identify covalent compounds that are likely to dissolve in water

Distribution of Water on Earth

Sources of Drinking Water US: Virginia: EPA Public Water System Inventory, FY2000

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.

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? In moles/liter? 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 (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 Cl Mg Li O Al Na+ Cl- Mg2+ Li+ O2- Al3+

Polyatomic Ions

Formulae for Ionic Compounds Mg + O Mg + Cl Al + O magnesium sulfate calcium phosphate Mg2+ & O2- → MgO Mg2+ & 2(Cl-) → MgCl2 2(Al3+) & 3(O2-) → Al2O3 Mg2+ & SO42- → MgSO4 3(Ca2+) & 2(PO43-) → Ca3(PO4)2