Chapter 5: The Water We Drink

1. Chapter 5: The Water We Drink

2. “Water has never lost its mystery. After at least two and a half millennia of philosophical and scientific inquiry, the most vital of the world’s substances remains surrounded by deep uncertainties. Without too much poetic license, we can reduce these questions to a single bare essential: What exactly is water?” Philip Ball, in Life’s Matrix: A Biography of Water, University of California Press, Berkeley, CA, 2001, p. 115 Do you know where your drinking water comes from? Do you know if your drinking water is safe to drink? How would you know?

3. Where Does Potable (fit for consumption) Drinking Water Come From? Surface water: from lakes, rivers, reservoirs Ground water: pumped from wells drilled into underground aquifers 5.2

4. The average American usesalmost 100 gallons of water a day. Nearly ¾ of the water enteringour homes goes down the drain. Much of our clean water comes from underground aquifers. The Ogallala Aquifer is shown in dark blue. While normally free of pollutants, groundwater can be contaminated by a number of sources: Abandoned mines Run off from fertilized fields Poorly constructed landfills and septic systems Household chemicals poured down the drain or on the ground. 5.2

5. A solution is a homogeneous mixture of uniform composition. Solutions are made up of solvents and solutes. Substances dissolved in a solvent- usually present in the lesser amount. Substances capable of dissolving other substances- usually present in the greater amount. When water is the solvent, you have an aqueous solution. 5.3

6. 5.3

7. Concentration Terms Parts per hundred (percent) Parts per million (ppm) Parts per billion (ppb) 20 g of NaCl in 80 g of water is a 20% NaCl solution 5.4

8. Molarity (M) = moles solute liter of solution [ ] = “concentration of” 1.0 M NaCl solution [NaCl] = 1.0 M = 1.0 mol NaCl/L solution Also – this solution is 1.0 M in Na+ and 1.0 M in Cl- [Na+] = 1.0 M and [Cl-] = 1.0 M 5.4

9. What is the concentration (in M and mass %) of the resulting solution when you add 5 grams of NaOH to 95 mL of water? 95 mL H2O = 95 g H2O mass % : 5 g NaOH/100 g solution 95 mL H2O = .095 L = 5% NaOH 5 g NaOH = 0.125 moles NaOH 0.125 mole NaOH/0.095 L = 1.3 M solution of NaOH 5.4

10. What is the molarity of glucose (C6H12O6) in a solution containing 126 mg glucose per 100.0 mL solution? 6.99 x 10-3 M 5.4

11. How to prepare a 1.00 M NaCl solution: mol solute M = L of solution Note- you do NOT add 58.5 g NaCl to 1.00 L of water. The 58.5 g will take up some volume, resulting in slightly more than1.00 L of solution- and the molarity would be lower. 5.4

12. Different Representations of Water Lewis structures Space-filling Charge- density Region of partial negative charge Regions of partial positive charge Charge-density 5.5

13. Electronegativity is a measure of an atom’s attraction for the electrons it shares in a covalent bond. EN Values assigned by Linus Pauling, winner of TWO Nobel Prizes. On periodic table, EN increases 5.5

14. Difference in Electronegativity • Examples • D EN equal or greater than 2.0 = ionic bond NaCl • D EN 0.4-1.9 = polar covalent bond HF, H2O • EN 0.0-0.3 = non-polar covalent bond N2, O2 • Molecules can have individual polar bonds and still be non-polar overall because the bond dipoles (red arrows) cancel out. Examples: CO2, CH4 • O C O

15. A difference in the electronegativities of the atoms in a bond creates a polar bond. O H H A polar covalent bond is a covalent bond in which the electrons are not equally shared, but rather displaced toward the more electronegative atom. Partial charges result from bond polarization. 5.5

16. H2 has a non-polar covalent bond. A water molecule is polar – due to polar covalent bonds and the shape of the molecule. NaCl NaCl has an ionic bond-look at the EN difference. Na = 1.0 Cl = 2.9 DEN = 1.9 5.5

17. Polarized bonds allow hydrogen bonding to occur. A hydrogen bond is an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule. The H atom must be bonded to an O, N, or F atom. Hydrogen bonds typically are only about one-fifteenth as strong as the covalent bonds that connect atoms together within molecules. H–bonds are intermolecular bonds. Covalent bonds are intramolecular bonds. 5.6

18. Forming ions 5.7

19. When ions (charged particles) are in aqueous solutions, the solutions are able to conduct electricity. • Pure distilled water (non-conducting) • Sugar dissolved in water (non-conducting): a nonelectrolyte • NaCl dissolved in water (conducting): an electrolyte 5.7

20. Substances that will dissociate in solution are called electrolytes. Ions are simply charged particles-atoms or groups of atoms. They may be positively charged – cations. Or negatively charged- anions. Dissolution of NaCl in Water The polar water molecules stabilize the ions as they break apart (dissociate). H2O NaCl(s) Na+ (aq) + Cl-(aq) 5.7

21. Some atoms form more than one stable ion 5.7

22. Naming simple ionic compounds is easy- Name the metallic element (cation) first, followed by the non-metallic element (the anion) second, but with an –ide suffix. Mg is the metal, O is the non-metal MgO magnesium oxide Na is the metal, Br is the non-metal NaBr sodium bromide 5.7

23. Ions that are themselves made up of more than one atom or element are called polyatomic ions. NaSO4 (sodium sulfate) dissociates in water to form: The sulfate group stays together in solution. Na+ and Sodium ions Sulfate ions 5.7

24. Naming polyatomic ionic compounds is also easy- Name the cation first, followed by the anion second. Mg+ is the cation, OH- is the anion MgOH magnesium hydroxide NH4+is the anion, Br- is the anion NH4Br ammonium bromide 5.7

25. Simple generalizations about ionic compounds allow us to predict their water solubility. *Insoluble means that the compounds have extremely low solubility in water (less than 0.01 M). All ionic compounds have at least a very small solubility in water. 5.8

26. Covalent molecules in solution A sucrose molecule – when dissolved in water, sugar molecules interact with and become surrounded by water molecules, but the sucrose molecules do not dissociate like ionic compounds do; covalent molecules remain intact when dissolved in solution. They will not conduct electricity; they are non-electrolytes. 5.9

27. Like dissolves like 5.9

28. Maximum Contaminant Level Goal (MCLG) and Maximum Contaminant Level (MCL) 5.10 5.10

29. Hard water contains high concentrations of dissolved calcium and magnesium ions. Soft water contains few of these dissolved ions. A pipe with hard-water scale build up Not in 6th ed.

30. Because calcium ions, Ca2+, are generally the largest contributors to hard water, hardness is usually expressed in parts per million of calcium carbonate (CaCO3) by mass. It specifies the mass of solid CaCO3 that could be formed from the Ca2+ in solution, provided sufficient CO32- ions were also present: Ca2+(aq) + CO32–(aq)  CaCO3(s) A hardness of 10 ppm indicates that 10 mg of CaCO3 could be formed from the Ca2+ ions present in 1 L of water.   Not in 6th ed.

31. Schematic drawing of a typical municipal water treatment facility 5.11

32. Getting the lead out: Schematic of a typical spectrophotometer Using a plot of absorbance vs. concentration Calibration graph 5.12

33. Access to safe drinking water varies widely across the world. 5.14

34. Two water purification techniques: Reverse osmosis Distillation 5.14

35. Water, water, every where,And all the boards did shrink;Water, water, every where,Nor any drop to drink. And every tongue, through utter drought,Was withered at the root;We could not speak, no more than ifWe had been choked with soot. The Rime of the Ancient Mariner Samuel Taylor Coleridge