Department of Chemistry CHEM1020 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1020/General Chemistry _________________________________________ Chapter 13. (L25)-Chemistry of Water Today’s Outline …Water (H 2 O) as a chemical substance Basic physical and chemical properties of water Effect of temperature on the structure of pure water … Water as a natural body: The hydrosphere …Origin of water and water body on earth surface...Water as a natural resource and life preservation system …Some basic physical properties of natural waters …Some basic chemical properties of natural waters

Chapter 13. (L25)-Chemistry of Water Water as a chemical substance … The H-O-H bonding angle: 104˚ 31’ (not 109.5˚ for a perfect cube) The H-O distance: Å in water vapor, 0.99Å in ice Polar nature of water molecule …2 pairs of unshared e - forming two arms in the opposite direction, representing the regions of negative electrification, which would attract the positive partial charge of H of nearby water molecules (the H-bonding), thus binding water molecules together.

Chapter 13. (L25)-Chemistry of Water Water as a chemical substance … Significance: Without the two negative arms in the water molecule, water molecules would not be held together to form the liquid water as it is, and they become water vapor, and consequently, no liquid water, no oceans, no seas, no lakes, rivers, etc. Then, no life! H-bonding is strong; upon heating ice, only ~15% of H- bonding is broken in ice Water is anomalous, perhaps unique

Chapter 13. (L25)-Chemistry of Water Water as a chemical substance …Basic physical and chemical properties of water b.p.: very high max density: 4 ˚C (not at f.p.), so water is heaviest at 4 ˚C water expends upon freezing surface tension: very high (water droplets) solvent: excellent polarity: very high a very special medium (aqueous media)

Chapter 13. (L25)-Chemistry of Water Water as a natural body: the hydrosphere …Water occupies ~70% of the earth surface, while land only ~30%. Hence, the hydrosphere is extremely important in earth surface system. …Natural waters: Complex biogeochemical systems a mixture of liquid water, solid particles, and various kinds of natural and anthropogenic chemical substances organized as a system the hydrosphere interacting with other compartments of the earth surface system, which is powered by solar radiation; the main-stream of the hydrosphere is controlled by global water cycle.

Chapter 13. (L25)-Chemistry of Water Water as a natural body: the hydrosphere …Liquid water surface water atmospheric waters (rain, fog, dew, cloud) …Solid water ice snow sleet hail …Gaseous water water vapor

Chapter 13. (L25)-Chemistry of Water Water as a natural body: the hydrosphere …Type of natural waters freshwaters (fresh, because of outflow, open aquatic systems) river stream spring brook lake reservoir pond wetland ground water soil water atmospheric water

Chapter 13. (L25)-Chemistry of Water Water as a natural body: the hydrosphere …Type of natural waters salty waters (salty, because of no outflow except evaporation, closed aquatic systems) sea (estuary) ocean stagnant waters running waters

Chapter 13. (L25)-Chemistry of Water Origin of natural water The earth formed from a cloud of ionized gas around the sun. The gases condensed and formed the primitive earth. The other small condensed pieces and bodies containing water hit the early earth in its making, and the collision produced heat, which, plus the internal heat from radioactive decay, released the water from minerals and silicates along with CO 2, CO, N 2, NH 3, and CH 4, and it was retained on the earth by gravitation in the steaming early atmosphere. Later, with gradual cooling, the water condensed and fell to form ancient oceans. Meteorites kept heavy bombardment from 4.5 to 3.8 billion years and removed the water in the earth surface layers. After the bombardment ended, life appeared shortly about 3.5 byr. Water formed mainly during the formation of the earth itself. Since then, only very small additions or losses of water have occurred. Water is lost by UV- photodissociation to H and O in the upper atmosphere. Cold-trap at ~15 km of the atmosphere, where it is so cold that water becomes ice and thus remained in the atmosphere.

Chapter 13. (L25)-Chemistry of Water Water as a natural resource and life preservation system drinking water agriculture civil use industrial use life design: Photosynthesis via splitting water and respiration via recovery of water by means of water redox chemistry Q: Oil or water, which would have more profound and enduring role in the human society?

Chapter 13. (L25)-Chemistry of Water Some basic physical properties of natural waters -Color (the presence of fine particles, and Ca 2+, causes to change from blue to green; yellow to brown caused by humic substances, or Fe(III)) -Turbidity (particles) -Odor (volatile substances, natural or anthropogenic) -Temperature -Taste (appropriate amounts of salts and pH give a good teast. But e.g., pH > 8, soapy; Fe and Mn, bad taste; Mg and SO 4 2-, bitter)

Chapter 13. (L25)-Chemistry of Water Some basic chemical properties of natural waters -pH/Acidity/Alkalinity -Hardness -Dissolved oxygen (DO) -Conductivity -Salinity -Redox potential -Inorganic ions (cations, anions) -Total and Dissolved organic carbon (TOC/DOC) -Nutrients (N, P, K, Fe, Mn, etc.) -COD (chemical oxygen demand) -BOD (biological oxygen demand)

Chapter 13. (L25)-Chemistry of Water Quiz Time Homework TBA