Structure and Properties of Water

1. Structure and Properties of Water • Molecular Structure • Physical Characteristics • Implications for Life on Earth • www.sbu.ac.uk/water/anmlies.html

2. Why do we care? • Molecular structure and physical characteristics determine important properties • Physical characteristics serve as a foundation for measurement systems • Physical characteristics support life on the planet

3. - 104.5° + Bipolar molecule • H2O in purest form • Two hydrogen atoms and one oxygen atom • Bipolar structure due to orientation of hydrogen and oxygen atoms held in covalent bond

4. Significance of bipolar structure • Covalent bonds result when electrons are shared by atoms • Oxygen is larger than hydrogen so sharing is unequal • The result is an asymmetric distribution of charge, leading to a positive and negative end of the molecule

5. Solubility of Chemical Compounds • Efficient solvent • Two mechanisms – ionic and hydrogen bonding • Ionic bonding: charged ions of a compound are isolated by oppositely charged poles of water molecules and separated (the compound dissociates) • Hydrogen bonding: compounds are partially dissociated and held in solution

6. Water as solvent Solvent: a substance that dissolves other substances, thus forming a solution. Substances may mix thoroughly so that solute cannot be seen, or the solution is visibly changed • Almost all chemical substances have been found in solution in water • Transfers gases to earth surface • Dissolved nutrient transfers sustain life

7. Attraction between ions and partial charges of poles

8. Hydrogen bonding • Charge-based bonds • Weaker than covalent bonds and temporary • Strong enough to develop forces that determine many physical properties

9. Hydrogen bonding • Results in surface tension • Determines density and molecular structure of water and ice • Determines wetting ability and solubility of surfaces and compounds

10. Surface tension The cohesive force of water or the work that needs to be done to pull it apart Affects: • Groundwater flow • Infiltration into soils • Ability of water surface to support an object Examples: • water rise in a capillary tube • needle floating on water • the Jesus lizard

12. Density and molecular structure of water • Density: Mass per unit volume • Typically used density of water: 1 g/cm3 • Actual range of densities of liquid: 1 - .987 g/cm3 (at 4º and 100º C)

13. Density and Molecular Structure • As many liquids cool, they decrease in volume (alcohol) • Hydrogen bonds are temporary, and the overall number of bonds formed is dependent upon molecular motion • Molecular motion decreases as temperature decreases

14. Density and Molecular Structure • Increased rigidity and alignment lead to slight expansion below 4ºC Implications • Ice is less dense than liquid water and floats • Water bodies freeze from the surface down, rather than the bottom up

15. Why is this important? • Very few life forms can withstand freezing and thawing cycles • Many can withstand cold conditions • Ice remains on the surface of water bodies • Life on the bottom is preserved • Lakes turn over • Allows for aquatic life in the higher latitude regions

16. Trends in boiling points in similar compounds

17. Wetting: Hydrophobic and hydrophilic surfaces • Hydrogen bonding can occur between molecules of chemically diverse substances • Tendency for water to coat or bead on a surface is related to hydrogen bonding that takes place on the surface • Hydrophilic surfaces allow coating because they have oxygen with unpaired electrons • Surface tension forces, caused by hydrogen bonding, cause beading on hydrophobic surfaces

18. Water as a reference substance • Bounds for liquid state of water define temperature measurement scales • Example: Fahrenheit and Celsius scales • Energy: specific heat and specific heat capacity

19. Fahrenheit Scale • Developed by Gabriel Fahrenheit – a thermometer maker • Scale replaced a previous scale based on body temperature • Divided temperature difference between water in solid state and in gas state into 180º, with 32º as the reference for ice • Depended upon changes in density of fluids (mercury and alcohol)

20. Celsius (Centigrade) Scale • Introduced in 1742 by Andreus Celsius • A decimal system, with 100º between solid and gas phases • Reference temperature is 0º at freezing Question: at what temperatures are Celsius and Fahrenheit readings the same?

21. Modulus of Expansion Liquid: • Density varies with temperature (mass/volume) • Liquid to solid: volume expands ~+9% upon freezing If temperature changed from 4ºC  4ºC, and we had a completely full container, how would the volume change?

22. Modulus of Compression • Relatively incompressible • A 1 psi decreases volume by about 3.4•10-4% •  a pressure of 100 psi would decrease volume by about 3.4 •10-2% • Water is efficient in transferring kinetic energy when enclosed in a rigid structure

23. Water and Energy Transfer • In liquid phase, water absorbs 1 calorie per temperature rise of 1º C/cm3 • An additional 539 calories are needed to go from liquid to gas phase • Going from liquid to solid phase releases 80 calories per cm3 • Change in state releases or absorbs significant amounts of energy • Liquid water can absorb and store large amounts of energy

24. Applications Sweating: vaporization for cooling • Liquid water on a hot free surface requires 539 calories per cm3 to become vapor • The result is substantial loss of thermal energy (cooling) Heat transfer: car cooling systems • Flowing water efficiently transfers heat away from hot spots created by friction

25. Global energy transfer • Equatorial zone receives much more solar energy than poles • Energy transfer from equator is accomplished by evaporation, condensation and ocean currents • Oceans have tremendous capacity to absorb and transfer solar energy

26. Summary • Chemical and physical properties of water are important and different than many others • Hydrogen bonding leads to internal cohesion and helps in dissolution of some chemicals • Stability in some properties makes it an ideal reference substance • Also a very good energy transfer medium, especially between liquid and vapor states

27. Next time • Watershed Delineation • Readings: p. 175-180 • Homework due Friday • NOAA visit next Wednesday 9/14