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Physical and Chemical Properties of Water

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1 Physical and Chemical Properties of Water
Advanced Environmental Geochemistry – Lecture 3 – Spring 2010 In a 2008 paper in Nature, Philip Ball says, “No one really understands water. It’s embarrassing to admit it, but the stuff that covers two-thirds of our planet is still a mystery.” Much more work remains to be done on water, but we can examine the properties of water, especially as they affect the earth. The physical and chemical properties of water are anomalous with respect to almost all other known liquids, and the degree of this anomalous behavior is often large. We have seen that the melting and boiling points of water are much higher than would be expected, and have explained this behavior on the basis of hydrogen bonding. We have also seen that the increase of density upon melting may be explained by the rigid tetrahedral structure of H2O in the solid state resulting in a coordination number of IV. The rigidity of this structure is due at least in part to hydrogen bonding. In the liquid state it has been demonstrated that some hydrogen bonding persists and accounts for the loose chains of water molecules observed in x-ray studies. The lack of rigidity allows the coordination to increase to approximately 4.4 at 4̊C. Freeze/thaw properties account for potholes, as well as some physical weathering. We will now examine some other physical and chemical properties of water to ascertain in what way the behavior of water is anomalous, and how such anomalies affect the geochemical and geological phenomena associated with water.

2 Heat Capacity Heat capacity is the amount of heat required to increase a given quantity of material by a given temperature at constant pressure and volume This is usually expressed as calories per degree Celsius

3 Confused about Energy Units?
For those who want some proof that physicists are human, the proof is in the idiocy of all the different units which they use for measuring energy. -Richard Feynmann The late Richard Feynmann was a Nobel prize winning physicist

4 Energy Units A calorie is defined as the amount of heat required to raise one gram of water one degree Celsius at a pressure of one atmosphere and a temperature of 15 ̊ C. Equivalent to: joules (international standard) x 10-3 British Thermal Units (BTU’s) Molar heat capacity - the quantity of heat necessary to raise the temperature of one molecular weight of a substance by one degree Celsius The calorie is equivalent to joules (NBS standard). Using the absolute system of electrical units to define the conversion factor results in one calorie equaling joules, which is the international standard. One BTU (British Thermal Unit) is equal to international calories. Molar heat capacity is defined as the quantity of heat necessary to raise the temperature of one molecular weight of a substance by one degree Celsius.

5 Heat Capacity Comparison
Substance Heat Capacity (calories/gram) Mercury (at 0°C) Bromine 0.113 (at 25°C) Water 1.00 Ammonia 1.23 One explanation for this effect is that, as water is heated, the increased movement of water causes the hydrogen bonds to bend and break. Energy absorbed in bond bending and breaking processes is not available to increase the kinetic energy of the water, so it takes considerable heat to raise water's temperature.

6 Heat Capacity Application
Water has a very high heat capacity, but sand, dirt, trees, etc. have a low heat capacity As a consequence, the land heats up during the day and cools at night, while the temperature of the ocean remains constant This explains the direction of sea breezes –the air over land warms (during daytime), pressure drops, and air accelerates from sea to land in response With a land breeze, the air over land cools (during nighttime), pressure rises, and air accelerates from land to sea in response Source: Lectures/Heat%20and%20Entropy.ppt Sea breezes and land breezes are similar in that they are both created by changes in temperature near the ground.

7 Biological Effects of Heat Capacity
Water will "feel" distinctly colder to us than air at the same temperature Due to the much higher heat capacity of water than of air Serious implications for the maintenance of body temperature and the prevention of hypothermia in warm-blooded animals submerged in water One needs only think of the sinking of the Titanic. People who entered the water froze to death well before rescue arrived. Those fortunate enough to be on life rafts were rescued, mostly in good shape. High heat capacity of water is also important in living organisms. It has often been claimed that life depends on these anomalous properties of water. Organisms control their body temperature by employing the large heat capacity, and high thermal conductivity, along with their high water content to thermally regulate themselves, and distribute heat more evenly, preventing local temperature fluctuations.

8 Energy Transfers Water’s large heat capacity produces large energy transfers from region to region by the movement of water, in both the hydrosphere and atmosphere This further serves to regulate the climate, and maintain a more uniform temperature over the entire surface of the earth than would otherwise be the case Water’s high heat capacity is extremely important in regulating and moderating temperature. As the average temperature of the earth increases, the oceans consume vast quantities of heat without changing temperature dramatically. This effect is especially evident in coastal areas near large bodies of water, but it affects the entire hydrosphere and atmospheric systems. Lithosphere contains huge quantities of water, which serve to regulate the temperature of that sphere as well

9 Density Currents Most sharp atmospheric fronts, including land breezes, sea breezes, and coastal fronts, are density currents Density currents have a sharp leading edge, called a "head", where the air ascends rapidly Behind the head is a region of turbulent mixing between the warm and cold air. Density currents are a common phenomenon in many aspects of earth science, and we will encounter them again later in the course

10 Phase Diagram - Water Figure 3-1
Image: Figure shows a phase diagram for water, illustrating the various phase transitions. Phrase transitions involve gain or loss of large amounts of energy. Another name for melting is Fusion

11 Latent Heat of Fusion (Lf)
The quantity of heat necessary to change one gram of solid to one gram of liquid with no temperature change, usually measured in calories per gram Except for ammonia, water has the highest known value for the heat of fusion For water at 0̊ C the value is calories per gram This property exerts a strong thermostatic effect on the earth due to the high quantities of heat released as ice melts or absorbed as ice freezes This mechanism is extremely important when considering glacial episodes and other large swings of the climate The reverse process, changing from liquid to solid, is called solidification As the temperature decreases, the latent heat of fusion increases because the hydrogen-bond strength of ice increases At lower temperatures, the vibrational energy in the hydrogen bonds is lower, which gives an increasing difference (as temperature is lowered) between the enthalpy of the water and ice, to a maximum value at 256K The reverse process, changing a vapor to a solid, is called condensation, and the resultant change in heat content is called the Latent Heat of Condensation (Lc). It is actually the large scale energy release when water condenses that drives most large storms.

12 Latent Heat of Vaporization (Lv)
The quantity of heat necessary to change one gram of liquid to one gram of vapor with no temperature change, again measured in calories per gram Water has the highest value of all substances ( cal/g at 100̊ C). This property is extremely important in heat and water transport in the atmosphere. Hurricanes are one example of large scale phenomena due largely to the latent heat of vaporization of water For life, the high latent heat of evaporation helps to prevent dehydration, and also contributes considerable cooling for small amounts of water evaporated.

13 Lv Compared to Lf When a solid is heated, turning it into a liquid, the kinetic energy of its molecules is increased, moving them further apart until the forces of attraction are reduced to allow the liquid to flow freely However, the forces of attraction still exist When a liquid is heated, turning it into a gas, the kinetic energy of the molecules are increased to a point where there are no forces of attraction between the molecules The energy required to completely separate the molecules, moving from liquid to gas, is much greater than the energy required to just to reduce their separation, solid to liquid

14 Latent Heat of Sublimation (Ls)
This quantity describes the heat required to change ice to vapor The value is 680 calories per gram (cal/g) of water The reverse process (vapor to ice) is known as the Latent Heat of Deposition (Ld)

15 Energies Associated with the Phase Changes of Water
Process Changes Heat gained (+) or lost (-) by the air From To J/g Cal/g Condensation Vapor Liquid 2255.3 539.55 Vaporization Deposition Ice 2840 680 Sublimation -2840 -680 Fusion -333.2 -79.71 Solidification 333.2 79.71 Table summarizes the energies associated with the phase changes of water. Table 3-1

16 Thermal Expansion The coefficient of volume expansion for liquids is the ratio of the change in volume per degree to the volume at 0̊C If V equals volume, t is the temperature, and β is the coefficient of expansion equation 3-1 gives the formula for computing Vt knowing the initial volume, V0 V =  V0 t

17 β Values for Freshwater
For water the coefficient is negative between zero and four degrees Celsius. For salt water the coefficient is negative between the melting point and the point of maximum density, but both decrease as salinity increases Beta is a function of temperature, pressure, and salinity This property is important in controlling temperature distribution and circulation, especially in lakes It is also critical in assessing future sea-level rise

18 Surface Tension from Capillary Tube Measurements
T = {rhdg}/2, Where: T is the surface tension d is the density of the liquid g is the acceleration due to gravity h is the height the liquid rises height in a capillary tube r is the internal radius of the tube Two fluids in contact exhibit this phenomenon, due to molecular attractions that appear to arise from a tension in the surface of separation It may be expressed as dynes per centimeter or ergs per square centimeter The surface tension is often determined by means of capillary tubes.

19 Surface Tension of Pure Water in Contact with Air
Temperature (EC) Surface tension (dynes/centimeter) 75.6 10 74.22 20 72.75 30 71.18 40 69.56 50 67.91 60 66.18 70 64.4 80 62.6 100 58.9 Data after Camp and Meserve, 1974, p.10 The surface tension of water against air at 0̊C is 75.6 dynes per centimeter, one of the highest of all liquids (Not the highest as is often mistakenly reported. Mercury has a value of dynes per centimeter at 20̊C. Many other elements are also higher than water.) Table 3-2

20 Surface Tension vs. Temperature Plot
Image source: Figure 3-2 is a graphical plot of the same data Surface tension occurs because of the strong hydrogen bonds between molecules in the liquid state. Surface tension is important in cell physiology It is also important in drop formation and behavior, which greatly affects the formation of clouds Surface tension plays a big role in the capillary ascent of brackish water in arid regions that do not receive enough water either by nature or by irrigation to flush the salts downward toward the water table Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. This is why “steam cleaning” is sometimes employed. Soaps and detergents further lower the surface tension.

21 Water Strider Some bugs utilize the high surface tension of water

22 Helictites Start out as small tubes, like stalactites
Unlike stalactites, which grow down because of dripping water, beaded helictites grow from small drops of water that are forced into the cave through cracks The surface tension of small drops is stronger than the force of gravity For this reason, beaded helictites can grow in any direction. Source:

23 Slope Stability Perhaps the most important aspect of the surface tension of water in geology is in slope stability Water molecules can act as glue, holding soils together The bonding between water molecules often stabilized a slope.

24 Water’s Roles Water acts as “glue” Water acts as a “lubricant”
A child playing with a bucket will find that dry sand, scooped into the bucket and poured out, will form a rounded mound. Damp sand can be formed into a tower by quickly inverting the bucket and placing it upside down on the beach If the sand is too wet, however, inverting the bucket will cause the sand-fluid mixture to flow over a larger area Thus, water can act as both glue and lubricant. The weight of water can also act to trigger mass movement. Water acts as “glue” Water acts as a “lubricant”

25 Water As Glue - Surface Tension
Small amounts of water in sediment, which is essentially all surface, act to hold the sediment together Dry sand in a bucket, when turned over quickly, will form a pile whose edges slide Damp sand will hold the sand together, even though the slope angles are quite steep

26 Cohesion Hydrogen bonds constantly form and break
Each hydrogen bond lasts for a fraction of a second, but the molecules continuously form new bonds with other water molecules around them At any time a large percentage of water molecules are bonded to neighboring water molecules which gives water more structure than most other liquids Collectively, the hydrogen bonds hold water together by the property of cohesion

27 Cohesiveness Video Source:
Video: 36 Drops of Water on a penny.wmv Running time: 1 min 21 seconds

28 Wave Formation Cohesion due to hydrogen bonding contributes to the formation of waves and other water movements that occur in lakes Water movements are integral components of the lake system and play an important role in the distribution of temperature, dissolved gases, and nutrients These movements also determine the distribution of microorganisms and plankton

29 Thermal Conductance Time rate of heat transfer by conductance through a unit thickness across a unit area with a unit difference in temperature Unit: calories per second per square centimeter with a thickness of one centimeter and a temperature difference of one degree Water has the highest value of all liquids However, in liquids currents transport mass and heat much faster than thermal conductivity, so this property is unimportant outside living cells. In cells the high heat conductivity of water is often important in ridding the cell of excess heat.

30 Transparency – Lambert’s Law
Transmission factor = I/Io – e-kx Where: I = intensity of transmitted radiation I0 = intensity of the incident radiation X = thickness of the absorber k = absorption coefficient Transparency is the ability of a substance to transmit electromagnetic radiation without absorbing it. The higher the transmission factor, the greater the transparency. Pure water has high transparency in the visible part of the spectrum, but high absorption in both the infrared and ultraviolet portions of the spectrum. This has implications for the greenhouse effect. If temperatures start to rise, more water will evaporate and will absorb more heavily in the IR and the UV, at least until saturation is reached.

31 Spectrum of Water Source:
Figure shows the visible ( nm), UV (below 400 nm), and IR (above 700 nm) spectra of liquid water Water exhibits little selective absorption (absorption at a particular wavelength) in the visible portion of the spectrum and thus appears almost colorless However, although very weak, there is a slight increase in absorption at the red end of the spectrum. Together with a five-fold increase in the scattering of blue light versus red light, this accounts for the blue color of lakes. Ice is blue due to similar effects.

32 Vapor Pressure Definition
The vapor pressure of water is the pressure of the water vapor in contact with liquid water at which vapor molecules condense on the liquid surface as fast as they evaporate from it Vapor pressure varies with temperature and increases slowly with increasing pressure

33 Vapor Pressure of Water
Image: vpvst.gif Vapor pressure of water as a function of temperature A substance’s vapor pressure dictates when the substance will boil. Boiling occurs when the vapor pressure of the substance equals the surrounding pressure. In Yellowstone National Park, near 8000 feet altitude, water boils at 93̊C. The vapor pressure of water is much lower than analogous compounds because hydrogen bonding holds the molecules together much more strongly, as discussed earlier.

34 Relative Humidity The amount of water vapor in the air at any given time is usually less than that required to saturate the air The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density Equation shows the relationship Relative Humidity = Actual Vapor Density x 100 (%) Saturated Vapor Density RH is important for several reasons It helps to determine how comfortable we are at any given temperature. The dew point is the temperature at which the relative humidity would equal 100%.

35 Evaporative Cooling Evaporative coolers offer a large energy savings compared with normal air conditioners in areas where they work but do use water to operate By curtailing energy consumption, we help to lessen pollutants introduced by power plants into the atmosphere, biosphere, and hydrosphere The dew point often controls the low temperature in a humid region, such as South Florida. When the dew point is reached, water begins to condense, releasing large amounts of energy into the atmosphere This prevents temperatures from falling much below the dew point It is only possible for South Florida to get truly chilly when northwest to north winds bring a mass of cold, dry air into the region. The dew point needs to be below about 13̊C for these to work

36 Relative Humidity Calculation
If the actual vapor density is 10.0 gm/m3 at 20°C compared to the saturation vapor density at that temperature of 17.3 gm/m3 , then the relative humidity is: R.H. = x 100 = 57.8% 17.3

37 Relative Humidity Example
Source: Example assumes actual vapor density is 6.0 gm/m3

38 Fog When the temperature reaches the dew point, water droplets are likely to form A common result if fog, seen here obscuring visibility at Salt Lake airport Image: Salt Lake City Tower in Fog

39 Viscosity Definition It is sometimes called internal friction
It is a measure of the ease with which molecules can move relative to each other It depends on the forces holding the molecules together This cohesivity is large in water due to its extensive three-dimensional hydrogen bonding

40 Viscosity of Water Table 3-4
The usual unit is the poise, defined as dyne-seconds per centimeter squared In SI units, 1 Poise = 0.1 Newton Second/m2 Viscosity of pure water is a function of temperature but is relatively pressure-independent at normal surface pressures Viscosity indicates the extent of hydrogen bonding present The number of hydrogen bonds decreases as temperature increases The exact number of hydrogen bonds present is not easily measured and a widespread divergence of opinion exists on how many are present at any given temperature. Table 3-4

41 Early Discovery Bridgeman (1925) described this behavior:
“Water is unique among the substances investigated in that, at low temperatures and pressures, its viscosity decreases with rising pressure instead of increasing. At low temperatures the viscosity passes through a pressure minimum and then increases. With increasing temperature the minimum flattens out, eventually disappears, and at temperatures above approximately 25° the viscosity increases with rising pressure from the beginning. This anomalous behavior of water has been already suspected from previous measurements of viscosity at low pressures. The anomaly is doubtless connected, as are many of the other anomalies of water with a high degree of association, which changes rapidly with pressure and temperature.” P.W. Bridgeman, The Viscosity of liquids Under Pressure, Proceedings of the National Academy of Sciences 11, , September 10, 1925. Before hydrogen bonding had been discovered, its effects were observed The explanation required more time. When hydrostatic pressure is applied to water, the viscosity begins to decrease. At higher pressures the viscosity begins to increase. At 2̊C the applied pressure must be about 1200 kg/cm2 to show a viscosity increase. At 30̊C, the applied pressure need only be about 600 kg/cm2. Water is the only substance known to show this behavior. It is attributed to the breakup of clusters, (H2O)n. As pressure increases the clusters first breakup (n decreases) but later reform at higher pressures (n increases).

42 Viscosity The viscosity of water is by no means the highest known. Many oils, for example, have higher viscosities than water. Water’s viscosity is higher then some other fluids. Near room temperature, water has a viscosity of around 1 centipoise. Gasoline has a viscosity between 0.4 and 0.5 cP; the viscosity of air is cP. Sea-water has a higher viscosity than fresh water. The property of a substance to offer internal resistance to flow; its internal friction

43 Flow Viscosity Viscosity increases from left to right
After the same interval of time, water has reached the lowest level, rubbing alcohol is slightly higher, ethylene glycol (antifreeze) is quite a bit higher, and vegetable oil is the highest Viscosity increases from left to right in the photo Viscosity increases from left to right

44 Magma Viscosity Video shows a rod being poked in hot, viscous magma on Kilauea, Hawaii The magma is hot, but still has considerable viscosity Video phhrod.avi from One major effect of viscosity in geology is the behavior of magmas The viscosity of magma affects the type of eruption, from the relatively benign style of the Hawaiian shield volcanoes to the explosive eruptions associated with converging plate volcanoes like the Cascades and the Aleutian Islands Many factors influence the viscosity of magma, including the amount of dissolved gases Water is the major dissolved gas in most magmas More information may be found at at the web site Physicochemical Controls on Eruption Style by Victor Camp.

45 Chemical vs. Physical Properties
The difference between chemical and physical properties is hard to define and the distinction is surely blurred occasionally There is often pronounced overlap between chemistry and physics concerning certain properties Nevertheless, the following properties are mainly chemical in nature

46 Dissolving Power The ability to act as a solvent to many substances, and the ability to take in large quantities of many different solutes, is one property that makes water unique In large part this is due to the partly covalent, partly ionic bond between hydrogen and oxygen within the water molecule

47 Polarity The molecule is polar enough to be a good solvent for all ionic-bonded substances The ability of water to dissolve ionic substances has mandated radical changes in the surface of the earth since it was first formed Water is non-polar enough to allow some solubility for non-ionic substances After the initial loss of all surface waters the earth began to accumulate liquid water oceans as the result of volcanic outgassing Through the action of the hydrologic cycle water evaporates and precipitates over the land It infiltrates the surface and begins to dissolve salts (ionic-bonded substances) and eventually carries them to the sea This has gradually increased the salinity of the sea (which is now buffered, at least with respect to certain ions) and has helped to keep the land's surface free of salts, which are injurious to plants. In addition the solvent power of water is important in cellular processes. It is also critical in many physical processes, including soil-water interactions.

48 Permittivities The potential energy is V (volt)
q1 and q2 are two charges separated by distance r The potential energy is V (volt)

49 Polar Molecules Polar molecules, whose centers of positive and negative charge are separated, possess dipole moments This means that in an applied electric field, polar molecules tend to align themselves with the field Although water is a polar molecule, its hydrogen-bonded network tends to oppose this alignment The degree to which a substance does this is called its dielectric constant and, because water is exceptionally cohesive, it has a high dielectric constant

50 Dielectric Constant ε = permittivity of a medium, such as water
ε0 = permittivity of vacuum, εr is the dielectric constant The dielectric constant (er) of the medium (also known as the relative permittivity) is defined as shown above The high dielectric constant allows water to act as a solvent for ionic compounds, where the attractive electric field between the oppositely charged ions is reduced by about eighty-fold, allowing thermal motion to separate the ions into solution. On heating, the dielectric constant drops, and liquid water becomes far less polar, down to a value of about 6 at the critical point The dielectric constant similarly reduces if the hydrogen bonding is broken by other means, such as strong electric fields.

51 Amphoteric Substance Electrolytic dissociation is the ability of water to split into two ions, H+ (hydrogen ion) and OH- (hydroxide ion) This behavior makes water an amphoteric substance, defined as a substance capable of behaving as both an acid and a base Actually H+ , a bare proton, does not exist in solution Protons quickly combine with water molecules to form H3O+ ions, sometimes called oxonium ions This tendency is very small.

52 Electrolytic dissociation
[H+] [OH-] = 10-14 The equation shows the solubility product of these two ions at 25°C In neutral water, [H+] = [OH-] = 10-7 Image source: Equation 3-8 lists the solubility product of these two ions. KW is strongly temperature dependent, and the value shown is for 25̊C. KW = [H3O +] [OH-] = (eq 3-8)

53 Value of - log KW as a function of temperature
Values from Chemical Rubber Corporation, Handbook of Chemistry and Physics, 90th edition, page 8-79, 2009. Table lists the value of - log KW as a function of temperature. Since [H3O+] = [OH-] for a neutral solution, the pH of a neutral solution actually depends on temperature The oft-quoted value of pH = 7.0 for a neutral solution applies to a temperature of 25̊C Table also shows the value of a neutral solution for all of the various temperatures.

54 Water in Biology Running time: 12 minutes 8 seconds

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