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Physical and Chemical Properties of Water

Advanced Environmental Geochemistry – Lecture 3 –

Spring 2010

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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

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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

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 3 C.

• Equivalent to: 4.18674 joules (international standard) 3.96832 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

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Heat Capacity Comparison

Substance Heat Capacity (calories/gram)

Mercury 0.033382 (at 0°C)

Bromine 0.113 (at 25°C)

Water 1.00

Ammonia 1.23

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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

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

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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

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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

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Phase Diagram - Water

Figure 3-1

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 03 C the value is 79.71 calories per gram

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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 (539.55 cal/g at 1003 C).

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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

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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)

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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 Liquid Vapor -2255.3 -539.55

Deposition Vapor Ice 2840 680

Sublimation Ice Vapor -2840 -680

Fusion Ice Liquid -333.2 -79.71

Solidification Liquid Ice 333.2 79.71

Table 3-1

Thermal Expansion

• The coefficient of volume expansion for liquids is the ratio of the change in volume per degree to the volume at 03C

• 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

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β Values for Freshwater

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Temperature, °C (10 -6 K -1)

1 -50

4 0

10 88

20 207

30 303

40 385

50 457

60 522

70 582

80 640

90 695

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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

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Surface Tension of Pure Water in Contact with Air

• Table 3-2

Temperature (C) Surface tension

(dynes/centimeter)

0 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

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Surface Tension vs. Temperature Plot

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Water Strider

• Some bugs utilize the high surface tension of water

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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.

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.

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Water’s Roles

• Water acts as “glue”• Water acts as a “lubricant”

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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

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

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Cohesiveness Video

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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

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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 liquids29

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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

Spectrum of Water

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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

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Vapor Pressure of Water

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Relative Humidity

Relative Humidity = Actual Vapor Density x 100 (%)

Saturated Vapor Density

• 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

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.

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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. = 10.0 x 100 = 57.8%17.3

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Relative Humidity Example

• Example assumes actual vapor density is 6.0 gm/m3

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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

Salt Lake City Tower in Fog

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

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Viscosity of Water

Table 3-4

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.”

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Viscosity

• The property of a substance to offer internal resistance to flow; its internal friction

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Flow Viscosity

Viscosity increases from left to right

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Magma Viscosity

• Video shows a rod being poked in hot, viscous magma on Kilauea, Hawaii

• The magma is hot, but still has considerable viscosity

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

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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

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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

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Permittivities

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

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

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Dielectric Constant

ε = permittivity of a medium, such as water ε0 = permittivity of vacuum, εr is the dielectric constant

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

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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

KW = [H3O +] [OH-] = 10-14 (eq 3-8)

Value of - log KW as a function of temperature

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t/C - log KW Neutral pH t/C - log KW Neutral pH

0 14.938 7.47 55 13.152 6.57

5 14.727 7.37 60 13.034 6.51

10 14.528 7.27 65 12.921 6.45

15 14.340 7.17 70 12.814 6.40

20 14.163 7.08 75 12.712 6.35

25 13.995 7.00 80 12.613 6.30

30 13.836 6.92 85 12.520 6.26

35 13.685 6.84 90 12.428 6.21

40 13.542 6.77 95 12.345 6.17

45 13.405 6.70 100 12.265 6.14

50 13.275 6.63

Water in Biology

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