MEA 200 Exam 2 Study Guide

Explain how the unique asymmetric structure of the water molecule results in a dipolar electrical distribution of charges on the molecule.

-A water molecule is formed when two hydrogen atoms covalently bond with one oxygen atom. The oxygen atom has 3 pairs of electrons in the outer shell. What's weird is that both hydrogen atoms bond with only ONE of those 3 pairs creating an asymmetric molecule.

-The bond angle between the two hydrogen atoms is 105, rather than the bond angle of a symmetric molecule, which is 180 degrees.

-The fact that these two hydrogen atoms are on the same side of the molecule, that side is biased with a more positive charge (because of the nuclei of the hydrogen atoms).

-The other side of the molecule (without the hydrogen atoms) is biased with a more negative charge.

What is the net (i.e., positive minus negative) charge of this molecule?

Even though the net charge on the water molecule is 0, the dipolar charge distribution creates a polar molecule.

What are hydrogen bonds and how are they associated with the structure of the water molecule? How do they differ from the chemical bonds (covalent and ionic)?

intermolecular bonds that form between water molecules and join the molecules together

How do the hydrogen bonds result from the dipole structure of this molecule and cause the unique properties found in water (e.g., surface tension, dissolving power, thermal properties, existence of water in liquid form at normal earth temperatures, etc.)?

-Hydrogen bonds are the result of the dipolar structure of the water molecule and the fact that the negative side of one molecule is electrostatically attracted to the positive side of another molecule.

-They are the single unifying bond that accounts for all the properties of water.

What are the three states of matter for H2O?

Liquid, Gas, Solid

If the structure of the water molecule was symmetric, what is its estimated freezing and boiling temperatures as projected by plots of other molecules with similar structures? -- see the figure in Chapter 5, Part 1.

-The freezing point would be -90 and the projected boiling point would be -68C. If this were true, water would exist as a gas at normal earth temperatures.

-*Water has a very high freezing and boiling point compared to other substances due to the polarity of the molecules, which allows water molecules to bond with each other and exist in liquid form at normal Earth temperatures.

Define heat capacity in terms of heat absorption.

-Heat capacity is the ability of a substance to absorb heat without a large rise in temperature.

-Absorbed heat is used to break the bonds and change the physical state rather than create kinetic energy and raise the temperature.

Define heat capacity in terms of raising the temperature.

-The amount of heat required to raise the temperature of 1 gram of liquid water by 1 degree Celsius is 1 calorie.

-The heat capacity of ice and vapor is half that of liquid water.

How many calories are required for ice or water vapor and, therefore, how do these heat capacities compare with that of water?).

-The temperature of the ice remains constant at 0 C until an additional 80 calories is added, then the 1 gram of ice changes to 1 gram of water.

-Again the temperature of the 1 gram of water remains constant (at 100 C) until an additional 540 calories is added, and then the 1 gram of water changes to 1 gram of water vapor.

Use heat capacity to explain why the air temperature range, between winter and summer extremes, is much smaller over the ocean than over land.

The heat absorbed by the pool water or the ocean is used to break hydrogen bonds and is less available to raise the kinetic activity of the water and the temperature doesn't rise much.

Why is the concrete area around a swimming pool so much hotter than the water in the pool, if they both receive the same solar radiation - i.e., which has the higher heat capacity?

Concrete or land mass doesn't have hydrogen bonds. All the absorbed heat is converted into kinetic energy and the temperature increases.

What is Latent Heat?

-The heat required to convert a solid into a liquid or vapor, or a liquid into a vapor, without change of temperature.

-Latent heat is hidden in the sense that added calories do not affect the temperature

Latent Heat of Melting

It takes 80 calories per gram to change water from solid (ice) to liquid (water).

What are the magnitudes of latent heats involved in the changes of phase between ice and liquid, and between liquid and vapor? Recall that the latent heat of evaporation is higher than that shown for vaporization.

It takes 540 cal/g to turn water into a vapor.

Sublimation

A change directly from the solid to the gaseous state without becoming liquid

Vaporization

The change of state from a solid or a liquid to a gas, only occurs at the boiling temperature

Evaporation

Type of vaporization, takes place below the boiling temperature, the change of a substance from a liquid to a gas

Define RH and explain what, for instance, 25% RH means.

-A measure of the actual amount of water vapor in air compared to the saturation value of that air if both have the same temperature.

-If air is only holding half of the water it is capable of holding for a given temperature, it is ½ saturated and its RH (relative humidity) is 50%.

To what is the humidity of air relative?

The rate of evaporation is directly dependent upon the degree of saturation and only indirectly dependent upon temperature.

How does unsaturated air reach the dew point?

By one of two ways:

1- By holding the amount of water vapor constant and lowering the temperature of the air

2- By holding the air temperature constant and evaporating more water vapor

What is the dew point temperature?

-The temperature at which unsaturated air is cooled to saturation by cooling alone- without adding additional water vapor.

-Another way to describe the degree of saturation of air → Low RH has a low DPT

How do you find the dew point temperature from the Table of Air Saturation values given in class?

-You determine the maximum saturation value of water at the same given temperature and then use the RH equation.

-RH = [(Amount of Water Vapor in the Air)/Saturation Value of the Air at the Same Temperature)] X 100

Why do we overheat under conditions of high humidity (i.e., R.H. > 60%)? What retards evaporation?

-The human body uses evaporative cooling (perspiration) as the primary mechanism to rid itself of waste heat.

-The rate at which perspiration evaporates on the skin under humid conditions is lower than under arid conditions.

-Because humans perceive a low rate of heat transfer from the body the same as a higher air temperature, the body experiences greater distress of waste heat burden at a lower temperature with high humidity than at a higher temperature at lower humidity.

What is the definition of salinity? How is a measure of salinity obtained from a 1 kg sample of seawater?

-The salts in seawater are really dissolved "ions" of elements and molecules, and we define seawater salinity in terms of those ions.

-The measure is obtained by taking 1kg of saltwater, letting all the water evaporate, and measuring how many grams of salt are left.

For a salinity of 34.7 parts per thousand (ppt), approximately how many grams of "salt residue" will be found in 1 kg of seawater?

Leave it in the sun all day and you will be left with 34.7 grams of salt residue→ 34.7 g salt residue in 1000 g of water→ 34.7 ppt (parts per 1000)

What are the six major ionic constituents in seawater? List by sign of ionic charge and the percentage of each found in seawater.

Sodium (Na+)

Magnesium (Mg++)

Calcium (Ca++)

Potassium (K+)

What is the rule of constant proportions?

Regardless of the total concentration (i.e., the total salinity), the relative proportions of the major ionic constituents of seawater are constant.

How can the "rule of constant proportions" be used to determine seawater salinity?

Sodium is always 30.61% of the total salinity (total ion concentration_ of seawater no matter what, and chloride is always 55.04%, etc.

Knowing the concentration of one of the solutes allows you to determine the others.

Contrast and compare the two ways salinity is determined.

-The crude method we used above to determine the salinity not only takes a long time, but it is highly inaccurate (a small amount of water would remain within the crystals of salt residue, giving a salinity value that was too high).

-The Rule of Constant Proportions is faster and more accurate.

What is the "standard salinity" value for the ocean?

standard value for salinity of the ocean is 35 ppt (or 35 g of salt in 1 kg of seawater)

What is the definition of density?

Mass (in grams, g) per unit volume in cubic centimeters, cm3)

For freshwater, what is the proportionality relationship of density to temperature and pressure?

-Density of pure water (zero salinity) is directly proportional to pressure (P) and inversely proportional to temperature (T), for T greater than or equal to 4 C →

-DENSITY prop to P, 1/T (for T greater than or equal to 4 C),

-Which we would read as "Density is directly proportional to P and/or inversely proportional to T, for temperatures greater than or equal to 4 C."

-Therefore, an increase in pressure would increase DENSITY, and an increase in temperature would decrease DENSITY

For seawater, what is the additional proportionality relationship of density to salinity?

-The density of seawater has the same pressure and temperature relationships as fresh water but, with the addition of "salt", its mass is increased, so the density of seawater also is directly proportional to salinity (S), and this is for any T; i.e., DENSITY prop to P, 1/T, S (for any T).

When fresh (zero salinity) water is cooled, the maximum density of the water is reached at a higher temperature than the freezing temperature. What are these two temperatures for fresh water, and how do they change for water with salinities greater than zero?

Freshwater or any other substance that reaches its maximum density at a higher temperature than its freezing temperature, it behaves in an abnormal manner so, we say, it exhibits a "density anomaly."

Why do we call the fact that maximum density is reached at a higher temperature than the freezing point a "density anomaly"?

Seawater salinities are all greater than 24.7 ppt, so no density anomaly exists for seawater. Therefore, everywhere in the OCEAN, density will always be inversely proportional to temperature

Why does this anomaly no longer exist for salinities greater than about 24.7 ppt?

This density anomaly does not exist above 24.7 ppt because all seawater salinities are greater than 24.7

How does the change of water density with depth affect the stability of the vertical water column?

-Most dense water is on top of the less dense water, which creates an unstable condition.

-The heavier water sinks to the bottom, seeking its own density, replacing the deeper water, which then rises to the top.

-These vertical convection currents are characteristic of unstable conditions.

What is the difference between neutral, stable and unstable stability conditions, and in which will vertical convection occur?

-Unstable Conditions→ density decreases with increasing depth

-Dense water on top of less dense water

-Vertical convection current created, characteristic of unstable conditions

-Stable Conditions→ density increases with increasing depth

-Neutral→ water density is constant with depth

Use an understanding of the density anomaly discussed earlier, and of the stability conditions above, to explain the process of freezing ice on a fresh water lake. Why is the water temperature no lower than 4 C a few feet below the ice at the moment when ice freezes on the lake surface?

1. The 6 C water temp. is higher than the temp. of maximum density, so: Density prop 1/T

2. Heat is lost through the surface

3. Boundary layer temp. decreases

4. Density increases

5. Unstable conditions and vertical convection are established and persist until the temp. of the entire basin reaches 4 C, the temp. of maximum density of freshwater.

6. Any further cooling at the surface BL will only create less dense water, because for temperatures lower than 4C, Density prop T, and stable conditions are created.

7. One convection stops, only the temp. of the BL is cooled below 4 C.

8. Thereafter, the thin BL will quickly lose heat and reach a temperature of 0 C and, after an additional 80 cal/g of latent heat of fusion is lost from this BL, ice will freeze on the surface

Generalize the observations about the freezing process just described to apply to all bodies of water with salinities < 24.7 ppt. In particular, at what temperature will the water beneath the ice be when ice first freezes for these salinities?

4 C

→ remember, once you cool the basin water to 4 C, only the surface BL cools to the freezing temperature after stable conditions are reached

How is the process of freezing ice on the surface of water with salinities > 24.7 ppt, different from that for salinities < 24.7 ppt ?

-No density anomaly exists for seawater (S > 24.7 ppt), cooling at the surface BL will ALWAYS result in unstable conditions and convective overturn

-For seawater (S > 24.7 ppt), the entire body of water must be decreased to the freezing temperature

Why is the temperature of the water below newly frozen ice on the ocean (which has a salinity of about 35 ppt) lower than the temperature of water at the same depth below newly frozen ice on a fresh water lake?

-Vertical convective currents?

-In a lake, when the basin is cooled to 4C, stable conditions persist, and only the BL continues to cool and ultimately freeze.

In what ways do the freezing temperature and the vertical stability condition of this ocean water explain this lower temperature?

-The freezing temperature of seawater is much lower than freshwater, but equally important;

-Seawater has no density anomaly, so its entire water mass must be cooled to the freezing temperature, while the entire water mass in freshwater must only be cooled to 4 C (then its BL cools to 0 C).

Thermocline

temperature slope or gradient with depth

Pycnocline

density slope or gradient with depth

Halocline

salinity slope or gradient with depth

Isothermal

constant temperature with depth

Isopycnal

constant density with depth

Isohaline

constant salinity with depth

At which latitude does temperature have the largest affect on density?

-Τemperature is more important than salinity in determining the density profile in the equatorial region (with a very strong permanent thermocline)

-Tropical latitudes- a 5 C increase in the temperature at tropical latitudes has an effect on density that is some three times larger than a 5 C increase in temperature at higher latitudes

At which latitude does salinity have the largest affect on density?

Salinity is more important than temperature at higher latitudes

What feature of the electromagnetic spectrum does Wein's displacement law define, and how is it related to the temperature of a radiating body?

-Used to obtain Lmax

-Lmax is about 0.5 micrometers, and that there are three major divisions of the solar spectrum

-UV (ultra-violet) portion at wavelengths less than 0.4 micrometers

-Visible portion between 0.4 and 0.7 micrometers

-IR (infra-red) portion at wavelengths greater than 0.7 micrometers

What are the upper and lower wavelength limits of the Visible portion of the solar energy spectrum?

0.4-0.7