The Earth -- Introduction

Class Notes - Groundwater

Introduction

Terminology

Karst Topography and Cavern Formation

Therefore, when CO2 increases calcite goes into solution - dissolves.

The reverse is also true. Any process that reduces the amount of CO2 in the system will cause calcite to precipitate.

Temperature

Changes in Pressure

    An increase in load pressure (due to the mass of the overlying material) increases the solubility of gases in liquids. This causes [2] to produce more carbonic acid and [1] to shift to the right causing calcite to dissolve.

Organic Activity

    Green plants remove carbon dioxide in the process of photosynthesis. Again, a loss of carbon dioxide will result in favoring the precipitation of calcite.

Decay

    When plants decay they release carbon dioxide and the solubility of calcite is favored.

The "Acidity" of the Environment

    As noted previously, the "acidity" of a solution is a measure of the amount of H+. Water dissociates into H+ and (OH)- as shown below.

    H2O = H+ + (OH)- [4]

    The measure usually adopted for the acidity of a solution is pH:

      pH = -log(H+)

      Let H+ = 10-10 (units of moles per liter)

      A logarithm is an exponent. In this case the exponent is -10.

      Therefore, pH = - (-10) or 10.

    Which is bigger: 10-10 or 10-5?

    As pH decreases, the amount of the hydrogen ion increases. Thus, low pH values are acidic and high pH values are basic. The world's oceans have a pH of about 8 - a weak base. Rainfall has a pH of about 6 - a weak acid.

    In general, carbonates go into solution in an acid solution and precipitate in a basic solution. From equation [2], an increase in the acidity of the solution causes the system to shift to the right and calcite dissolves.

Test Your Understanding

    CaCO3 + H2CO3 = Ca+2 + 2HCO-3

  1. The system above is at equilibrium. An additional amount of Ca+2 is added to the system. Predict what will happen

    the reaction will shift to the right and calcite will precipitate
    the reaction will shift to the left and calcite will dissolve
    nothing will happen because the system is at equilibrium
    the reaction will shift to the left and calcite will precipitate

    H2O + CO2[2]
    ||
    CaCO3 + H2CO3 = Ca+2 + 2HCO-3 [1]

  2. When CO2 increases:

    calcite dissolves
    calcite precipitates
    nothing will happen because the system is at equilibrium

  3. Water flowing in a glacier is under pressure. If this water is fully saturated with CO2 while it is in the glacier, what will happen when this water flows out on the surface?

    calcite precipitates because at low pressure the water can contain more CO2
    calcite precipitates because at low pressure the water can contain less CO2
    nothing will happen because the system is at equilibrium

  4. Vegetation is decaying in a restricted portion of Galveston Bay. Therefore, you predict that :

    calcite dissolves
    calcite precipitates
    nothing will happen because the system is at equilibrium

  5. Which is bigger: 10-10 or 10-5?

    10-10
    10-5

  6. A solution has a concentration of H+ of 10-3 moles per liter. The amount of H+ can be written as:

    0.1moles per liter
    0.01 moles per liter
    0.001 moles per liter
    0.003 moles per liter
    0.0001 moles per liter

  7. A solution has a concentration of H+ of 10-5. The pH of the solution is:

    -5
    5
    10

  8. A solution has a concentration of H+ of 10+2. The pH of the solution is:

    -2
    2
    4

  9. When CO2 decreases:

    calcite dissolves
    calcite precipitates
    nothing will happen because the system is at equilibrium

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Copyright by John C. Butler, July 29, 1995

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