Geoscience Resources Multiple choice
Precipitation and Solution of Calcium Carbonate

The behavior of calcium carbonate (for example, the mineral calcite) in a near-surface environment is controlled primarly by equilibrium in the reaction:

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

    where:

  • CaCO3 is a solid - calcite
  • H2CO3 is carbonic acid - a relatively weak naturally occuring acid that forms by the reaction between water and carbon dioxide:

      H2O + CO2 = H2CO3 [2]

  • Ca+2 Calcium is a positive ion (a cation) in solution and
  • 2HCO-3 is a negative ion (an anion) in solution - the so-called bicarbonate ion

The Forward Direction is to the right as you view this page. The Reverse Direction is to the left.

A System will be defined as some portion of the universe that you are interested. Everything else is part of the surroundings. The system and the surroundings are separated by a "boundary". Equation [1], for example could be a system.

Imagine the following experiment. A sealed bottle is half full of water. Gaseous CO2 from the air dissolves in the water and dissolved CO2 in the water escapes into the air.

    CO2 in the air = CO2 dissolved in water [3]

The reaction continues until a balance is established between these two processes and an equilibrium condition is reached. At a constant temperature the ratio of :

    CO2 dissolved in water / CO2 in the air

is constant at a constant temperature - the so-called equilibrium constant.

Chemical equilibrium is dynamic. If we disturb equilibrium by increasing the amount of CO2 in the air the reaction [2] adjusts by "reversing" the nature of the disturbance - that is, the amount of CO2 in the air is reduced by dissolving some of the "excess" in water. That is, in reaction [2] the Forward Direction is favored. The reaction continues until the balance between CO2 in the air and CO2 dissolved in water is that given by equation [3].

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

Equations [1] and [2] clearly relate to each other. Think of the | | representing equilibrium just as = represents equilibrium. Both equations have their own equilibrium constant.

Imagine that this system is at equilibrium. Any process that increases the amount of CO2:

  1. promotes the production of more H2CO3 (the equilibrium shifts to use up the increased CO2).

  2. The increased H2CO3 causes reaction [1] to shift to the right which means that CaCO3 will dissolve.

    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

      Gases like CO2 are less soluble in warm solutions than they are in cold solutions. Therefore, if the system is at equilibrium, warming the water will result in a reduction in the amount of the gas, equation [2] will shift so as to produce more CO2 which results in a decrease in the amount of carbonic acid. Equation [1] will shift to the left to produce more carbonic acid and calcite will precipitate.

      Calcium carbonate, for example, is often found precipitating in proximity to hot springs.

      In contrast, carbon dioxide is more soluble in cold water than in warm. In the deep ocean basins (water depths greater than about 11,000 meters), equation [2] is shifted so as to produce more carbonic acid which results in a shift to the right in equation [1] resulting in the solution of calcite. Calcite is not found in sediments in the deep marine environment.

    Changes in Pressure

      An increase in load pressure (due to the mass of the overlying material) is to increase 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 dissolve
      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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