EarthLabs > Corals > Lab 3: Building a Reef > 3A: Building a Skeleton

Building A Reef

Part A: Building a Skeleton

In Lab 2, you learned that coral polyps get nutrition from the zooxanthellae algae that live in their stomach tissue. The coral polyps use the oxygen and sugars produced by zooxanthellae photosynthesis for growth and energy, and release heat, waste, and carbon dioxidea process called respiration (the same process humans use in breathing). In fact, you can use your own exhaled breath to simulate the role of respiration in coral reef growth.

  1. Gather the following materials:
    • clear plastic cup partially filled with lime water
    • empty clear plastic cup
    • two drinking straws
    • small (#2) coffee filter
    • water
    • white vinegar
    • eyedropper

  2. Examine the lime water and describe its appearance. Lime water is the common name for saturated calcium hydroxide solution, Ca(OH)2 (aq).

  3. Place one of the drinking straws into the lime water and blow gently into the liquid. DO NOT INHALE OR BLOW TOO HARD. Continue exhaling through the straw until a white precipitate (solid) forms.

  4. Place the coffee filter over the empty cup. Carefully pour the lime water into the cup through the filter to separate the precipitate from the liquid.

  5. Put the white precipitate aside and allow it to dry and solidify.

  6. Now place a drinking straw into a cup of regular water (instead of lime water) and blow gently. Observe what happens.

    Checking In

    • Describe how lime water and regular water react differently when carbon dioxide gas is added to each.

      When CO2 is added to lime water, it becomes milky due to the production of calcium carbonate (CaCO3). Calcium carbonate is insoluble and precipitates (settles out of solution):

      Ca(OH)2 (aq) + CO2 (g) CaCO3 (s) + H2O (l)

      The regular water bubbles as the CO2 is added, but no precipitate forms.


  7. Calcium carbonate, the substance that makes up coral skeletons, reacts with weak acid (such as vinegar). To prove that the substance you filtered out of the lime water is indeed calcium carbonate, use the eyedropper to add a small amount of white vinegar to the precipitate. If it is CaCO3, there should be a fizzing reaction from the release of carbon dioxide.

    Stop and Think

    1: Based on what you observed in the lab, explain where you think the raw materials for coral skeletons come from and what role the biological processes of coral polyps and zooxanthellae play in the formation of coral reefs.



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