I enjoyed a visit with 3rd and 4th graders from Park Forest Elementary School last week when I talked with students in Mrs. Hoffman’s Learning Enrichment classroom. Polar Ambassadors Anna and Emily came along to help me teach a lesson about ocean acidification.

    Students observing coral
    Students from Mrs. Hoffman's class make observations of coral skeletons

    During the lesson, we asked students to find the mass of a small piece of coral skeleton and then put it in a vinegar solution. Vinegar is acidic and has a pH of about 4.0. Remember that the pH scale goes from 0-14. Solutions with pH of 7 are neutral. Solutions with a pH below 7 are acidic and solutions with a pH above 7 are basic.

    pH scale
    The pH scale

    During the lesson, we addressed three main questions:

    • What is ocean acidification?
    • How does it work?
    • Why is ocean acidification a problem?

    What is ocean acidification?

    Ocean acidification is the increase in acidity of the Earth’s oceans. The oceans are an important natural resource and are part of the carbon cycle, which is a complex series of processes through which all the carbon atoms in existence circulate. Oceans absorb about one third of all anthropogenic (manmade) carbon dioxide or CO2 . This totals about 7 billion metric tons per year. That is about 15,435,000,000,000 pounds!

    The oceans absorb CO2 at the surface and transfer it to deeper waters, out of contact with the atmosphere for hundreds of years. Since the invention of the combustion engine and the improvement of techniques for mining fossil fuels, like coal and oil, humans have been burning more fossil fuels and introducing larger amounts of CO2 into the Earth’s atmosphere. As atmospheric CO2 levels increase, excess CO2 is forced into the surface waters of the ocean. As CO2 dissolves in surface seawater, the pH of the water decreases. Remember that pH is the measure of the amount of hydrogen ions in a solution. As the pH decreases, the water becomes more acidic.

    Students observing coral in acid
    Students from Mrs. Hoffman's class observing coral in an acidic solution

    Student with Nell
    A student talking with me about vinegar, an acidic solution

    How does it work?

    When CO2 dissolves in seawater, it forms carbonic acid. This reaction is shown in Equation 1.

    Equation 1
    carbon dioxide and water combine to form carbonic acid

    Some of the carbonic acid dissociates, or separates, in water, releasing hydrogen ions and bicarbonate. This is shown in Equation 2.

    Equation 2
    carbonic acid dissociates in seawater, resulting in hydrogen ions and bicarbonate

    Some of the hydrogen ions stay in the ocean water. Others react with carbonate ions already present in the water to make bicarbonate. This is shown in Equation 3.

    Equation 3
    hydrogen ions combine with carbonate ions to create bicarbonate

    Why is ocean acidification a problem?

    Ocean acidification removes carbonate ions from the water and negatively impacts organisms that need calcium and carbonate ions to make their shells or skeletons. Examples of these organisms include marine snails, corals, mussels, clams, sea urchins and many others. Other organisms that rely on these species for food are also negatively impacted.

    Mrs. Hoffman's students will find the mass of their piece of coral again in a week. What do you think they will find?

    Mrs. Hoffman's Students
    Mrs. Hoffman's Students with me and Polar Ambassadors Emily and Anna

    Author
    Date
    Location
    State College Area High School

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