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Anytime Lesson Plan: Coral and Chemistry

Abstract

By testing whether increased carbon dioxide makes ours oceans more basic or more acidic, students will learn how human alterations of the atmosphere are changing the pH of the ocean, impacting coral reef survival.

Objectives

Students will:

  1. learn that coral reefs need a certain pH to survive.
  2. learn that human alterations of the atmosphere are changing the pH of the ocean.
  3. test whether increased carbon dioxide is making our oceans more basic or more acidic.
  4. learn how a change in oceanic pH is impacting coral reefs.

Materials

  • Large, clear jars or cups (3)
  • Small, clear jars with lids - baby food jars work well (1 per student)
  • straws (1 per student)
  • red cabbage juice (enough to fill each small jar about 1 inch high and each large jar about 2
  • inches high)
  • common household acid (vinegar or lemon juice)
  • common household base (baking soda or ammonia cleaning product)
  • 5% hydrochloric acid solution (5:100 dilution, which means 5 parts acid and 95 parts water)
  • gloves
  • goggles
  • eyedropper
  • container or tray into which you will drop a small amount of 5% hydrochloric acid solution
  • Tums (a few tablets)
  • Coral and Chemistry Worksheet (1 per student)
  • Map - Where in the World is Coral?
  • coral reef video (optional)

Vocabulary

  • aqueous: relating to or dissolved in water
  • acid: a compound that forms hydrogen ions (H+) when dissolved in water, and whose aqueous solutions react with bases and certain metals to form salts
  • base: a compound that forms hydroxide ions (OH-) when dissolved in water, and whose aqueous solutions react with acids to form salts
  • acidification: the process of becoming more acidic
  • pH: a measure of the acidity of a solution; described on a scale ranging from 0 (most acidic) to 14 (most basic), where 7 represents the neutrality of pure water

Activity

Preparation

  1. Cut red cabbage into pieces and boil with the lid on until water is a dark color.
  2. Strain the cabbage and discard. Save the cabbage juice.
  3. Pour a small amount of cabbage juice into each small jar. Fill each jar less than 1 inch high.
  4. Pour a slightly larger amount of cabbage juice into the 3 larger jars. Fill each jar no more than 2 inches high.

Introduction

  • Show the map, Where in the World is Coral?
  • Discuss what corals need to survive: (1) warm water (in tropical latitudes); (2) salinity (in oceans, not too close to big river outlets); (3) sunlight (not too deep because their symbiotic algae, zooxanthellae, need light to photosynthesize); (4) low levels of sediment (murky water blocks the sun’s rays and doesn’t allow photosynthesis); and (5) slightly basic pH.
  • Give a brief explanation for how the ocean absorbs carbon dioxide. Humans are releasing carbon dioxide gas into our atmosphere, primarily by burning fossil fuels. Some carbon dioxide stays in the atmosphere, contributing to climate change, but some of it is actually taken up by the ocean because carbon dioxide freely diffuses into water.
  • Tell students that when carbon dioxide is added to the oceans, it undergoes a chemical reaction and changes the oceans’ pH. The class will perform an experiment to determine whether the ocean is becoming more acidic or more basic.

Procedure

  1. Pass out to each student:
    • small jar with a small amount of red cabbage juice inside
    • lid
    • straw
  2. Tell students not to touch their materials yet, but to look at them as you explain what they are going to do.
  3. Tell students that they are going to mimic carbon dioxide absorption by blowing into these jars. As we exhale carbon dioxide into the cabbage juice water, some of it will be absorbed, change into its aqueous form, and change the pH of the water.
  4. Tell students that the jars contain water in which red cabbage has been boiled. Red cabbage is a natural pH indicator, meaning that red cabbage changes color to indicate changes in pH.
  5. In front of the class, show all three large jars of cabbage juice.
  6. Show students the household acid and ask “Is this acidic or basic?”
  7. Show students the household base and ask “Is this acidic or basic?”
  8. Then add the acid to one large jar of cabbage juice and the base to a second large jar (leaving one jar with regular cabbage juice as the control). Make sure to add enough of the acid and base so that you see a color change.
  9. Discuss what this test means. Acids make cabbage juice more pink/red and bases make cabbage juice more blue/green.
  10. Pass out a Coral and Chemistry Worksheet to each student.
  11. Tell students to make a hypothesis answering this question: Will the increased carbon dioxide from my breath make the cabbage juice more acidic (pinkish) or more basic (greenish)?
  12. Help students to write out the procedure they will use to answer this question.
  13. Have students place their straws in the jars and then cover them with the lids to prevent spraying the juice.
  14. Tell students to blow lightly into the jars for 4-6 minutes. (It might be a good idea to have something for students to watch while blowing into their jars, like a few minutes of a coral reef video).
  15. The color will change from a dark purple to a purple pink as the water becomes more acidic.
  16. Tell students to write down their results.

Discussion

  • Ask your students what this means about how CO2 is affecting the oceans. (It is making them more acidic by adding hydrogen ions to the water. It is causing acidification of the oceans.)
  • Ask your students what effect this might have on plants and animals that live in the ocean. (Plants and animals have certain needs, including pH ranges in which they can live. Corals and many other animals that have calcium carbonate shells or skeletons cannot live in environments that are too acidic. Note that not all acids are harmful to life. Some organisms thrive in acidic environments, but many organisms have adapted to the specific pH of their environment and have a specific pH range in which they can survive.)

Procedure

  1. Tell your students that you will now do a demonstration to see how acid affects coral.
  2. Gather students around you and place a tablet of Tums in a container or on a tray.
  3. Explain to students that Tums tablets are made of calcium carbonate, the same substance that corals use to build their skeletons. So, the tablet represents a piece of coral.
  4. Put on gloves and goggles.
  5. Ask students what they think will happen when you put a few drops of acid onto the Tums tablet.
  6. Warn students not to touch the demonstration because the acid is dangerous.
  7. Use an eyedropper to put a few drops of 5% hydrochloric acid solution on the Tums tablet.
  8. Watch it fizz and explain that a property of acids is that they dissolve carbonates and coral skeletons are made of calcium carbonates.

    2HCl + CaCO3 ↔ CaCl2 + CO2 + H2O
    Hydrochloric Acid + Calcium Carbonate ↔ Calcium Chloride + Carbon Dioxide +Water

  9. Explain that the fizzing is the carbon dioxide gas in the equation above.
  10. You can try dropping the acid on other objects to see how they respond (try some with calcium carbonate like seashells and some without like a marble or a pencil).
  11. After witnessing this demonstration and discussing the impact, have students answer the final question on their worksheet, “How does increased carbon dioxide in the oceans impact coral?”
  12. Small amounts of the 5% acid solution can be flushed down the sink with water.

Extensions

Have students research ocean acidification and/or climate change’s impact on coral and write brief reports. Short articles on these topics can be found at http://www.sciencedaily.com.

References

California Content Standards

Grade Six

Investigation and Experimentation

  • 7a. Develop a hypothesis
  • 7e. Recognize whether evidence is consistent with a proposed explanation

Grade Eight

Physical Sciences, Reactions

  • 5e. Students know how to determine whether a solution is acidic, basic, or neutral.

Chemistry of Living Systems (Life Sciences)

  • 6a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms.
  • 6b. Students know that living organisms are made of molecules consisting largely of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

Grades Nine Through Twelve

Chemistry

  • 5a. Students know the observable properties of acids, bases, and salt solutions.
  • 5b. Students know acids are hydrogen-ion-donating and bases are hydrogen-ion-accepting substances.

Life Sciences

  • 6b. Students know how to analyze changes in an ecosystem resulting from changes in climate, human activity, introduction of nonnative species, or changes in population size.

 

Background

Acid and Base Chemistry: Although we don’t usually think about it on a daily basis, acids and bases are found all around us: in our foods, cleaning supplies, gardening materials and much more. Acids and bases are chemical compounds differentiated by their location on the pH scale, which measures the concentration of hydrogen ions (H+) floating around in a solution. Ranging from 0 to 14, the pH scale is rooted in the center at 7.0, representing the neutrality of pure water. High concentrations of H+ result in a pH lower than 7 (acids), and low concentrations of H+ result in a pH higher than 7 (bases).

One can often recognize acids because their chemical formula begins with hydrogen (HCL, H2SO4, and H2CO3). Acids release this hydrogen to form hydrogen ions (H+) in water. Acids thus increase the H+ concentration of a solution, which decreases the pH. Bases, on the other hand, either release hydroxide ions (OH-) in water, which combine with hydrogen ions (H+) to form water, or have a structure that is able to react with lone hydrogen ions. Either way, bases remove H+ from the solution and increase the pH value.

With regards to physical properties, acids generally have a sour taste and are known to corrode metal, whereas bases generally taste bitter and feel slippery to the touch.

Ocean Chemistry: Acids and bases are important in ocean chemistry because the plants and animals that live in the ocean require certain pH levels to survive. Although different areas in the oceans have slightly different pH levels, normal ocean pH is slightly basic, with the typical ocean pH being close to 8.2. Corals and other ocean animals have adapted to live in water in this pH range.

The pH of the ocean has been changing recently. As people burn fossil fuels and release carbon dioxide (CO2) into the atmosphere, some of that carbon dioxide is taken up by terrestrial vegetation via photosynthesis and some is absorbed by the oceans. Scientists estimate that each of these sectors, terrestrial vegetation and oceans, has taken up about one third of human-induced carbon dioxide emissions. If all carbon dioxide emissions remained in the atmosphere, we would be experiencing climate change much more quickly. Thus, the uptake of carbon dioxide by terrestrial vegetation and the oceans is climatically beneficial. However, there are ramifications of increased carbon dioxide in the oceans.

When the ocean absorbs carbon dioxide from the air at its surface, the gas changes into an aqueous form (one dissolved in water) and begins to chemically react with water to form carbonic acid (H2CO3). As an acid, carbonic acid releases H+ to its surroundings, becoming a bicarbonate ion (HCO3-). This bicarbonate ion can further break down into carbonate (CO32-) by releasing the remaining hydrogen. The following chemical equation expresses this series of reactions:

CO2 (aq) + H2O ↔ H2CO3 ↔ HCO3- + H+ ↔ CO32- + 2H+

Carbon Dioxide + Water ↔ Carbonic Acid ↔ Bicarbonate + Hydrogen ↔ Carbonate + Hydrogen

All of the absorbed carbon dioxide does not follow this trajectory to become carbonate, but rather all of these states exist simultaneously. The proportions of carbonic acid, bicarbonate, and carbonate produced are dependant on many factors, but in general in the oceans more than 85% of dissolved carbon exists in the form of bicarbonate (HCO3-). The conversion of carbon dioxide and water into bicarbonate produces hydrogen ions and thus results in a decrease in pH. This means that increasing carbon dioxide emissions to the atmosphere leads to the ocean becoming more acidic. Scientists estimate that since 1750 the oceanic pH has lowered by about 0.1 units and that it will continue to lower as the oceans absorbs carbon dioxide. Acidity is a problem for coral and other animals that have shells or skeletons made of calcium carbonate (CaCO3) because calcium carbonate dissolves in acid. Thus, lower pH makes it more difficult for these animals to construct their body parts and can dissolve and weaken the parts that they have already created.

Corals are currently struggling to survive for a number of reasons including increased sea temperatures, over harvesting, pollution, and acidification. These threats impact coral animals, but they also impact the whole food web, including humans who depend on coral reefs for food, materials, coastal protection, and beauty.

 

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