Seawater Chemistry: the Carbonate System and pH · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why extra CO2 troubles shells

Science · Earth's Systems · ages 17-18
Name ______________________   Date ____________

  1. Extra carbon dioxide enters seawater. What happens to carbonate ions?

    • They multiply quickly
    • They get scarcer
    • They turn into fresh water

  2. What does it mean to call seawater a buffered solution?

    • Its pH resists change through coupled equilibria
    • It never changes under any addition
    • It holds no dissolved salts at all

  3. Seawater pH is held by a set of coupled carbonate equilibria.

    Circle one:   True   False

  4. An eggshell sits in vinegar for days and thins away. What does this show?

    • Vinegar adds carbonate ions to water
    • Eggshells turn into living coral in acid
    • Acid dissolves calcium carbonate, as in more acidic seas

  5. Why does a small pH drop threaten coral?

    • Coral dislikes the taste of the water
    • Carbonate ions get harder to find, so shells struggle
    • Lower pH freezes seawater solid

  6. Two seawater samples differ only in dissolved CO2. Which one has the lower pH?

    • They must share the same pH
    • The one with less dissolved CO2
    • The one with more dissolved CO2

  7. Below a certain depth, dredged shells come up pitted and thin. What does that tell you?

    • The water there is warmer and calmer
    • The water there favors dissolving over building
    • Shells there grow thicker instead

  8. Air CO2 climbs on one graph. What pattern do you expect on the ocean CO2 and pH graphs?

    • Ocean CO2 climbs while pH slips down
    • Ocean CO2 falls while pH climbs
    • Neither of the other curves moves
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Why extra CO2 troubles shells W1-mt_BrdrNZmBzE-s1

  1. They get scarcer · The pushed equilibria leave fewer carbonate ions available.
  2. Its pH resists change through coupled equilibria · The carbonate system absorbs pushes, so pH moves only a little.
  3. True · That coupled system is exactly what buffering means here.
  4. Acid dissolves calcium carbonate, as in more acidic seas · The kitchen acid mimics what acidic seawater does to shells.
  5. Carbonate ions get harder to find, so shells struggle · Shell building needs carbonate ions, which scarcity takes away.
  6. The one with more dissolved CO2 · More dissolved CO2 pushes the equilibria toward acidity.
  7. The water there favors dissolving over building · Past a threshold depth or pH, the carbonate balance runs backward.
  8. Ocean CO2 climbs while pH slips down · The three curves move together in that linked pattern.
Worksheet · LightMySky