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 increase because CO2 adds carbon
    • They decrease because added CO2 shifts carbonate into bicarbonate
    • They stay fixed because buffering holds every carbonate-system concentration constant

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

    • Its pH resists change through coupled equilibria
    • Its pH cannot change when acid is added
    • Its pH depends only on dissolved salt, not acid-base reactions

  3. Using seawater's approximate apparent pKa values of 6 and 9, bicarbonate is more abundant than both CO2* and carbonate at pH 7.5. CO2* means dissolved CO2 plus carbonic acid.

    Circle one:   True   False

  4. Vinegar dissolves an eggshell. Which conclusion about seawater is supported?

    • Seawater must fall below pH 7 before calcium carbonate can dissolve at any depth.
    • Every small fall in ocean pH dissolves every type of shell as fast as vinegar does.
    • Acid can dissolve calcium carbonate, but vinegar is not a seawater rate model.

  5. Why can a small pH drop caused by added CO2 hinder coral skeleton growth?

    • Added CO2 raises carbonate, so precipitation depletes dissolved calcium.
    • Lower carbonate availability reduces mineral saturation, which can hinder skeleton growth.
    • Lower pH removes dissolved calcium but leaves carbonate unchanged.

  6. Two seawater samples have the same temperature, salinity and total alkalinity. At equilibrium, one is exposed to more CO2 than the other. Which has the lower pH?

    • Both samples, because buffering keeps their pH equal
    • The sample with less CO2, because lower CO2 increases H+
    • The sample with more CO2, because added CO2 increases H+

  7. Two seawater samples come from the same depth and have the same pH. One is saturated with respect to a shell mineral; the other is undersaturated. Which favors net dissolution of exposed shell material?

    • The saturated sample, because reaching saturation forces more shell material to dissolve
    • The undersaturated sample, because its mineral saturation favors net dissolution
    • Both samples equally, because matching depth and pH always means matching saturation

  8. Atmospheric CO2 rises while air-sea exchange continues and other conditions stay unchanged. A student predicts that dissolved ocean CO2 will rise but pH will stay fixed because buffering neutralizes the added CO2. Which evaluation fits the carbonate system?

    • The CO2 prediction fits, but pH falls because added dissolved CO2 shifts the equilibria toward more H+.
    • Both predictions fit, because buffering prevents any change in H+ concentration.
    • The CO2 prediction fits, but pH rises because added CO2 lowers the H+ concentration.
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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 decrease because added CO2 shifts carbonate into bicarbonate · Added CO2 shifts the carbonate system toward bicarbonate, reducing carbonate-ion availability.
  2. Its pH resists change through coupled equilibria · Carbonate-system species react with added acid or alkali, limiting the pH change. Buffering does not prevent all change.
  3. True · pH 7.5 is above the first apparent pKa and below the second. Both comparisons favor bicarbonate over its neighboring forms.
  4. Acid can dissolve calcium carbonate, but vinegar is not a seawater rate model. · The demonstration shows acid dissolving calcium carbonate. It does not establish ocean dissolution rates or a pH 7 threshold.
  5. Lower carbonate availability reduces mineral saturation, which can hinder skeleton growth. · Added CO2 reduces carbonate availability and mineral saturation. This can make calcium carbonate skeleton growth harder.
  6. The sample with more CO2, because added CO2 increases H+ · Under these controlled conditions, more CO2 increases H+ and lowers pH. Buffering limits the change but does not stop it.
  7. The undersaturated sample, because its mineral saturation favors net dissolution · Undersaturation favors net dissolution of that mineral. Equal depth and pH do not establish equal mineral saturation.
  8. The CO2 prediction fits, but pH falls because added dissolved CO2 shifts the equilibria toward more H+. · The student's CO2 prediction fits air-sea exchange, but buffering resists pH change rather than preventing it. Added CO2 increases H+, so pH falls instead of staying fixed.
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