Ocean Acidification

Biology, Chemistry:	CO2 Levels, Acids and Bases
Grade Range:	Elementary School, Middle School, High School
Format:	Hands-on

This space is intended to describe the demonstration that is here. Please use this space to describe in short the demonstration, making sure to mention here if it will require any additional safety equipment or how easily it can be adjusted for varied grade levels. Use this page as a base layout for editing the wiki. Feel free to copy the layout onto other pages, and to put the content as needed into the page.

Materials

• Two 1000mL Beakers
• Water
• Dry Ice
• Vinegar
• Carbonated Water
• Two Eyedroppers
• 1.0M Sodium Hydroxide (NaOH) Solution (See Preparation)
• 0.25M Calcium Chloride (CaCl₂) Solution (See Preparation)
• Chalk or Limestone

Safety Precautions

Please read the Liquid Chemical section of the Demonstration Safety page before performing this demonstration.

This demonstration requires: Safety goggles, disposable gloves

Demonstration

Preparation

• Prepare the NaOH solution: for every liter of water, dissolve 40g of NaOH. Once dissolved, store and label the solution.
• Prepare the CaCl solution: for every liter of water, dissolve 27.75g of CaCl₂. Once dissolved, store and label the solution.

Presentation

1. Set up the beakers: Fill each beaker with 500mL of water. In one beaker, drop in a few pieces of chalk (crushed) or limestone, as well as a piece of dry ice. In the other beaker, mix in 100mL of the 0.25 CaCl solution.
2. When students come up, show them the beaker with the chalk/limestone in it. Ask them to look at the chalk/limestone and look for evidence of it breaking down. If they see it, ask them to think of possible causes for it.
3. Show them a separate piece of chalk/limestone, and use the eyedropper to put 1-2 drops of vinegar on it. The chalk/limestone will start to fizz! Ask the students for ideas on the connection between the vinegar and the beaker, being sure to note that there is no vinegar in the beaker. What else could be causing it?
4. Now show the beaker with the dissolved CaCl. Add 250mL of carbonated water to the beaker (filling it to the 850mL mark), and use an eyedropper to slowly add the NaOH solution to the beaker. After several drops, small particles will start to appear in the solution!
   • Once the second solution gets too milky from precipitation, you'll need to rinse it out and remake the solution. Both solutions can safely go down drains.

Why This Works

Short Explanation

Carbon Dioxide levels are commonly talked in terms of global warming. Something else that these CO₂ levels effect, however, are the acidity of the oceans. This effects ocean life, especially coral reefs, since they are made of calcium carbonate. Calcium carbonate is something a lot of ocean life uses to make shells or structures, and we use it as chalk and limestone. it breaks down when exposed to acids, as we see when the vinegar is dripped on it. When CO₂ dissolves in water, it creates an acid called carbonic acid. If carbonic acid levels build up, it can cause these coral structures to start breaking down, creating havoc in the ecosystems that rely on the coral reefs to survive. This is what happens in the first beaker as the dry ice dissolves, and we see the chalk/limestone breaking down.

In the second beaker, we can see small particles forming in the water as we put in drops of NaOH. NaOH is Basic, or the opposite of an acid, so it can neutralize the carbonic acid in the solution from the club soda. This causes calcium carbonate to precipitate! This is to demonstrate why we should make efforts to decrease the CO₂ levels, so to protect the ocean life in coral reefs and around the world.

Full Explanation

Corals are animals that live on the sea floor and are an important part of the Earth's ecosystem. They're not like most of the animals you think of - they plant themselves in the sand and don't move around. They can grow to form enormous reefs large enough to see from space! These reefs are terrific habitats for many species of fish, so coral reefs help support many types of ocean life. They have very strong skeletons made of calcium carbonate, the same chemical that makes an eggshell, chalk, and limestone. These reefs are endangered by the increasing levels of Carbon Dioxide (CO₂) in the atmosphere, which correlate strongly with human industrial activities. The reason why CO₂ levels in the air effect ocean life is because of the reactions that take place.

Carbon Dioxide in the Ocean

CO2(aq) + H2O(l) → H2CO3(aq)

H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq)

H2CO3: Carbonic Acid

CaCO3: Calcium Carbonate

Ca(HCO3)2: Calcium Bicarbonate

When excess CO₂ dissolves in water, it weakly reacts to create carbonic acid. Carbonic acid is common in the water; if there is CO₂ in the air, some of it will inevitably dissolve in the water. Under normal circumstances this is of no concern, especially since it is of such small concentrations. All the water we drink has it in very tiny amounts, and it is actually a part of the buffer system in our blood!

If there is calcium carbonate available, the carbonic acid will react with it to make calcium bicarbonate. Calcium bicarbonate is also quite common, due to how easy it is to find calcium carbonate. Calcium bicarbonate, however, is different in that it only exists in solution, and cannot be precipitated out of the solution. One can think of it as the "more soluble" version of calcium carbonate; normally it only very weakly dissolves, but if there is extra carbonic acid in the solution, it will dissolve readily.

Ocean reefs are built from calcium carbonate, and they provide shelter to a huge variety of wildlife. As CO₂ levels rise, this results in a rise of carbonic acid levels in the ocean, which results in an increase of dissolved calcium bicarbonate. Coral reefs in particular have been hit hard from the increase of CO₂ levels, with many of them becoming "bleached", meaning that the organisms that make the reefs are dying off.

To counter the effects of increased CO₂ in the oceans, we need to find ways to remove the dissolved CO₂ and make the ocean more basic. The ocean is normally slightly basic, with a pH that is slightly above 7. As CO₂ levels increase, the pH levels in the ocean have been slowly decreasing, in some areas dropping just below 7. This has been catastrophic to the coral reefs, since this change in acidity affects the ability for the coral to rebuild itself.

CaCl2(aq) → Ca2+(aq) + 2 Cl-(aq)

Ca2+(aq) + 2 H2CO3(aq) + 2 H2O(l) → Ca(HCO3)2(aq) + 2 H3O+(aq)

In excess OH-(aq), Ca(HCO3)2(aq) → CaCO3(s) + CO2(g) + H2O(l)

In the second half of the demonstration, the beaker precipitates calcium carbonate. Even though it isn't prepared with calcium carbonate, the calcium bicarbonate is prepared by the available calcium and the carbonic acid in solution from the carbonated water. When we add the base to the solution, it neutralizes the excess acid and leaves the solution slightly basic. This causes the calcium bicarbonate to decompose, which is why the calcium carbonate precipitates out.

This part of the demonstration is to showcase how important the pH of the solution is, and ultimately how important the pH of the ocean waters is to the wildlife that lives in them. If the water is too acidic, the calcium carbonate will continue to dissolve. Likewise, if the water is made basic again, then we can expect the reefs to recover and rebuild.

Additional Information

• Information on this demonstration can be expanded on with the explanation for the Rainbow Connection.
• This demonstration pairs well with the Food Chain Jenga.