Shell Shifts

Discover how pH affects seashells

Click to watch video on how to teach the lesson

Implement this lesson:

After completing ‘Rainbow of pH’ lesson

Learning objective:

Students will explore how the pH of a solution dictations whether carbon is present in the form needed to make seashells.

Next Generation Science Standards (NGSS)

MS-ESS3-4

Science and Engineering Practices

1-LS1-1 Use materials to design a solution to a human problem by mimicking how plants and/or animals use their external parts to help them survive, grow, and meet their needs.

Disciplinary Core Ideas

ESS2: Compare multiple solutions designed to slow or prevent wind or water from changing the shape of the land.

ESS3 Construct a scientific explanation based on evidence for how the uneven distributions of Earth’s mineral, energy, and groundwater resources are the result of past and current geoscience processes.

PS1.1 Develop models to describe the atomic composition of simple molecules and their structures

Crosscutting Concepts

MS-PS1-2 Cause and Effect Cause and effect relationships may be used to predict phenomena in natural or designed systems.

Texas Essential Knowledge and Skills (TEKS)

K.6A use the senses to explore different forms of energy such as light, thermal, and sound

K.9B examine evidence that living organisms have basic needs such as food, water, and shelter for animals and air, water, nutrients, sunlight, and space for plants

K10B identify basic parts of plants and animals

1.6(A) identify and discuss how different forms of energy such as light, thermal, and sound are important to everyday life

1.10(A) investigate how the external characteristics of an animal are related to where it lives, how it moves, and what it eats

2.9(A) identify the basic needs of plants and animals

2.9(B) identify factors in the environment, including temperature and precipitation, that affect growth and behavior such as migration, hibernation, and dormancy of living things

2.9(C) compare the ways living organisms depend on each other and on their environments such as through food chains

3.9(A) observe and describe the physical characteristics of environments and how they support populations and communities of plants and animals within an ecosystem

5.9(A) observe the way organisms live and survive in their ecosystem by interacting with the living and nonliving components.

5.9(B) describe the flow of energy within a food web, including the roles of the sun, producers, consumers, and decomposers

5.9(C) predict the effects of changes in ecosystems caused by living organisms including humans, such as the overpopulation of grazers or the building of highways

5.9(D) identify fossils as evidence of past living organisms and the nature of the environment at the time using models

Overview:

A wide variety of ocean organisms—from shellfish and corals to certain kinds of algae—contain calcium carbonate in their exoskeletons. There needs to be a sufficient concentration of carbonate ions available for these creatures to construct their shells.

Increasing levels of CO2 in the atmosphere are creating an increase in levels of dissolved inorganic carbon and a decrease of the pH in the oceans, a phenomenon called ocean acidification. The carbon species you worked with in this activity are all in a dynamic equilibrium described by these equations:

CO2 + H2O ⇔ H2CO3 ⇔ HCO3− + H+ ⇔ CO32− + 2H+

The equilibrium ratios of different carbon species in seawater depend on the pH, shown by the graph below (click to enlarge). The concentration of carbon acid (H2CO3) is low, so it has been left off the graph. At ocean pH (shown by the vertical blue bar), the bicarbonate form (HCO3−) represents over 90% of the dissolved inorganic carbon.

The minerals in the ocean contain large amounts of carbonate, so another reaction that occurs is:

CO2 + H2O + CO32−⇔ 2HCO3−

These two reactions show how increasing CO2 can lower the pH and reduce the concentration of available carbonate ion. The lower concentration of carbonate reduces the amount available for calcifying organisms that span the food chain and also drives the equilibrium to dissolve more CaCO3 rocks in the ocean.

The primary component of most seashells is calcium carbonate (CaCO3). You may have noticed bubbles forming when you initially covered the seashell with vinegar. The bubbles are carbon dioxide (CO2), which is created when the CaCO3 in the shell is exposed to an acid such as vinegar. The reaction that describes this process is:

CaCO3 + 2H+ ⇔ Ca2+ + CO2 + H2O

Sodium bicarbonate (NaHCO3) and calcium chloride (CaCl2) both dissolve well in water, and the solutions you made with them should have appeared relatively clear. These dissolutions can be expressed as:

NaHCO3 → Na+ + HCO3−

CaCl2 → Ca2+ + 2Cl−

In water, bicarbonate (HCO3−) is never present by itself, but exists in equilibrium with other forms of dissolved inorganic carbon: carbonic acid (H2CO3), carbonate (CO32−), and carbon dioxide (CO2). The balance between all of these species is shown by these equations:

CO2 + H2O ⇔ H2CO3 ⇔ HCO3− + H+ ⇔ CO32− + 2H+

When you mix the NaHCO3 and CaCl2 solutions together, the carbonate ion (CO32−) reacts with the calcium ion (Ca2+) to form CaCO3 through the following reaction:

CO32− + Ca2+ ⇔ CaCO3

The mixture should turn cloudy since CaCO3 is not very soluble in water and will precipitate out of the solution upon forming. You have just made little bits of shells!

When you add vinegar to this mixture, the excess hydrogen ions (H+) will dissolve the calcium carbonate (CaCO3) particles—just like it did to your seashell—and the solution should turn clear again. There should be many bubbles because the bicarbonate in solution will also react with the vinegar to form CO2.

Adding a base such as sodium hydroxide (NaOH) will then shift the equilibrium back to solid CaCO3, forming a cloudy precipitate again. This shows how sensitive CaCO3 is to the pH of its environment.

Materials:

Vinegar
Calcium chloride (CaCl2, sold as Damp Rid in stores)
Sodium bicarbonate (NaHCO3, also known as baking soda)
0.25 molar sodium hydroxide (NaOH, sold as lye in stores)
Several clear cups or glass container
Water (not shown)
Marker that will write on plastic or glass
Safety goggles
Optional: pH indicator such as cabbage juice, bromothymol blue, or phenol red

Advanced Prep:

(optional) Prepare cabbage juice
Make a sodium bicarbonate solution by adding one spoonful of baking soda to a cup of water. Stir well. This can be created in advanced prep or by the students.
Create a separate calcium chloride solution by adding one spoonful of calcium chloride to one cup of water. Stir well. This can be created in advanced prep or by the students.
Review effects of acids and bases on oysters shells using a microscope https://docs.google.com/presentation/d/19ImfY80agilDZtVto-UgQ-fL4qX-0JuadaqW6XQXHsk/edit?usp=sharing

Procedure:

Make a sodium bicarbonate solution by adding one spoonful of baking soda to a cup of water. Stir well. This can be created in advanced prep or by the students.
Create a separate calcium chloride solution by adding one spoonful of calcium chloride to one cup of water. Stir well. This can be created in advanced prep or by the students.
In a four cup add equal parts calcium chloride solution and sodium bicarbonate. Mix well. Have students write down observations.
(optional) Add a small amount of pH indicator to keep track of the pH throughout the experiment. The experiment will still work without this.
Add small amounts of vinegar to the mixed solution solution until you notice a change. Stir after each addition.
Add in small amounts of 0.25M sodium hydroxide to the mixed solution until you notice a change. Stir after each addition.

Questions to Ask:

Pre-experiment

What is the chemical formula for seashells?

Post-experiment

How would you describe the three solutions?
What did you notice when the two solutions mixed?
How much sodium hydroxide did you add to the solution before you noticed a change?
How much vinegar did you add to the solution before you noticed a change?