Carbon Dioxide Removal

Part of the Climate Change exhibition.

Introduction

The carbon cycle is a fundamental Earth process. Carbon moves continuously through the atmosphere, plants and animals, soils, the ocean, and the rocks of the solid Earth, in time-spans ranging from hours to tens of millions of years. Throughout Earth's history, the balance of carbon in those various reservoirs has kept the atmosphere's carbon dioxide (CO₂) level—and Earth's temperature—within relatively narrow ranges.

In this experiment, students will observe a natural process that removes carbon dioxide (CO₂) from Earth's atmosphere. This process is a part of the carbon cycle and results in temperature suitable for life.

Objective:

Students will:

develop an understanding of a part of the carbon cycle: animals exhale carbon dioxide and photosynthetic plants consume carbon dioxide
develop an evidence-based understanding of the connection between plants and animals through the carbon cycle

Skills:

observation
experimentation
interpretation

Materials:

bromothymol blue (BTB), a carbon dioxide indicator in a dropper bottle
small jars with lids (baby food jars)
water
drinking straws
sprigs of Elodea (or other water plant, commonly found in aquarium supply stores)

Procedure

Prior Knowledge

Explain to students that air is a mixture of many different gases (e.g., nitrogen, oxygen, and other trace gasses). Ask: What gases do animals breathe in and breathe out? What gases do plants take in and release through photosynthesis? (Answer: Animals' bodies use oxygen and produce carbon dioxide; breathing out, or exhaling, releases carbon dioxide. Through the process of photosynthesis, plants do the opposite: they take in carbon dioxide for building their cells and release oxygen. Animals and plants are, then, dependent on each other for survival.)

Exploration

Tell students that they will conduct an experiment that will demonstrate a natural process that removes CO₂ from Earth's atmosphere.
Divide the class into teams of 2 or 3 students.
Hand out two jars and lids, water, straw, sprig of Elodea, and the bromothymol blue (BTB) pre-mixed solution to each group.
Instruct teams to half fill both jars with a pre-mixed mixture of water and BTB (the mixture should be about 6 drops of BTB to 8 ounces of water). This step will insure that the liquid in all the jars is a uniform blue color. (See Figure 1)
Ask each team to choose 1 member to blow into the jars through a straw until the others notice a change. (The water will turn yellow). NOTE: It is important to stress that students should blow OUT through the straw, not suck in.
Ask students what they have observed. (Answer: The BTB solution has turned yellow.) Explain that BTB is a chemical indicator, something that shows when a particular chemical is present. BTB turns yellow in the presence of carbon dioxide.
Have students place a sprig of Elodea in one of the jars, cover both jars and place them in a sunny window or in direct sunlight for 1 hour. Explain that Elodea is a plant that grows underwater in lakes and ponds.
After an hour or the next day, have students observe the contents of the jars and report their observations and interpretations. (Answer: The BTB solution with Elodea turned blue again. The blue color means that there is no carbon dioxide in the water. The plant must have removed the carbon dioxide from the water. The solution in the other jar remained yellow because there was no plant to remove the carbon dioxide and the carbon dioxide is still in the water.)

Wrap-Up

Show and discuss with students the Carbon-Oxygen Exchange diagram (Figure 2).
Ask students: "What do you think will happen to the carbon dioxide levels in the air if a forest is cut down and burned?" (Answer: CO₂ would increase.)
Ask: What effect do you think this would have on Earth's climate? (Answer: Since trees and other plants build their cells using carbon from the carbon dioxide they absorb, cutting and burning them releases this carbon back to the atmosphere as carbon dioxide, increasing its concentration in the air. Since carbon dioxide is a greenhouse gas. The greater the concentration of carbon dioxide in the air, the warmer the temperature, resulting in the possibility of global warming.)