Carbon Dioxide Can Make a Solution Acidic

Materials for the Demonstration

• Universal indicator solution
  
• Water
  
• 2 clear plastic cups
• Straw

Teacher Preparation

Make indicator solution for student groups

• Make a dilute universal indicator solution for this demonstration and for each student group by combining 625 mL water with 25 mL universal indicator solution.
• Pour at least 80 mL of this dilute universal indicator solution into a clean plastic cup for each student group.

Note: Your local tap water is likely fine for the demonstration and activities in this lesson. If the indicator solution you make is not green, this means that your water is either acidic or basic. If this happens, use distilled water, which is available in supermarkets and pharmacies.

Note: In the activities, each group will need 80 mL of indicator solution. Check to make sure that you prepare enough solution. You will need about 50–60 mL of indicator solution for your demonstration. If 650 mL of solution is not enough, make more using the same proportions.

Give each student an activity sheet.

• Lesson 6.10 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 6.10 Activity Sheet Answers PDF | DOCX | Google Doc

Download the student activity sheet, and distribute one per student.

All Downloads

The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms & Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

Question to Investigate

Will carbon dioxide from carbonated water change the pH of an indicator solution?

Materials for Each Group

• Universal indicator solution in a plastic cup
• Water
• Carbonated water (club soda or seltzer water) in a wide clear plastic cup
• 1 wide, clear, plastic cup
• 2 taller, clear, plastic cups
• Graduated cylinder
• Universal Indicator pH Color Chart

Teacher Preparation

Pour 25 mL of carbonated water into a wide, clear, plastic cup for each group.

Expected Results

The indicator inside the cups with water remained green, while the indicator with the carbonated water turned yellow.

Question to Investigate

Will carbon dioxide gas produced in the baking soda and vinegar reaction change the pH of an indicator solution?

Materials for Each Group

• Universal indicator solution in cup
• Universal indicator pH color chart
• Water
• Baking soda in wide clear plastic cup
• Vinegar in cup
• 2 small clear plastic cups
• 1 wide clear plastic cups
• 2 taller clear plastic cups
• Graduated cylinder

Teacher Preparation

• Pour about 50 mL of vinegar in a wide plastic cup for each group.
• Place about ½ teaspoon of baking soda into a small clear plastic cup for each group.

Expected Results

The indicator inside the cup with only vinegar remained green while the indicator inside the cup with the vinegar and baking soda reaction turned yellow.

Ask students:

• Did either indicator change color?
  Only the indicator with the chemical reaction changed color.
• Why did one set of cups only have vinegar in the bottom?
  It is possible that vinegar by itself causes the indicator to change color. Since this indicator did not change color, it must be the carbon dioxide gas produced by the chemical reaction, and not just the vinegar that caused the color change. The indicator solution in the set of cups with only vinegar in the bottom serves as a control.
• What does the color of the indicator solution tell you about the pH of each solution? Is it acidic, neutral, or basic?
  The color change shows that the indicator solution is slightly acidic.
• What could you add to the acidic indicator solution to neutralize it?
  Because the indicator solution is acidic, students should suggest adding a base. Tell students that baking soda is a base.

Tell students that carbon dioxide reacts with water to produce carbonic acid. Students may count the number of atoms on each side of the equation to show that it is balanced. Point out that the double arrow in this equation means that carbonic acid breaks down readily to form carbon dioxide and water again.

Explain to students that too much CO₂ in the atmosphere causes Earth and its atmosphere to become warmer. But excess CO₂ can do something else which they have seen in the chemical equation and in their experiments. Carbon dioxide can make water more acidic which is causing a big problem in the oceans. The excess acid in ocean water, called ocean acidification, makes it difficult for some organisms to form shells and is especially damaging to coral.

Explain that the ocean is actually basic. The pH of the ocean is about 8.2. The term “ocean acidification” means that the ocean is tending to become more acidic or less basic. It has moved from about 8.2 to 8.1. This may not seem like a big change, but it is a very big change to organisms in the ocean which are very sensitive to changes in pH. When ocean water becomes more acidic it causes two main problems for shell-making organisms like clams, oysters, and coral:

1. It becomes harder for these organisms to make their shells
2. If the water becomes too acidic, normal shells can react with the more acidic water causing the shell to break down

Clams, oysters, coral and other shell-making organisms make their shells out of two ions: the calcium ion (Ca⁺²) and the carbonate ion (CO^-2). When these two ions join, they make calcium carbonate (CaCO₃) which is the main substance for the structure of the shell. Ocean acidification affects the carbonate ion. Here’s how:

The site also offers a wealth of resources on ocean acidification and climate change.

Project the illustration Carbonic acid and Carbonate ion.

A hydrogen atom from the carbonic acid gets into the water as a hydrogen ion (H⁺). This hydrogen ion bonds to the carbonate ion in ocean water and creates bicarbonate ion (HCO-₃) which the shell-making organisms can’t use. This means there are fewer carbonate ions for the creatures to attach a calcium ion to, making it harder for them to make the calcium carbonate they need to make their shells.

Extra hydrogen ions in the water also makes the water more acidic. If the water eventually gets too acidic it might react with the calcium carbonate in the shells causing them to break down.

Show the video Ocean Acidification.

Note: The narration and action of the video go by pretty quickly so you might want to stop the video in a few places to help students understand what is being presented.

The vast majority of the excess carbon dioxide in Earth’s atmosphere is from burning fossil fuels such as petroleum, natural gas, and coal. Most of this fuel is used for cars, trucks and other forms of transportation, for running power plants that produce electricity, and for heating homes and businesses.

Have students research alternative sources of energy that could help in burning less fossil fuel. Students could present their research in a short paper with illustrations, power point, tri-fold board, or in any way you feel will work. Students should describe how the renewable energy source works and the advantages and challenges of the technology.

Some possible topics could be:

Renewable energy sources

• Wind
  
• Solar
• Geothermal
• Biofuels
• Hydroelectric

New transportation technology

• Electric cars
• Hydrogen fuel cells