Summary: Dry ice is added to a solution of bromothymol blue indicator and water. The solution changes color. When sodium hydroxide is added, the solution temporarily changes back to the original color. [At an elementary level, this is another way for us to "see" carbon dioxide and see it at different states. At the middle/high school level, this deals with acids and bases.]

---

Estimated Time: 10 - 15 minutes

Materials Needed:

• 1 500-mL graduated cylinder
• Small chunks of dry ice (in a small cooler)
• 1 pair of tongs
• 1 pair of gloves
• 1 pair of safety goggles
• 100 mL Bromothymol blue solution
• 100 mL 0.1 M NaOH solution
• tap water

Safety Notes: Students should not handle dry ice! Holding it can cause frostbite. Use gloves and tongs when handling dry ice. Do not ingest any lab materials. Bromothymol blue solution stains. Use care to avoid staining clothes, carpet, etc. Clean any spills immediately. NaOH is a strong base. If contact occurs with skin or eyes, immediately wash with water. Follow standard chemical safety procedures when dealing with chemicals and clean up. We recommend using a plastic graduated cylinder. If glass is used, follow proper procedures for glass clean up and disposal if it breaks.

---

Introduction: So far we have only experimented with carbon dioxide as a gas. Carbon dioxide can also be a solid. When carbon dioxide is a solid, we call it dry ice. This is what it looks like. It is very cold so we must use protection when handling it. Do not touch dry ice because it is so cold it can burn your skin and cause frostbite. [At the middle/high level, you may want to review/introduce acids and bases.]

Procedure:

• Fill the graduated cylinder 3/4 full with tap water.
• Add a few drops of bromothymol blue solution to the water.**
  • What color is the water now? Why?
• Wearing safety goggles and using tongs and gloves, remove a small chunk of dry ice from the cooler and add it to the water and bromothymol blue solution.
  • What color is the solution now? Any idea why?
  • What else can you observe happening?
  • What does the dry ice look like?
  • What do you think you are seeing above the cylinder and in the bubbles? Is anything else there?
  • Where is the "smoke" going?
  • Being careful not to knock over the cylinder, feel the smoke. How does it feel?
    
• Add a few drops of NaOH solution to the graduated cylinder.**
  • What color change happened?
  • How long did it last?
  • Why do you think this is happening?
• Repeat adding dry ice and NaOH several times while discussing.**

**This step and related questions (about the color, acids and bases) may be omitted at the elementary level. Add more dry ice as needed to see bubbles and "smoke" throughout the discussion.

Think About It:

Elementary Level:

What did you notice happen when we put the dry ice in the water? What do you think the bubbles contain? The bubbles have carbon dioxide inside. The water is warmer than the dry ice and so it changes from a solid to a gas. This is just like the dry ice does when it's exposed to the air. The process of changing from a solid to a gas without changing to a liquid is called sublimation. Look at the piece of dry ice in the water. It looks kind of shiny. That is because ice has formed on the outside of the dry ice. Feel the "smoke" on top. What does it feel like? What do you think it is? The smoke is made up of two parts. The part you see is actually water vapor. The carbon dioxide is so cold that it causes some of the water to condense in the cold air and form water vapor (like when you set a cold glass of water out and see the water vapor on the glass). The part you don't see is carbon dioxide. The water vapor "smoke" can show us where the carbon dioxide is going. Where is the smoke going? It goes down the side of the column. What does this tell us about carbon dioxide? It seems like it is "heavier" than the rest of the air. We say that it is more dense than the mixture of air around it.

Middle Level:

First complete the elementary level observations.

What did you notice about the color of the water after we put the dry ice into the cylinder? This same thing happened in our last activity. The yellow color means that the water became acidic. The water became acidic because the CO₂ reacted with the water to yield carbonic acid. The dry ice turned into CO₂ gas that acted the same way that our breath did before. The water turns back to blue because we put a base in it to neutralize it.

High School Level:

When solid CO₂ (dry ice) is added to water containing NaOH, the solution becomes a mixture of H₂CO₃, which is a weak acid, and NaHCO₃, which is a weak base. H₂CO₃ is carbonic acid, which makes the fizz in soda pop, and NaHCO₃ is sodium bicarbonate, which is an antacid (another word for base). NaHCO₃ is also known as baking soda, because it is used in cooking. The reaction can be written as follows:

CO₂ (g) + NaOH (aq) ------> H₂CO₃ (aq) + NaHCO₃ (aq)

Because the resulting mixture includes both a weak acid and a weak base, adding either strong acid or strong base (NaOH, or sodium hydroxide) does not change the pH very much. If a strong acid is added, the pH does not become much lower because the acid is neutralized by converting some of the base, NaHCO_3, to H₂CO₃. The H₂CO₃ does not give a very low pH because it is a weak acid.

If a strong base, such as sodium hydroxide, is added, the pH does not become very much higher because some of the H₂CO₃ is converted to NaHCO3. The NaHCO₃ does not give a very high pH because NaHCO₃ is a weak base. Thus the pH can neither become very low nor very high. The mixture of H₂CO₃ and NaHCO₃ acts as a buffer. A buffer resists a change in pH.

The bromothymol blue, an indicator, is used to track the change in pH of the solution by its color change. Bromothymol blue is blue in water. When CO₂ is added to the bromothymol blue and water solution, the pH is decreased as the CO₂ is dissolved slightly, and carbonic acid is formed. This slightly acidic solution becomes yellow. When the NaOH is added to the solution, the pH increases slightly and the solution becomes blue. As the solution returns to a lower pH value, it becomes yellow again.

There are some other important things to notice in this experiment. Clearly, the dry ice is very cold. You can see that ice formed around the pieces in the cylinder. Immediately the dry ice sublimed into carbon dioxide gas. What does this tell us about the phase diagram of carbon dioxide? At standard pressure and temperature, carbon dioxide is a gas. The solid phase is obtained by putting carbon dioxide under pressure at a low temperature. The same is true of the liquid phase. This means that the triple point of carbon dioxide occurs at a higher pressure and lower temperature than standard. One last thing that is important to notice is the vapor. The "smoke" you see is not carbon dioxide, rather it is water vapor, caused by the cold CO₂ cooling the air. However, the water vapor does indicate where the carbon dioxide is flowing. What does this tell us about carbon dioxide? Since it flows down the cylinder, it is more dense than the air around it. In fact carbon dioxide is much more dense than the air around it (even though some carbon dioxide is already in the air).

---

Teaching Tips:

• We strongly suggest this activity be completed as a demonstration.
• Bromothymol blue can be purchased from a store that sells aquarium supplies.
• Omit or simplify the acid and base discussion for younger students. Although we feel young students can understand acids and bases, the goal of the activity is to explore carbon dioxide.
• To dispose of dry ice, simply leave the cooler open and away from students and animals in a well ventilated area. The dry ice will sublime on its own.