Summary

In this demonstration, students will understand the factors affecting solubility of both a solid and a gas in a liquid through the process of making root beer.

Grade Level

High School

Objectives

By the end of this demonstration, students should be able to

• List the factors that affect dissolving a solid in a liquid.
• List the factors that affect dissolving a gas in a liquid.
• Differentiate between a solute and a solvent.

Chemistry Topics

This demonstration supports students’ understanding of

• Solubility
• Solute and Solvent
• Temperature
• Pressure

Time

Teacher Preparation: 15 minutes

Lesson: 30 minutes

Materials

• Large plastic container with lid (ex: gallon size Tupperware container)
• Weights for lid (ex: Textbooks, brick, etc…)
• Dry ice (5lb bag)
• Distilled water (at least 1 gallon/2 classes)
• Small cups – clear plastic is best (1 per student)
• Granulated Sugar (5lb bag)
• Sugar cubes (1 box)
• Root beer concentrate (1 container)
• Measuring cups
• Stirring spoon
• Ladle

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wear gloves. The dry ice is extremely cold and can cause frostbite if directly touched with skin. Only maneuver the dry ice with proper handling equipment.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• Give students time to answer all of the pre-lab questions.
• Discuss answers to questions:
  • Be sure that the students have included both sugar and CO₂ as solutes for question 3.
• Tell students you will be making very basic root beer in class using some of the ingredients they have listed. Have all of the ingredients hidden behind your desk, and take them out one at a time as needed.
  • Pour about a half a gallon of distilled water in your container
• Ask students what one of the main ingredients is in all soda. They should say that it is sugar. Lead the students to tell you that sugar is a solid and that you will first be discussing the factors that affect dissolving rate of a solid.
  • Get out your box of cubed sugar and place a few cubes in the water. Then just stare at it.
  • Ask the students what you could do to affect the dissolving rate of the sugar cubes.
  • Lead the discussion towards crushing the sugar cube which will increase the surface area. Then get out your bag of granulated sugar and place about 1-2 cups of sugar in the water. Do Not Stir!
  • At this point the students should be telling you that you need to stir the solution to help increase the dissolving rate. I like to refer to stirring a solution as agitation of a solution.
  • Finally, lead the students to tell you that an increase in temperature of the solvent would also increase the dissolving rate of the solid sugar. You do not want to actually heat up the water, however, due to the next step in the demonstration.
• At this time students should have the left side of their chart filled out.
• You will also want to add about a tablespoon of the root beer concentrate to your water at this time.
• Now lead the discussion about CO₂ being a gas that is dissolved in sodas, and the factors that affect the dissolving rate of a gas.
  • Ask the students to tell you where you could find CO₂.
  • Some might say that they breathe out CO₂. I then jokingly ask the students if someone wants to volunteer to stick their head in the container and blow bubbles!
  • Lead the students to tell you that dry ice is a solid form of CO₂. This is a great time to review the term “sublimation.”
  • Place the dry ice in the container.
  • Allow the student to conclude that the CO₂ is escaping from the container.
  • A student might then tell you to put a lid on the container, which you will do (leaving a small gap for some gas to escape.)
  • Lead the student to understand that by placing a lid on the container you are increasing the pressure inside the container.
  • Next, discuss the conditions under which they like to drink their favorite soda. Bring up the term “flat” and what that means. Lead the discussion towards the fact that a cold soda will hold more CO_2­, while a hot soda will go “flat.”
  • Place the container somewhere in the room, and place weights on the lid. The container must sit like this for at least 20 – 30 minutes in order to have some of the CO₂ dissolve.
  • Typically at this time I introduce solubility graphs.
  • The root beer is served with the ladle and small cups at the end of class.
• Do not expect this root beer to taste like A&W. It will be very lightly carbonated. If using clear plastic cups, the students will be able to see the CO₂ bubbles on the sides of their cups.