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The Sewer Slugs

An Air Density Experiment

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Introduction and Credits

The Sewer Slug experiment is one of my personal favorites of the many science demonstrations available. While I will give the example here in relationship to weather and air pressure, the experiment has excellent parallels for physics, chemistry, biology, and more. To credit the person who came up with the experiment is difficult, because like many experiments and ideas for teaching, I heard about this years ago and have had little success in determining the person responsible for this wonderfully creative idea. To my knowledge, however, this is the first representation of using this experiment for demonstrating air pressure and air volume.

The idea of the experiment is to set up a small scenario for students. The experiment itself is like a discrepant event. Students are presented with a scenario and asked to brainstorm reasons for the event. By offering up explanations, students are using their intelligences to understand something new. Later, they will relate this event to something in the atmosphere.

Materials

  • Several Brands of Soda - Include clear and brown soda colors.
  • Tall glass jar - An old pickle jay works great. The experiment can also be done on a large scale with a fish tank.
  • Raisins - The drier the raisins, the better the experiment will work.
  • A funny attitude

Age Level

The Sewer Slug experiment works best for elementary and middle school level students. The experiment is also a cheap and easy homeschool experiment.

Instructions

  1. Pour several different types of soda into a large jar until the color has the appearance of dirty water. Make sure the pop is still translucent so that the Sewer Slugs can be seen through the liquid. Leave at least an inch or two of air space above the liquid and seal the jar with a lid.
  2. Place 10-15 dried raisins in the soda mixture. Initially, the raisins will sink to the bottom of the jar. The wrinkles in the raisins will eventually cause carbonation bubbles from the soda to get trapped. As the amount of carbonation builds on the raisin, the density of the raisin-bubble combo will change causing the raisin to float to the top of the jar. As the raisin breaks the surface, the carbon dioxide bubbles will burst and the raisin will again sink to the bottom.
  3. Ask for a student volunteer to record reasons why these Sewer Slugs are able to move up and down in the water so easily.
  4. Tip: The jar must be held relatively still. Walk around the room explaining the scenario (below) to students. Have some imaginary facts and figures ready. As students observe, they will be amazed to see the Sewer Slugs working on 'eating' the pollution in the water.
  5. Tip: The mixture must be made fresh prior to the start of the demonstration. If the soda loses some of its carbonation, the experiment will not work as well. If the raisins are suspended in the liquid too long, they will absorb liquid and stop working because the wrinkles will be smoothed out.

The Scenario

Your role as a teacher is to play up the following scenario for students.
Students are lead into believing there is a 'new technology' for cleaning up dirty water. The Sewer Slugs are a new breed of insect found in the year XXXX. Scientists were amazed to find that a tiny breed of creature actually uses and processes pollution in water. The bugs have no legs or fins of any kind, but can somehow move about in the water! Once a group of sewer slugs are 'released' into dirty or polluted water, they will clean the water to a drinkable level within days.
Keep the jar at a safe distance from students pretending as if the Sewer Slugs are rare. Also remind students that these insects are aggressive and can bit if provoked. By doing this, you are setting the students up to be curious about the event. They will also keep a relative distance from the jar. That way, the surprise is not ruined. You don't want the students too look too closely at the jar.

The Explanation

Carbon dioxide is the gas in the carbonation of soda. When under pressure, carbon dioxide is not visible in soft drinks. Once the pressure is released by taking off the cap, the carbon dioxide is released in the solution as bubbles. These bubbles are lighter than the surrounding liquid and will float to the top. This is Archimedes' Principle.

The raisins in the soft drink have deep crevasses or wrinkles where carbon dioxide bubbles will cling. As the bubbles cling to the raisin, the density is changed. Remember, Density is Mass divided by Volume or D=M/V. The volume of the raisin increases with added carbonation while the mass stays relatively the same.

Prior to an explanation of the reason the Sewer Slugs move in the water column, students must recognize that water has mass. To make the transfer to the atmosphere, they must also understand air has mass. In the atmosphere, the buoyant force is also the reasons a hot air balloon will float in the sky. Atmospheric density changes with height and with temperature. This is also another reason why hot and cold air masses will move differently relative to each other to create warm and cold fronts.

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