Gas Diffusion Model and Formaldehyde in Wood products

Gas Diffusion Model and Formaldehyde in Wood products


Subject (Focus/Topic):  Physical Science and Life Science

Grade Level:  6-9 Grade

Lesson Summary (Overview/Purpose):  Students will participate in collaborative lab groups to discover the mechanism for gas diffusion and apply that mechanism to the issue of formaldehyde outgassing in some wood composite products.

Overall Concept ( Big Idea/ Essential Question):  How does gas diffusion occur and how can gas diffusion impact health?

Focus Questions:

  • What is an acid?
  • What is a base?
  • What is an indicator?
  • What is diffusion?
  • What constitutes a semipermeable membrane?
  • Why is diffusion important to cell function?
  • Can gases diffuse?
  • What is pH?
  • What are some uses for formaldehyde (past and present)?

Objectives and Learning Goals:

  • Students will be able to differentiate between acids and bases.
  • Students will be able to collect data and interpret results for acids and bases using cabbage juice as an indicator.
  • Students will be able to describe the movement of gas molecules across a semipermeable membrane.
  • Students will be able to discuss the issue of diffusion of toxic gases into lung and nasal passages and cells in general.

Background information:  Acids and bases can affect our everyday life.  Students will test the properties of some substances with a simple indicator to determine whether a substance is an acid or a base.   When one boils purple (red) cabbage, the juice contains a type of pigment known as anthocyanins.  These pigments give many fruits and vegetables their rich red to purple colors.  The purple cabbage juice changes color in response to a change in free hydrogen concentration in solution.  The purple color changes to pink in the presence of an acid, which will donate available H+ while bases cause a change to a deep green because of the hydroxide ions in solution.  The actual intensity of color change is dependent on the strength of the acid or the base.  The pH scale is a negative log of H+ ions.  PH 1-6 is considered an acid with 7 being neutral.  PH of 8-14 is considered basic.
Diffusion of molecules through a semipermeable membrane is essential to the functioning of cells.  Molecules must be the correct size to move across membranes and be in the correct concentrations to move without using energy.  Diffusion is a concept that is difficult for students to grasp and models with food coloring or cornstarch abound.  Since gas molecules are so small and abstract to students, the concept of diffusion of oxygen or carbon dioxide into and out of our cells is not a tangible one.

Semipermeable membrane can be modeled using dialysis tubing and ammonia in a 25% solution can be used as a gas.  The ammonia is a volatile liquid that releases gas molecules easily.  It has a distinctive odor that lets you know this even if you do not understand the chemistry of outgassing.  This activity combines many concepts: pH, indicators, volatile gas, semipermeable membranes, and diffusion.

The concept of gas diffusion is directly related to a tangible Maine resource,wood.  Some composites contain formaldehyde and have outgassed into the atmosphere.  Especially in newer homes and trailers that have little air flow in an attempt to be more energy efficient.  Below are links to three articles for the teacher to read as background information.  The first link for USA TODAY is an article that middle school students could use as well.

This lesson would probably be best to have students complete after they have done a general cabbage juice as an indicator exploration lab with a variety of common acids and bases (lemon juice, milk, vinegar, soda, soap).

Teacher Preparation:
Obtain one small head of purple cabbage.  Cut into eighths and place into a large pot.  Cover with approximately 2 liters of water and boil slowly for 20 minutes.  Allow the pot and contents to cool.  Remove big pieces of cabbage.  You may want to strain the remaining mixture to remove smaller pieces of cabbage.  Store the liquid in jars and refrigerate until ready to use with the students.  The cabbage juice is best if used within 2-3 days, but will last for 2 weeks (the color change becomes less dramatic as time goes on).

Make a 25% ammonia solution (NH3).  The amount you need will depend on the number of students you have and how many students are in the lab groups.  Household ammonia is strong and the 25% solution works in the experiment and is safer for students to handle.  Ammonia should not be smelled directly or inhaled.  Read the caution on the bottle when making the solution.  Be sure your solution is labeled, dated, and tightly covered.  When it comes time for the experiment, have students get the ammonia from you.  DO NOT LEAVE AMMONIA ON LAB BENCHES FOR STUDENTS TO POUR EVEN IF TIGHTLY COVERED.

Dialysis tubing is available through a scientific catalog company.  The tubing is thin and has a plastic feel to it. The 25mmx16mm size works well with 15ml test tube that has a 15mm diameter. You may opt to prepare the tubing ahead depending on the age of the student you are working with.  If so follow these directions:

  1. Cut a length of tubing 12 inches or 30 centimeters.
  2. Place this length in a beaker of water to wet and soften the tubing.  Leave in the water for a minute.
  3. Take it out and rub the tubing between your thumb and index finger to loosen the two layers.
  4. Use a pair of scissors and cut lengthwise along the seam to open the tubing up.
  5. Cut the opened tubing into 2 inch or 5centimeter pieces.
  6. The tubing will dry out, but will become pliable again if moistened.

Obtain the following materials for each lab group:

  • two test tubes (15ml with a diameter of 15mm),
  • two 125ml Erlymeyer flasks ,
  •  aluminum foil pieces that will cover the students flasks
  • two squares of precut dialysis tubing (have extra on hand in case students do not get the seal right the first time)
  • two 2 inch balls of clay
  • two rubber bands to secure the tubing square to the test tube
  • 14 ml of cabbage juice
  • 25ml of 25% ammonia solution
  • 25ml of vinegar
  • 10ml graduated cylinder
  • Goggles for each student

Create a model test tube set up with the dialysis tubing on securely on the test tube for the students to see as well as a finished test tube flask combination.  Use the procedure below to create the examples for the students.

Student Directions:
Introduction:  The exploration you will be completing should provide you with a model for the diffusion of gases.  Keep in mind some important concepts: acids, bases, diffusion, semipermeable, and indicators.  If  you need to check your lab notebook and review your results for the earlier completed Cabbage Juice Exploration.


  • 2 15ml test tubes
  • 2 125 ml Erlymeyer flasks
  • 2 balls of clay
  • 14 ml of indicator (purple cabbage juice)
  • 25 ml of vinegar
  • 25ml of ammonia
  • Foil piece to cover flask with ammonia in it


  1. Put on your goggles.
  2. Pour 7 ml of cabbage juice into each of the test tubes.
  3. Put filled test tubes into test tube rack.
  4. Wet your dialysis tubing  (the clear plastic square).
  5. Place the wet square over the top of the test tubes filled with cabbage juice.
  6. Fold the edges down along the side of the test tube.  Don’t be afraid to use more water to secure the seal the edges.  Your goal is to create a sealed test tube with this dialysis tubing and a rubber band.
  7. Once you have a smooth, sealed piece of tubing over the test tube, wrap the rubber band around to keep everything in place.  Your teacher should have a finished product to show you.
  8. Test your seal by tipping the tube upside down.  Nothing should leak out.  If it does try setting it up again.
  9. Repeat for your second test tube.
  10. Pour 25ml of vinegar into one flask.
  11. Working with your partner, turn the tube upside down and lower it into the flask until it is at the 75 ml level.  Have your partner put clay around the tube so it will not fall into the vinegar. The opening to the flask should be all closed up.
  12. Go to the teacher’s lab station and bring your empty flask as well as your foil piece.  The teacher will pour 25ml of ammonia into your flask and you will cover it immediately with the foil securing around the edges.  Ammonia is an irritant and should not come in contact with your eyes, skin, or nasal passages (do not smell it).
  13.  When you and your partner are ready, remove the foil and repeat step #11.
  14. Record any changes to the cabbage juice in your lab notebook.
  15. Students Tasks:  Please record your answers in your lab notebooks.
  16. Write an explanation of what happened in both test tubes.  Make a claim about the dialysis tubing and ammonia.  Provide evidence from the experiment to back up your claim.  Feel free to use a diagram to help you complete this task  (you must include a written description too).
  17. Predict what will happen to the vinegar test tube and flask set up.  Have one student group keep their set up to observe tomorrow.
  18. If you had used aluminum foil instead of dialysis tubing as a barrier, do you think we would have seen a change in color in either the vinegar or the ammonia test tube and flask set ups?
  19. How is the dialysis tubing like a cell membrane?  How is it different from a cell membrane?

A Framework for Science Education
From Practices for K-12 Science Classroom
Analyzing and interpreting data
Engaging in argument from evidence
From Seven Crosscutting Concepts of the Framework
Cause and Effect: Mechanisms and Explanation
Systems and System Models
Stability and Change
From Core and Component Ideas in the Life Sciences
LS1.A: Structure and Function
LS2.A: Interdependent Relationships in Ecosystems


  • Have students research why formaldehyde is no longer used as a preserving agent.
  • Have students predict which other fruits or vegetables could serve as indicators (blueberries do work…far more expensive to use then purple cabbage).

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