Starch and Iodine Diffusion (2 Day Investigation)

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SWBAT conduct an experiment to determine the manner in which items diffuse through a membrane.

Big Idea

This lesson serves as a hands on way to help students understand semipermeable membranes.

Engage (Day 1)

5 minutes

In prior lessons, the students reviewed the structure and function of the cell membrane.  They have also reviewed the ways in which materials enter and exit cells.  I begin class with a discussion of the cell membrane.  

I ask the students to describe the structure of the cell membrane and to explain how items pass through it.  I frame the discussion by asking the students to think about the simulations we completed online.  I have found that referring the students back to those simulations helps them remember the information more fully.  This discussion and the rest of the lesson address NGSS MS-LS1-2 and SP2 as we review the various structures of the cell and their functions and the models we have used to represent them.

Explain (Day 1)

10 minutes

I ask the students to take out their Chromebooks and access their lab sheet for the day.  We read through the instructions together and I ask the students to take a moment and type an answer to the first question on the Diffusion Lab - Part One lab sheet.

Once the students have answered the question, I ask for volunteers to share their answers with the class.  Most of the students are able to recall the basics of active and passive transport and many of them also describe diffusion and osmosis.

Explore (Day 1)

20 minutes

After reviewing their answers, I place the students in groups and provide them with background information regarding the chemicals we will be using in the lab.  I also review basic safety procedures with them.  The students then enter the lab and put on their safety gear before awaiting further instructions.

I wait until the students have their safety gear on and are seated and quiet prior to explaining further instructions regarding the lab in order to ensure that the students are able to focus their attention on the instructions.  I then demonstrate how to moisten, open, and tie the dialysis tubing.  I also read through the instructions with the students, to further ensure student understanding.

After the explanations and answering additional student questions, the students begin working on the lab.  The students are able to easily observe the chemical reaction that takes place when the starch and iodine are combined (NGSS MS-PS1-2).  They have more difficulty when it comes to opening, filling, and tying the dialysis tubes. As I move from group to group, I remind the students to keep the dialysis tubing in the water until they are ready to use it, so the tubing does not dry out.  This video is an example of that reminder.  Some of the students also have difficulty remembering to place the starch and iodine tubes into the beaker containing the alternate liquids. Sometimes the students have difficulty tying the dialysis tubing.  I suggest having the students pour some of the liquid out and try again, as seen in this video.  It is also important to make sure that the students are making the starch and iodine beakers correctly.

Wrap up (Day 1)

5 minutes

As the students finish up the first portion of the lab, I ask them to explain what they think the beakers and tubes will look like the following day.  I also ask them to provide reasoning for their explanation.  For the most part, the students are able to determine that diffusion will occur through the dialysis tubing and they are able to identify that they will know if diffusion has taken place if they observe a color change.  The students are not sure of which material will diffuse through the dialysis tubing.

Engage (Day 2)

5 minutes

As the students enter the room they take out their journals and respond to the prompt: What do you think our experiment from yesterday will look like today?  I circulate through the room as the students write their responses.  Once the students have had an opportunity to respond, I ask for volunteers to share their thoughts with the class.  For the most part, the students think that a color change signifying diffusion will have occurred.  This color change is an example of a chemical change, addressing NGSS MS-PS1-2.  They have difficulty articulating which beaker and/or tube will have changed color.

Explain (Day 2)

15 minutes

The students take out their Chromebooks, put on their safety gear, and pick up their experiments from the previous day.  The students examine their experiments and begin to answer the questions from part one of the diffusion lab.  

One of the questions in the second portion of the lab requires students to make both quantitative and qualitative observations of the tubes.  The students are able to use electronic balances to find the mass of the tubes as a quantitative measure.

After allowing a few minutes for the students to examine their beakers, I show the students a correct example of the experiment.  We review the contents of the dialysis tubes and the beakers and the students explain what they think happened using what they know about the color change resulting from the combination of the starch and iodine solution.  I do not completely explain the experiment to the students until the wrap up portion of the lesson, as there are questions on the lab sheet for the students to answer.

Explore (Day 2)

15 minutes

As the students finish up answering the questions, they begin working on part two of the lab.  Part two of the lab requires students to complete three mini experiments to further explore the process of diffusion. As the students work on the experiment, I circulate through the room answering student questions and asking the students more probing questions about the experiments they are conducting. 

Wrap up (Day 2)

5 minutes

At the end of the class period, I review the experiment information with the students.  I ask the students to use evidence to explain what happened during part one of the experiment.  Furthermore, I ask the students to describe the coloration of the beakers and dialysis tubes as a way to confirm whether or not diffusion has taken place.  Based on the colors of the tubes, the students are able to identify that the iodine diffused while the starch did not diffuse.  I also asked the students to explain whether or not mass was a factor in determining whether or not diffusion had taken place.  For this experiment, the change in mass was minimal, so it was focused on more in terms of making quantitative observations than in examining diffusion.

We also discuss the manner in which the food coloring diffused through the water, moving from areas of high concentration to low concentration.  I ask for the students' observations on how this related to the way that the coloring from the M&M and the Skittle moved through the water.  From there we discuss the manner in which the raisins absorbed the water.  I ask the students to explain how these activities relate to cell membranes (MS-LS1-2, SP2).  The students are able to explain that diffusion takes place across cell membranes.  I conclude the discussion with the idea that the models we used in class are not exactly like cell membranes and we review some of the ways in which our models differ from the function of the cell membrane (Cross Cutting Concept Systems and System Models).