NAME OF LESSON: Diffusion and Osmosis

Subject Area: biology

Age or Grade: Middle school

Estimated Length: 1-2 class periods

Prerequisite knowledge/skills: Students will have already covered osmosis and diffusion in their textbook

Description of New Content: Observing and identifying osmosis and diffusion in action

Goals: Students will observe examples of osmosis and diffusion, and take part in the process themselves! Students should learn the definitions of each, including similarities and differences, the definition of semi-permeable membranes, and low vs. high concentrations.

Materials Needed:

4 medium (250 ml) beakers of tap water

2 large clear buckets of tap water

grapes

raisins

sugar

food coloring

sausage casing (or dialysis tape/tubing)

latex balloons

kool-aid (preferably red, blue and yellow!)

Procedure:

 Stations will be set up around the room, including a teacher demo station, where the lesson will start with a demonstration of diffusion of kool aid. (this also works well with a tea bag.) The two stations (several of each should be placed around the room) include: 1) grapes placed in a 1:1 mixture of sugar and water, which students will need to make and mix very well, raisins placed in the same solution, and grapes and raising placed in control beakers with just tap water. (the grape will shrink in sugar water and the raisin will expand in plain water, both illustrating osmosis, or movement of water from high to low concentration across a semi-permeable membrane). 2) Colored water (using red food coloring in some groups, blue koolaid in other groups) filled sausage casings or dialysis tubes (make sure these are sealed very well!!) Vs. colored water or koolaid in a latex balloon. Balloons and sausage/dialysis should be placed in separate buckets with plain, clear tap water, and may need to be observed over the course of a few hours or over two days. For koolaid, its effective to use yellow koolaid in water in the bucket, so when and if diffusion takes place, the color will become green due to the blue koolaid in the sausage/dialysis and the balloon!! (ideally students will observe diffusion through the sausage casing/dialysis tube, and NOT the balloon because the balloon isn’t permeable.)

Opener:

 Start the lesson by slowly pouring red koolaid into one of the large clear buckets filled with water at the front of the room. Ask the students to describe exactly what they see and the order of events. Once they report back to you, have them give it a definition: osmosis or diffusion and why? Then send them off to the stations to make othe similar investigations. (Another example of an opener that my classroom teacher Mrs. Belmonte uses, is to have the students all close their eyes, and standing in one back corner of the room she sprays some apple-cinnamon air freshener, and asks students to silently raise their hands when they smell something. After a few minutes she has all the students look around and see whose hands are up, and students explain what just happened.)

Development:

Students work together or solo at the two stations, then come together at the end.

Closure:

 End with a discussion of what they observed, why it happens, and examples of when osmosis and diffusion are important for life processes (use examples from nature and/or physiology).

Evaluation:

Quiz, lab report, lab notebook grading.

Extensions:

 Try the koolaid experiment with hot vs. cold water, or try similar experiments using blood cells in hypertonic, hypotonic or isotonic solutions.

References