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Observing Osmosis! Lab Activity Teacher Implementation Guide: Thank you for purchasing this product! I use this lab activity to supplement my unit on the Cell Membrane and Cell Transport and to reinforce my teaching of osmosis. It’s an excellent way for students to observe the impact of different concentrations of solutes on various food samples. Students should have prior knowledge of diffusion and osmosis before completing this activity. If possible, please consider writing a review of this product on my TPT page, in exchange for TPT credit. Your feedback is important to me! Happy teaching! -Kimberly B. It’s Not Magic It’s Science What’s Included? Osmosis Investigation Lab Student Handout (6 pages- PDF) Answer Key for Osmosis Lab (6 pages- PDF) Required Materials: (to be purchased) (enough for three classes of approximately 20-25 students) One Dozen Eggs (3 eggs per class) 1 Gallon White Vinegar 4-6 potatoes (any kind) Gummy Bears (60-75 total) Plastic Cups Karo Syrup (or any corn syrup, I have tried maple syrup and it did not work as well)- 250 mL per class 1 Gallon Distilled Water (optional: I have done this lab both with distilled water and tap water, and have honestly seen very little (if any) difference in results) Required Laboratory Equipment/Additional Supplies: 100 mL Graduated Cylinders (one per student group) Electronic Balance Scale (one per student group if available) Markers (one per group) Paper Towels 400-600 mL Beakers (3 total per class- to be used for egg demonstration) Calculators Pre-Lab Set-Up: 1. Soak a dozen eggs in white vinegar. I either place as many as I can fit in a large mixing bowl, or 3-4 eggs in 1000 mL beakers. As the shell is dissolving, carbon dioxide gas is released, so do not seal the bowls/beakers as pressure will build up! The shell can almost dissolve completely in 24 hours, but I prefer giving them 48 hours to ensure the reaction fully completes (the eggs will dissolve at different rates depending on their placement in the bowl). You can also GENTLY rub any remaining shell pieces that have not yet dissolved in order to remove them completely. NOTE: Eggs are very fragile! I always soak more eggs than I need because I always end up popping a few! This is the very reason the eggs are part of a teacher demonstration and not given to students to use. 2. Prepare baskets/containers of materials for each student group. In each basket place the following: a. 4 plastic cups, one marker, 2 gummy bears (of the same color to ensure validity), 2 potato slices, paper towels, 100 mL graduated cylinder 3. Pre-slice the potatoes right before class begins (if sliced too far ahead of time they will begin to turn brown). You do not need to peel them. Cut them into approximately 5 mm wide slices, then cut them in half to create two halfmoon shaped pieces (they fit better in the plastic cups this way). 4. Prepare saltwater solutions. Each group needs 180 mL of salt water. In a class of 24 students, split into 8 groups of 3, that would be 1440 mL. Rounding up for ease, I would prepare 3 separate 600 mL beakers each with about 500 mL of solution. To create a 10% salt solution, add 10 grams of salt to 100 mL of water. Demonstration Instructions: (15-20 minutes Day 1, 10-15 minutes Day 2) 1. This demo works best with ~500 mL beakers. Any smaller and the eggs won’t fit! To ensure validity I fill each beaker with the same volume of liquid (250300 mL is perfect). You can fill the beakers prior to student arrival or fill them as the demo progresses (my preference). a. Fill Beaker A with 250 mL of vinegar b. Fill Beaker B with 250 mL of distilled water c. Fill Beaker C with 250 mL of Karo Syrup 2. Before weighing the eggs, I will usually walk around the room to show students what the eggs look like up close. Most of them are extremely surprised an egg can be deshelled! Note: Modeling the entire procedure with the eggs throughout the demonstration makes it much easier for students to then do the same thing with their potato and gummy bear samples! 3. For each egg I ask a student volunteer to come up to the front and weigh its initial mass. We all record it together in our data table. We all also record the initial volume of each liquid. 4. When it comes time for students to make their hypotheses, I tell students to think about which solutions possibly represent solutions which are hypertonic, hypotonic, or isotonic to the eggs. 5. On day two, again ask student volunteers to come up to the front to weigh the eggs. The egg placed in Karo Syrup will be especially funny looking (extremely deflated), so I always walk around the room with that beaker for students to see it better! 6. Calculate the new volume of liquid in each beaker by estimating the volume on each beaker (not the most accurate method but pouring Karo Syrup into graduated cylinders can be extremely sticky and messy). a. Optional: Obtain a 500 mL graduated cylinder and pour the contents of each beaker into the cylinder to obtain a more accurate measure of the final volume. 7. Model to students how to calculate percent change of the mass of each egg. Make sure to point out that a negative number is okay, it just means it got smaller! Unexpected Result! The egg placed in the vinegar is intended to represent an isotonic environment. However, I usually always see a slight gain in mass in this egg. Student Laboratory Instructions: Day 1 (15-30 minutes depending on class size and academic level of students) 1. Each student group should receive a basket of prepared materials and one electronic balance. 2. Students should label their cups with their group initials and water type (saltwater or distilled water) and record all initial weights of each food sample. 3. Place large beakers of distilled water and saltwater in 2-4 different locations around the room for student groups to use to measure 90 mL of each water sample for each cup. a. NOTE: I recommend the students using only 90 mL of water in their plastic cups because it can be easily measured in a 100 mL graduated cylinder. In addition, each cup will either gain water or lose water due to osmosis. The gain of water never exceeds 10 mL, so the final volume of each cup can be measured in a 100 mL graduated cylinder as well. In my classroom I tend to always have a lot of 100 mL graduated cylinders in my supply closet, so this volume works well. However, feel free to alter the measurements to best suit your own equipment. Student Laboratory Instructions: Day 2 (30 minutes, including time to answer conclusion questions) 1. Stress to students to CAREFULLY remove their gummy bear from the distilled water. It will be very fragile and can easily break into several pieces, making it difficult to get accurate mass and volume measurements. 2. Make sure they place a paper towel on their balance scale before recording final weights, as food samples will be wet. Don’t forget to remind them to zero out their scales after the paper towel is placed! 3. Once each food sample is out of the cup, students will pour the remaining liquid in their cups back into their 100 mL graduated cylinders to record the final volume. 4. All solutions can be poured down the drain and food samples and cups should be thrown in the garbage. Unexpected Result! Students usually expect the gummy bear placed in salt water to lose mass, but in actuality, it gains mass! Although unexpected, it can lead into an interesting discussion as to why this result was obtained. There is more dissolved sugar in the gummy bear than there is dissolved salt in the water, making the salt water hypotonic to the gummy bear. It can also be used to point out just how much sugar is in the candy that we eat! Conclusion Questions: I like to go over questions 5 and 6 with the students together as a class, as they are a bit more difficult and require more critical thinking. I will often model the response I am looking for in question 5, and have students complete question 6 on their own. Suggested Group Sizes: I think this works best as an activity completed in groups of three. However, pairs of students work as well!. It all depends on how large your class size is and the amount of material you are able to purchase. Questions? Feel free to send me an email at [email protected] and I would be happy to help!