Title: Osmosis and Eggs: 7th Grade Science, Unit 3 – Structures and Processes of Living Things
Goal: To demonstrate how a selective-permeable membrane will allow certain molecules (water) to pass through, but not allow others (sugar, contents of egg). Also, to demonstrate the process of diffusion, the movement of particles from an area of high concentration to an area of low concentration.
L.OL.07.23 Describe how cells in all multicellular organisms are specialized to take in nutrients, which are used to make the materials that a cell or organism needs.
S.IA.07.14 Draw conclusions from sets of data from multiple trials of a
scientific investigation to draw conclusions.
S.IP.07.12 Design and conduct scientific investigations.
S.IP.07.13 Use tools and equipment (spring scales, stop watches, meter sticks and tapes, models, hand lens, thermometer, models,
sieves, microscopes, hot plates, pH meters) appropriate to scientific investigations.
Background: Students should have knowledge of both diffusion and osmosis and how a cell membrane is selective-permeable. The timeframe for this activity is 2-3 days.
Vocabulary: diffusion, osmosis, cell membrane, water, cell
Materials: 500ml beakers, de-shelled eggs (soak raw eggs in vinegar for 48 hours – a chemical reaction will occur between the vinegar and the calcium carbonate of the eggshell; the egg contents will remain in the egg membrane), water, water with food coloring, corn syrup, molasses, syrup, salt water, sugar water, etc., balances, metric cloth tape measure if available, paper towels.
Safety: The eggs are uncooked. Eggs should be disposed of properly when finished, and students should avoid contact with their faces and wash hands when finished. Students should be instructed to handle the eggs with care – they are easily broken.
Procedure: Explain to the students that it is necessary to have a permeable membrane in order to demonstrate osmosis. The membrane of the de-shelled egg provides this permeable membrane to work with: the contents of the egg cannot get out. Water molecules can move across. Each group should begin by making a prediction of how the mass of each egg will be affected by placing them into different solutions. Have students design a procedure that will demonstrate how water moves across a selectively-permeable membrane. Before each group begins, the procedure and data table should be approved by the teacher. Here are points to consider as you “approve” student plans:
Each group should record the initial mass (and circumference if equipment is available) of each egg and make observations about the egg’s appearance. Then, students will place one egg into different solutions and leave them in the solution for approx. 24 hours. Then, each group should record the new mass (and circumference) of each egg and make observations about the egg’s appearance.
It is not important that each group tests every solution available because groups can share data as a class on the second day.
How to Incorporate More Inquiry: Demonstrate a de-shelled egg in a container of water with food coloring. In only a few minutes, the colored water will diffuse into the egg for the class to see. From here, show the class the available materials and have them generate questions about diffusion and osmosis. Have them design an experiment to answer the question.
Expected Results:
Egg in water – the egg will gain mass and get larger
Egg in vinegar – the egg will gain mass and get larger
Egg in food colored water – the egg will gain mass and color and get larger
Egg in corn syrup – the egg will lose mass and shrink
Egg in molasses - the egg will lose mass and shrink
Egg in syrup - the egg will lose mass and shrink
Egg in salt water - the egg will lose mass and shrink
Egg in sugar water - the egg will lose mass and shrink
In each case where the egg gained mass, the concentration of water molecules was greater outside the egg. The water molecules moved from a higher concentration outside the egg to a lower concentration inside the egg – osmosis. The food coloring molecules are also small enough to move across the egg’s membrane. In each case where the egg lost mass, the concentration of water molecules was greater inside the egg. The water molecules moved from a higher concentration inside the egg to a lower concentration outside the egg – osmosis. The salt and sugar molecules are too large to pass across the membrane.
Conclusion: Here are questions for group discussion/individual conclusions –
1) Create a class data table on the board…
2) Why did the eggs in water and vinegar gain mass?
3) Why did the eggs in the sugar and salt solutions lose mass?
4) Why did the salt and sugar molecules not enter the egg?
4) Calculate the mass of water that moved into and out of the egg.
5) Why did you use a de-shelled egg for this activity?
6) What part of the egg controlled water’s movement in and out of the egg?