# Ward's World Activity Guides

## How to Teach Osmosis and Diffusion with Agar Cubes

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Agar cube cell model activity: 3. Fill a clear container with vinegar to a 3-cm depth. 4. Place one agar cube of each size in the vinegar, making sure the blocks are submerged. The untreated blocks (one of each size) will be used for comparison. 5. After five minutes, remove the cubes from the vinegar with a plastic spoon and place them on white paper or a white plate. 6. Compare the treated cubes to the untreated cubes and observe any color changes. 7. Carefully return all the treated cubes to the vinegar. Continue checking the vinegar-soaked cubes every five minutes by re- moving them to determine the percentage of the cube that has been penetrated by the vinegar. 8. Continue this process until the vinegar has fully penetrated the cubes. Make a note of the time when this occurs. 9. Calculate the percentage of the volume of the cube that has been penetrated by the vinegar to determine how much vinegar has been absorbed by each treated cube. (Hint: It may be easier to first consider the volume that has not been penetrated by the vinegar—the portion that has not yet changed color.) 10. Percentage of cube penetrated = (volume of penetrated portion/volume of original cube) x 100 11. Volume of penetrated portion = volume of original cube – volume of non-penetrated inner cube 12. Volume of non-penetrated inner cube is the volume of the inner cube that has not changed color Determine the surface area and volume of each cube: 13. Multiply the length of a side of a cube by the width of the side of a cube. This will give you the area of one face of the cube. 14. Multiply this number by 6 (the number of faces on a cube) to determine the total surface area. 15. To find the volume of the cube you need to know the length, width and height. Multiply the length by the width by the height. For a cube the measure of each of these dimensions is exactly the same. Therefore, you can multiply the length of any side three times. This results in the formula: Volume = side * side * side. 16. Divide the surface area by the volume for each cube to determine the surface-area-to-volume ratios. Expected Results: Results may vary based on materials used, setup, procedure, and other factors, however, here is an example on what to expect: • As an acid, vinegar has a large number of hydrogen ions. When the hydrogen ions come into contact with the pH indicator, the vinegar changes color. Diffusion occurs when molecules in an area of higher concentration move to an area of lower con- centration. • As hydrogen ions from the vinegar move into the agar cube, the color of the cube changes, allowing you to see how far they have diffused. While random molecular motion will cause individual molecules and ions to continue moving back and forth between the cube and the vinegar solution, the overall concentrations will remain in equilibrium, with equal concentrations inside and outside the agar cube. Follow-up: • While this activity investigates how the size of an agar cube impacts diffusion, the shape of each cube remains consistent. Bio- logical cells, however, come in different shapes. To see how different shapes of "cells" affect diffusion rates, try various shapes of agar solids. Ice-cube molds can be found in spherical and rod shapes in addition to cubes. How does the shape impact the surface-area-to-volume ratio? • You may have noticed that the bigger the vinegar-soaked cube gets, the time it takes for additional vinegar to diffuse into the cube also increases—but not in a linear fashion. In other words, if the cube dimensions are doubled, the time it takes for the hydrogen ions to diffuse completely more than doubles. When you triple the size, the time to diffuse MUCH more than triples, why would this happen? How to Teach Osmosis and Diffusion with Agar Cubes (continued) + ward ' s science Find materials for this activity at wardsci.com. Discover more free activities at wardsworld.wardsci.com

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