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How to Teach Osmosis and Diffusion with Agar Cubes

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How to Teach Osmosis and Diffusion with Agar Cubes + ward ' s science Recommended Grade Level(s): Appropriate for: High School Time Requirements: Activity Time: 30–45 minutes Teaching Topics & Concepts: • This activity fits into a series of investigations on osmosis and diffusion. • Students will be able to recognize the relationship between structure and function in living things. Using this activity, stu- dents will investigate how surface area, volume, and surface-area-to-volume ratio affects diffusion in cells. • Measurement • Interpreting data Materials: • Ward's® Prepared Agar Cubes • pH indicator, such as bromothymol blue or phenolphthalein • Small container • Clear container for immersing agar cubes • Vinegar • Calculator • Clear plastic metric ruler • Pencil and notepaper • Plastic spoon • White paper or white plate • Timers Safety • Consult safety data sheets for all chemicals used. • Agar cubes are stored in a sodium hydroxide solution. This can cause skin irritation and can be harmful if ingested. Avoid contact with skin and eyes. • Wash your hands with soap and hot water before and after activity. • Wear appropriate Personal Protective Equipment (PPE) – safety glasses and disposable nitrile gloves. • Phenolphthalein solution is a flammable liquid and should be kept away from heat and sources of ignition, preferably in a flammable-liquids cabinet. ! Background All biological cells require the transport of materials across the plasma membrane into and out of the cell. By infusing cubes of agar with a pH indicator, and then soaking the treated cubes in vinegar, you can model how diffusion occurs in cells. You will make different size and shaped agar cubes as a model for cells to study the impact of cell size and shape on diffusion rate. By observing cubes of various sizes, you can discover why larger cells might need extra help to transport materials. Procedure: Demonstrate how the pH indicators change from blue or pink to clear when exposed to neutral or acidic conditions: 1. Place a few milliliters of the pH indicator (either bromothymol blue or phenolphthalein) into a small container. 2. Using a dropper, add a few drops of vinegar.

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