Measuring Diffusion of a Solid in a Liquid Purpose: To measure the effect of temperature on the rate of diffusion of a solid dissolved in a liquid. Background: Diffusion is the tendency of molecules of a substance to move from an area where they are in high concentration to an area where they are in lower concentration. The process of diffusion is due to the random motion of molecules of the diffusing substance. The rate of diffusion is affected by the speed of the random molecular motion. The faster the random molecular motion, the higher the rate of diffusion. All cells must constantly exchange materials with the air, water or soil that surrounds them in order to maintain the delicate balance that keeps them alive and healthy. They must constantly acquire gas and food molecules that they need to live and constantly get rid of gas and waste molecules that would poison them. Diffusion is extremely important to this process. Diffusion “brings” molecules to the cell and “takes” molecules away from the cell. Diffusion, also, is one of the processes that helps move molecules in and out of the cell through pores or channels in the cell membrane. Materials: Petri dish bull’s eye card potassium permanganate crystal hot water thermometer cold water room temperature water graduated cylinder Procedures and Interpretations: 1. What is diffusion? 2. What causes diffusion to occur? 3. What affects the rate of diffusion? 4. How would high molecular motion affect diffusion? 5. How would low molecular motion affect diffusion? 6. What is the relationship between temperature and molecular motion? 7. Why is diffusion important to cells? 8. Through what cell organelle does diffusion occur? A. Place the bottom of a glass Petri dish on the bull’s eye card (card marked in circles that are 1 cm apart and numbered across the center of the dish) so that the cross is at the center of the dish. B. Obtain three equal sized spherical crystals of potassium permanganate (KMnO4) from your teacher and place them on a dry sheet of paper. Be careful not to wet them. C. Fill the Petri dish with 50 ml of water from the hot (70-80oC) water beaker. Measure the actual temperature of the water before you pour it into the Petri dish. Record it in your personal data table. D. One student should act as the timer and the other as the observer/recorder. Using a forceps, the observer/recorder should place one of the crystals of the KMnO4 in the dish at the point of intersection of the lines at the command of the timer. Take care to get the crystal right on the cross hairs. Take care not to bump the dish or table table while diffusion is occurring. E. As the purple color diffuses, record the time that the circle of color reaches the numbers on the lines. The circles may not enlarge at an equal rate in all directions; if you find this to be the case, pick the most rapidly moving edge on one of the lines and record the time which it reaches each number. Record the times in your Personal Data Table. F. Rinse the Petri dish, dry it with a paper towel, and place it over the numbered lines. Repeat the experiment this time using water from the room temperature beaker. Measure the temperature of the room temperature water and record it in your Personal Data Table. Record the times when the circle edge of KMnO4 reaches each number on the lines in your Personal Data Table. G. Rinse the Petri dish, dry it with a paper towel, and place it over the numbered lines. Repeat the experiment, this time using water from the cold temperature beaker. Measure the temperature of the cold water and record it in your Personal Data Table. Record the times when the circle edge of KMnO4 reaches each number on the ones in your Perso