Introduction

Molecules are in constant random motion. If molecules of one type are concentrated in one area of a container, they will move around until they are evenly distributed throughout the container. In this investigation you will observe the conditions that affect the rate at which various substances diffuse.

Part 1: Temperature and the Rate of Diffusion

Materials

Finger bowls (3), polar-coordinate graph paper (3), potassium permanganate (KMnO₄) crystals, forceps, watch or clock, water (hot, cold, and room temperature).

Methods

Fill one of the finger bowls about half-full with cold water. Fill another about half-full with room-temperature water, and the third about half-full with hot water. Record the temperature of each in the data table. Center each of the finger bowls on a piece of the graph paper. Using the forceps, place one crystal of KMnO₄ in the center of each finger bowl. Record the time. Make sure that this lines up with the center of the graph paper. Be careful not to touch the KMnO₄ as it can be toxic.

After five minutes, record the farthest distance that the KMnO₄ has traveled. Wait another five minutes, and again record the farthest distance that the KMnO₄ has traveled. Wait another five minutes and repeat the process. Record any observations concerning the color, intensity, or shape of the colored region. Pour the water into the designated waste container, rinse the dishes with water, and put them away.

Part 2: Molecular Size and the Rate of Diffusion

Materials

Glass tube (0.5 m x 1 cm), cotton balls, plastic dropping bottles with 1 M hydrochloric acid (HCl) and 1 M ammonium hydroxide (NH₄OH), meter stick.

Methods

Make sure that the glass tube is clean and dry. Place a small piece of the cotton ball in each end of the tube. Make sure that it completely fills the end of the tube, but do not stuff it too full. Lay the tube flat on the table. Put a paper towel under each end of the tube.

Obtain the dropping bottles with the HCl and NH₄OH. Be very careful not to spill either of these materials, as they can cause severe burns and will eat holes in clothing, books, etc. If you spill some of it on yourself or another person, quickly wash it off with lots of water and inform the teacher.

With your partner, simultaneously place ten drops of HCl on the cotton at one end of the tube and ten drops of NH₄OH on the cotton at the other end of the tube. Observe the tube until you see a faint white ring form on the inside of the tube.

Measure the distance from each end of the tube to the white ring, and record your data in a data table. Record the results obtained by the other members of the class in the same data table. Rinse out the tube with water, dispose of the cotton in the designated container, and put the remaining materials away.

Observations:

Part 1: Temperature and the Rate of Diffusion

Prepare a data table in which to record your results. You should have three rows (one for each temperature), and five columns in which to record the initial water temperature, the final water temperature, the distance that the KMnO₄ diffused during each five-minute period.

Prepare a histogram (bar graph) to show the maximum distance traveled by the KMnO₄ at each temperature in the fifteen-minute period.

Part 2: Molecular Size and the Rate of Diffusion

Record the distance from each end of the tube to the center of the white ring.

Conclusions

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