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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.
Methods (continued):
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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Science Education Connection Department of Biochemistry The University of Arizona Wednesday, February 12, 1997 johnmcc@dakotacom.net
http://biology.arizona.edu/sciconn/lessons/mccandless/ |
