Diffusion of a Solid Through A Liquid

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