SOLUTION OR NOT?*
MATERIALS: gelatin, table sugar, flashlight, 2 beakers, 2 spoons
PROCEDURE:
1. Nearly fill each beaker with hot water.
2. To the first beaker, add 2 teaspoons of dry gelatin and stir to dissolve.
3. To the second beaker, add 2 teaspoons of sugar and stir to dissolve.
4. Darken the room.
5. Shine a flashlight through each beaker.
6. Record the results.
7. Let the beakers sit until near the end of class.
8. Again darken the room and shine the light through the beakers.
9. Record the results.
ANALYSIS:
1. How did the 2 beakers differ when the light was shined through them?
2. In which liquid were the particles larger? Explain why you think so.
3. In either beaker, did anything settle to the bottom?
4. Use these clues to identify each beaker:
*suspensions have particles larger than 100 nanometers and settle to the bottom,
*colloids have particles between 1 and 100 nanometers in size and the particles do not settle, and
*solutions have particles smaller than 1 nanometer that do not settle to the bottom.
Is the first beaker a suspension, colloid or solution? _____________
Support your answer: ____________________________________
Is the second beaker a suspension, colloid or solution? __________
Support your answer: ____________________________________
Name 5 additional solutions that are part of your everyday life:
Name 5 additional colloids that are part of your everyday life:
ENRICHMENT:
The particles of a colloid are always in constant motion. This Brownian
movement is visible by using a microscope.
Using a light microscope, view a drop of gelatin at low power. Record your observations.
It requires even greater skill to observe the Brownian motion of mayonnaise. Repeat the microscope viewing. Record your results.
What interesting thing do you notice about the color of mayonnaise?
Would it be possible to better observe Brownian motion by adding a stain or dye to the colloid? Why or why not?
*Lab activity from Spectrum Science, 2001, Holt
(Teacher note: Scilinks NSTA; topic: colloids; www.scilinks.org;
keyword: HK1061
The clues for ANALYSIS question 4 are from Compton’s Concise Encyclopedia, 1994. The definitions for these three terms are not precise enough to specifically label the size of the particles involved. There are
photo-physical reasons to argue against the particles being smaller than 100 nanometers and still being able to deflect light.
So, if the concern is precise sizing this lab might be inappropriate; if, however, the concern is relative sizing then it is probably appropriate.
In fact, potting soil in water works well for relative sizing as it serves as a suspension, acolloid source and as a solution.