Materials needed: low power laser pointer, beakers labeled A through E, distilled water, milk (not skim), gelatin, NaCl, dirt
Preparation: Half-fill the five beakers with water. Fill the remainder of beaker A with water, the remainder of beaker B with milk, add a tablespoon of NaCl to beaker C, add a tablespoon of gelatin to beaker D, and finally add a tablespoon of dirt to beaker E. Mix all five beakers thoroughly.
Safety: Even low power lasers can be dangerous. The laser should be handled with care and should be aimed away from everyone's eyes.
Procedure: Turn off the lights and make the room as dark as possible. Shine the laser through each of the five prepared beakers. Write down observations.
To Think About: Which of the test tubes scattered the laser beam? What does this mean? Can this help determine what is a solution and what is a suspension? Why is it better to use whole milk for this experiment rather than skim, or fat-free, milk?
Results: Homogeneous solutions have small particle sizes and do not separate upon standing or filtration. These solutions (A and C) will not scatter the light. The remaining solutions (B, D, and E) are all heterogeneous consisting of solid particles and liquid solution. The presence of solid particles scatters the laser beam (Tyndall Effect).
Need Some Help?
If you ignore the cheesy background music, this video offers a nice visual demonstration about the differences between solutions, suspensions, and colloids. The music alone is enough to watch the whole thing.
This clip is very similar to the lab you just completed. Check it out.
Materials Needed: three zip lock bags, distilled water, salt, sugar, and a freezer.
Preparation: Label each bag (A through C) and fill with 1/4 cup water. To bag A add one teaspoon salt. To bag B add one teaspoon sugar. Mix the bags until all three are homogeneous solutions.
Procedure: Place all three bags on an empty shelf in the freezer. Record the time and observations of the contents every 10 minutes.
To Think About: Why would the same quantity of water take a different amount of time to freeze? Do salt and sugar have the same effect even though they are chemically different?
Results: Bag A (pure water) will freeze the fastest because its freezing point is 0ºC. The water in bags B and C have a lower freezing point because of the added solute particles (the salt and the sugar). Remember particles are the building blocks of matter. Particles can be atoms, molecules, or ions. In this lab, the particles in the salt solution are Na+ and Cl- ions, and the particles in the sugar solution are glucose molecules. There are more particles in a teaspoon of salt than in a teaspoon of sugar. Colligative properties depend only on the quantity of particles not the identity of the particles. Bag B might freeze slower than bag C but it might not be observable under these conditions.
Can't Get Enough of Freezing Point Depression?
This lab is very similar to the one described above, except they use fancy temperature probes and computers.
If you still can't get enough here is a longer video that will give a more in depth explanation about freezing point depression.
Materials Needed: four beakers, distilled water, cold water, room temperature water, hot water, corn syrup, food coloring
Preparation: Label the four beakers A through D. Half-fill beaker A with room temperature water, beaker B with cold water, beaker C with hot water, and beaker D with corn syrup.
Procedure: To each beaker add several drops of food coloring. Observe how the food coloring diffuses through the solution in all beakers. Write down observations.
To Think About: What is the difference between hot, cold, and room temperature water? How is the viscosity of corn syrup different than water?
Results: Particles in hot water have more kinetic energy than particles in room temperature water which means they move around faster in solution. As a consequence the food coloring will spread throughout the beaker more quickly. Beaker C will have the fastest rate of diffusion, followed by beaker B, then beaker C. Beaker D will have the slowest rate of diffusion due to the high viscosity of corn syrup. In a nutshell, the particles in corn syrup are more compact and stuck together causing the liquid to be thicker. This thickness causes a slow rate of diffusion.
Now Some Videos:
Here is a quick visual demonstration of the above experiment.
Check out this fun video. And they give a nice visual of the basic principles of the experiment, complete with sweet tunes.
The rate of diffusion is directly proportional to the temperature. It's all explained here.