Expt 025 -- Densi-Tee
A large cylinder contains water with a layer of coarse granular salt at the bottom and a golf ball resting on top of the salt layer. If left undisturbed, over the course of a few days (..weeks? ...months??) the top of the salt layer gradually descends as the salt dissolves, but the golf ball slowly levitates upward! Aside from the unique visual effect of seeing a golf ball hovering inside a solution, the overall slowness of the whole phenomenon is also quite remarkable.
The golf ball's density is about 1.15 g/mL, between that of regular water (1.00 g/mL) and that of saturated salt solution (1.20 g/mL).
- Whether an object floats or sinks in a fluid depends on whether that object's density is less than or greater than the density of the fluid.
- Most solutes increase a solvent's density as they dissolve into it; thus, salt water is more dense than regular water.
- The unassisted diffusion of a substance through a liquid is rather slow -- compared, for instance, to diffusion through a gas. This is a result of the relatively small spaces between the liquid particles and the subsequently small mean free path of the diffusing particles.
Use ordinary laboratory safety precautions.
- Pour 200 - 300 mL of salt into a 1 liter graduated cylinder, tennis ball container, or other tall thin container. Shake the cylinder to even the salt layer.
- Slowly add water, trickling it down the side of the cylinder so as not to disturb the salt layer. Fill the cylinder in this manner to within 1-2 cm of the top.
- Allow the salt to settle and the solution to clear. Most of the cloudiness is due to small air bubbles.
- !!!Click here to See Movie. Click |> or <| to step the slides forward or back.
- Carefully lower the golf ball below the water surface. Drop the golf ball down on top of the wet salt layer.
- !!!Click here to See Movie.
- Cover the mouth of the cylinder with plastic wrap, secured with a rubber band. (If a tennis ball can is used, simply snap on the plastic lid.)
- With a permanent pen, mark the three levels: the top of the water layer, the top of the golf ball, and the top of the salt layer. Note the top of the salt is not the bottom of the golf ball because the ball settles into the salt when it drops.
- !!!Click here to See Picture.
- Carefully place the set-up in a visible location where it can remain undisturbed for an extended period of time. Show the students the set-up. Ask them to predict what changes will occur to it over the course of time, and how long those changes will take.
- !!!Click here to See Movie. Click |> or <| to step the slides forward or back.
You may wish to set a second golf ball in a 1 liter beaker for comparison.
Large Scale Demonstration:
- Obtain a clear, 6-ft plexiglas pipe (1.75" ID), stopper it securely at one end, add about 30-35 cm of salt, then fill it with water and insert the golf ball. Mount the set-up to the wall.
- Mark the level of the top of the salt layer weekly or monthly. At the same time mark the level of the top of the golf ball.
- Keep a record over a long time.
- !!!Click here to See Movie. Click |> or <| to step the slides forward or back. 25Tall.ASD
- I started the one in the pictures during November of 1992. At the current rate of change, the salt layer will probably finish dissolving around the year 2000, but the ball will continue to ascend (if I've planned it out correctly) to within a few centimeters of the top and then descend back to the bottom -- touching down somewhere between 2030 and 2050!
- Predict what will happen to the salt and the golf ball over time?
- The same experiment is set up with a golf ball in a 1 liter beaker. Predict the differences you would observe. Explain the differences.
- What will happen when all of the salt dissolves?
- Predict the effect of stirring the salt a bit before adding the golf ball.
Name ___________________________ Class _______
BeckerDemos 025 Densi-Tee
Use the movies and pictures to answer the questions.
This experiment may be set up early in the year to see significant changes. Begin by discussing physical properties. Later in the year, discussions of mean free paths in liquids are appropriate. The slow diffusion is a dramatic illustration of the short mean free paths in liquids. When discussing average kinetic energy of molecules, you can contrast the average velocity with the actual movement of the ions in the solution.
Demonstration - Student or Teacher
- The demonstration may be set up by a teacher or as a student project. In order to see any change, it must be set up in the classroom near the beginning of the year and left undisturbed.
- It is suitable as a home project with a tennis ball can or tall narrow jar (Certain pickle or olive jars).
- See cooperative learning suggestions for student projects. Each project may use a different variation selected by the student and approved by the instructor.
When salt and water are used, this experiment requires minimal supervision. If your students are doing projects, preview their proposals for safety and ease of disposal.
Teacher Preparation: 15 minutes
Class Time: 10 minutes of discussion several times during the year.
- coarse grain kosher salt
- a 1-L graduated cylinder or a clear tennis ball can.
- a golf ball
- plastic wrap
- a rubber band
Solution may be disposed of at the sink.
- The diameter of the cylinder has a major effect on the rate of change. A narrow cylinder leaves relatively little room around the sides of the golf ball. Thus there is little surface area over which the saturated salt water can mix with the "fresh" water above. Setting out a beaker for comparison can be illustrative.
- See CoopLearn- below for variations.
- After a week or two, the salt layer drops by about a centimeter due to the gradual dissolving of the salt in the water above it. The golf ball, however, rises slightly - suspended above the salt layer on an invisible layer of dense, saturated salt solution. [If such a levitation device could be utilized on the golf course, it would no doubt be referred to as a "densi-tee!"] The reason for this levitation is simple: the golf ball's density is about 1.15 g/mL, between that of regular water (1.00 g/mL) and that of saturated salt solution (1.20 g/mL). Thus, although the ball sinks in regular water, it floats on the layer of saturated salt water forming below it. As more time passes, the salt level continues to drop, and the golf ball continues to rise. Interestingly, if less salt is used than is required to saturate the entire volume of water, then the golf ball's path through time might be more of a parabolic one: once the salt runs out and the saturated solution below slowly mixes with the rest of the water above, the ball's upward progress should gradually reverse itself and the ball may well end up descending back to the bottom!
- The slowness of the whole phenomenon is quite remarkable. Dissolving and diffusion are both processes which we attribute, at least in part, to molecular movement, and the velocity of the individual particles involved is, we believe, quite great. Yet over the course of an entire week, we are only able to observe changes on the order of magnitude of a few millimeters. These seemingly incongruous ideas can lead to a wonderful discussion on Brownian movement and the concept of mean free path, and to a better appreciation perhaps for the true scale upon which these processes are occurring.
- Q1. Predict what will happen to the salt and the golf ball over time?
- A1. Student predictions vary. Correct prediction: The salt will gradually dissolve. The ball will gradually ascend because it is less dense than the saturated salt solution.
- Q2. The same experiment is set up with a golf ball in a 1-liter beaker. Predict the differences you would observe. Explain the differences.
- A2. The beaker is much wider than the graduated cylinder. Thus there is much more surface area along the sides of the ball to allow mixing of the saturated solution and the water above it. This results in a much faster upward movement of the ball. Whereas, a graduated cylinder generally takes 1-2 years to reach completion, a beaker takes only 1-2 months!
- Q3. What will happen when all of the salt dissolves?
- A3. The ball will continue to rise because the concentrated salt solution will continue to diffuse upward to the level of the ball. The ball will sink to the bottom only if the salt solution's density drops below the density of the golf ball throughout the entire solution.
- Q4. Predict the effect of stirring the salt a bit before adding the golf ball.
- A4. It will be an Instant Densi-Tee. The ball will float on a layer of dense solution when added. The ball will still rise very slowly from the initial position.
- Each student in the class could set up his or her own unique "densi-tee", each focusing on a different parameter: using fine versus coarse salt, the amount of solute used, the type of solute used (sodium iodide, copper sulfate, sugar...), the type of solvent used (alcohol, mineral oil, glycerin...) the dimensions of the cylinder, the type of object used, the omission of the cellophane. The students could then make periodic markings of the different levels, and, at the end of the year, present their findings to the rest of the class, complete with graphs showing the various level changes as a function of time. If tennis ball containers are used, the students can rotate them slightly between each marking; thus the marks form a life-size graph of the experiment. When the experiment is over, the can may be cut open and placed on the overhead as part of the presentation.
- Note that disposal may be a problem with some solutes and solvents. Ask students to present a plan to you. Check before the project is started in the laboratory.
Acknowledgments: The concept for this demonstration first developed during a discussion with some middle school teachers involved in a summer workshop at Sacred Heart University. It was subsequently published in the
South-Western Connecticut Chemmunicator, a newsletter published by Dr. Babu George, Sacred Heart University Chemistry Dept. (Fairfield, CT) Volume 8, #2, Fall, '90.
Key Words 1-
density, mean free path, molecular velocity, velocity