Authored by Nick Thomas
Chemistry Department, Auburn University at Montgomery, Montgomery, AL
Demonstrating the densities of various liquids (oils, alcohol, detergent, syrup, etc.) in a tall glass container is a popular at-home science activity described on many websites.1-3 Colored sugar solutions of varying concentrations can similarly be layered to illustrate density and generally require the lower density solutions to be carefully poured onto the higher density solutions to minimize mixing the miscible layers4-7 although this is generally unavoidable. While solutions are usually prepared individually, in kitchen cups or glasses, with little graduated accuracy using tablespoon quantities of sugar,4-6 more careful preparations have been reported.7 The procedure described here uses accurate concentrations.
This article also describes a convenient procedure for filling a sugar density column using a slightly modified thistle funnel to fill a large graduated cylinder in reverse density order to produce remarkably distinct layers for an attractive classroom display of density (see Video 1). The sugar solutions are introduced sequentially into the cylinder via the funnel from the least to the greatest density. However, the funnel’s stem is generally too wide leading to rapid drainage and the partial mixing of layers as the cylinder fills. To narrow the funnel’s stem opening, a Pasteur pipet is attached via a short piece of rubber tubing producing a near perfect separation of colored layers.
Video 1: Density Column, Online Chemistry YouTube Channel (accessed 3/22/21)
Densities of each solution were measured which students may calculate using data provided in table 1 below. Students can use data to practice molarity and dilution calculations in the classroom.
A 500 cm3 graduated cylinder requires about 5-10 minutes to fill by the procedure described. This may be demonstrated during the class or, alternatively, could be filled prior to class and if carefully carried may be moved to the classroom without disturbing the layers.
100 cm3 and 500 cm3 graduated cylinders
Food colorings (6 colors, purple, blue, green, yellow, orange, red)
Thistle funnel with 30-cm stem (available from chemical supply companies and relatively inexpensive)
Disposable Pasteur pipet, 14 cm length, attached to a small (2-3 cm) length of rubber tubing to fit thistle funnel stem
Six beakers (150 to 200 cm3)
Place the six colored solutions next to the 500 cm3 graduated cylinder containing the adapted thistle funnel. Begin by pouring the red (least concentrated) solution into the funnel reservoir. As it empties into the graduated cylinder, top up the reservoir. Quickly add the next (orange) solution just before the funnel reservoir empties to avoid air bubbles forming in the funnel stem. Continue with successive solutions until the purple solution has drained from the funnel (500 cm3 graduated cylinders actually hold about 600 cm3). The total time to fill the cylinder will be 5-10 min, depending on the width of the pipet tip.
To remove the thistle funnel, first place a stopper tightly in the funnel to prevent any solution remaining in the stem from draining out when the funnel is removed. Slowly lift the funnel from the cylinder. If this is done carefully, there will be no mixing of the solutions when the funnel stem is withdrawn.
Safety Notes: Sugar solutions may be discarded down a sink. When breaking the pipet tip, hold it between a cloth or wear thick gloves.
- While Pasteur pipets taper to a narrow opening that will slow down the rate of graduated cylinder filling, this may be too slow to demonstrate during a class. To speed up the process and still retain a good separation of solution layers, the pipet should be cut about 2-3 cm from the tip by scoring with a triangular file and snapping off the lower piece of the pipet. Teachers should test the filling process with water before class so the cylinder will fill at a rate of about 75 -100 cm3 per minute. Note that the slower the filling rate, the better the layer separation will be.
- Commercial orange food coloring may look too yellow or red, so the orange color is best prepared by mixing a drop or two of yellow and red.
- A larger (1 liter) graduated cylinder may be used for an even more impressive display, but the quantities of solutions will need to be doubled.
- While the described demonstration may merely serve to illustrate density to younger students, a more quantitative approach could be used with older students when introducing molarity and dilution calculations. Teachers could display the column and invite students to calculate the molarities of each sugar solution layer using appropriate dilution factors or M1V1 = M2V2. Molarities are shown in the Table, along with densities calculated from the masses of 15.0 cm3 of each solution. If provided with masses and volumes, students at various levels could also calculate the densities of each layer during the demonstration.
- The column layers will remain separated for several weeks, but through osmosis will gradually merge. The cylinder may be left undisturbed over that period in a classroom or lab so students can observe the process occurring.
Attach a shortened Pasteur pipet (see Teacher Notes) to the stem of the thistle funnel with the piece of rubber tubing. Place the adapted funnel inside the graduated cylinder with the reservoir protruding from the top of the cylinder and the pipet tip resting on the bottom of the cylinder. No clamping is necessary.
For the six solutions, prepare a 2.0 M sugar solution by dissolved 274 g of sugar in 400 cm3 tap water in a 1 L beaker. The solution is stirred and heated at 40°C until clear, forming approx. 600 cm3 of solution, then cooled to room temperature. 100 cm3 of the concentrated solution is transferred with a 100 cm3 graduated cylinder to a beaker containing 1-2 drops of purple food coloring.
Next, 80.0 cm3 of the 2.0 M solution is added to the 100 cm3 graduated cylinder and diluted to 100 cm3 with tap water. That solution is placed in a second beaker with 1-2 drops of blue food coloring. The 2.0 M solution is diluted three more times into separate beakers by the amounts shown in the Table with different food colorings added. A sixth beaker containing 100 cm3 of tap water with red food coloring is also prepared. Dilutions, molarities, and densities of all solutions are shown in the Table 1.
Table 1: Dilutions for sugar solutions with molarity and density values
1. Steve Spangler Science, 2019. https://www.stevespanglerscience.com/lab/experiments/seven-layer-density... (accessed March 2021).
2. Helmenstine, A. M. ThoughtCo., 2020. https://www.thoughtco.com/make-a-density-column-604162 (accessed March 2021).
3. Krampf, R. The Happy Scientist, 2019. https://thehappyscientist.com/content/making-density-column (accessed March 2021).
4. The Kitchen Pantry Scientist, 2011. https://kitchenpantryscientist.com/sugar-water-density-columns (accessed March 2021).
5. Steve Spangler Science, 2019. https://www.stevespanglerscience.com/lab/experiments/colorful-sugar-dens... (accessed March 2021).
6. Haley, H. Carolina Biological Supply Company, 2017. https://www.carolina.com/teacher-resources/Interactive/sweet-and-colorfu... (accessed March 2021).
7. Davis, M.; Henry, C. The Sweeter Side of Density. J. Chem. Educ. 2008, 85 (8), 1088A-1088B.
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies