A density column consists of layers of liquids of different densities which do not mix with each other, and which are clearly distinguishable from each other. The highest density material is at the bottom of the column, and as you proceed up the column, the density of each successive layer decreases, with the lowest density layer on top. Three methods for construction of a five-layer density column are discussed in this write-up. All methods use the same five liquids. • Method 1 gives the names of the liquids and the order for adding them, and its goal is to directly construct the density column without any experimentation. • Method 2 assumes names and densities of the five materials to be unknown, and involves open-ended experimenting, with few instructions, to construct the column. • Method 3 also assumes names and densities of the materials to be unknown, and outlines a systematic way to approach the experimentation involved in constructing the column.
Obtain a clear plastic container whose size meets your needs, and carefully add appropriate amounts of the following materials in the order specified (the first material listed is the most dense, and should be added first: water -- color with an appropriate amount of food color before pouring canola oil -- don't add food color (it won't dissolve in oil) 60% isopropyl alcohol -- you can buy 70% isopropyl at drug stores and grocery stores -- it's used as rubbing alcohol and disinfectant -- you then have to dilute it to make it 60% -- add 2 ml of water to every 10 ml of alcohol, or 20 ml of water to every 100 ml of alcohol -- before pouring, color with an appropriate amount of food color different from the first one mineral oil -- baby oil is mineral oil, and can be used here, but ordinary mineral oil is easily obtained at pharmacies and is cheaper -- don't add food color (it won't dissolve in oil) 91% isopropyl alcohol -- you can buy 91% isopropyl at most drug stores, and sometimes at grocery stores -- it's used as a disinfectant, particularly for pierced ears -- before pouring, color with an appropriate amount of food color different from the previous two Helpful Hints: • Layers should probably be a minimum of about 1/2 inch thick, and pouring should be gentle, to avoid a layer falling through the layer it is being poured on and interacting with the layer below that one. If the container is small, consider using a pipette or eyedropper. This is easier than pouring, and allows you to add the liquids very slowly and gently to prevent unwanted mixing. • Sometimes the material the container is made of plays a role in how well the layers form, possibly due to surface tension effects. • Try tilting the container a little so that the liquid you are adding runs down the side more slowly. Or try laying the new liquid very gently on the previous layer by having the dropper tip just barely above the liquid surface, so that the new liquid doesn't fall and hit the surface hard. • Don’t use too much food coloring – depending on the amount of liquid you are using, even one drop of food color may be enough. If the color is too dark, you can't see through it, and it's not as effective.
Treat the five materials as unknowns. Play around with them (i.e., experiment!) to determine which ones will allow food color dissolve in them, and the relative order of their densities. Use your results to construct the density column.
Five-Layer Density Column.....9/29/05
© 2005 Exploratorium, www.exploratorium.edu
Don Rathjen....Exploratorium Teacher Institute....3601 Lyon St., San Francisco, CA 94123...firstname.lastname@example.org
Students receive liquids in containers labeled A, B, C, D and E. Neither the identity or density of any of the materials is given. Students also receive a copy of the Data Table below (or have them draw their own if desired). This is what students will fill in as they experiment. (NOTE FOR TEACHERS: In...