Density Reagents The students will be instructed to identify the plastics on the basis of their density. You may provide them with a balance and liquid suitable to determine the density directly from the mass and volume by displacement, or you may provide them with a set of differential density materials. If you use the direct method of mass and volume by displacement, you must provide a liquid with a density of less than .917. This generally means a volatile organic solvent. Since small tea candles will most likely be in use, this is not the preferred method. The preferred method is to provide them with a series of differential density liquids. A common set include: Liquid Density 1 Corn Oil .917 2 46% Isopropyl Alcohol .950 3 Distilled Water 1 4 10% NaCl 1.07 5 25% NaCl 1.19 6 Saturated NaCl 1.25 The Corn Oil is poured straight out of the container The 46% Isopropyl Alcohol is made by mixing 66 ml of 70% isopropyl alcohol with 24 ml of distilled water for every 100 ml of solution needed The distilled water is straight The 10% NaCl is mixed by adding 10 g of sodium chloride to 100 ml of distilled water. The 25% NaCl is mixed by adding 25 grams of sodium chloride to 100 ml of distilled water. The saturated NaCl is 5.4 M NaCl. The solution is mixed by adding salt to distilled water until no more will dissolve. This should be about 32 grams of salt for every 100 ml of solution. If you also add a drop of different color food coloring to the various solutions except the corn oil, it is quick and easy to see if the detectives have contaminated them. It is important to only add 1 drop of food coloring to every several hundred milliliters of solution so that you do not alter the density of the material significantly. It is handy to put the 6 liquids in 2 oz Solo cups with lids and then put the 6 cups that form 1 set into an egg carton that has been cut in half. Thus you can distribute the closed half egg cartons quickly and easily. The egg cartons will contain any mess and help prevent tip-overs. It is important to tell students to put the plastics directly into the cups and not pour the liquids out into something else. It is also important to tell the students to remove the plastics from the cups after they have determined if the plastic sinks or floats in the liquid.