This activity is always a hit with the students, and I love it, too. It is an excellent demonstration of the density of liquids and how density affects buoyancy. Students will calculate the densities of five liquids and then hypothesize what will happen when they are poured together and then what will happen when they are mixed. This activity uses readily-available liquids and a couple drops of food coloring to distinguish the liquids.
To begin, explain that you will be investigating various liquids and making hypotheses about them. Students will investigate the properties of the liquids and record their findings prior to calculating their densities. Discuss how scientists describe things using words with specific meanings.
For the next step, students will need to know something about density. This activity can be used at any time, but it does help to have some understanding of density going into it, or plan to spend some time explaining during this activity. To understand density, students will need to know about volume and mass (and mass versus weight). For this activity, they will need to be able to measure water using a graduated cylinder and weigh items using a scale.
Students will need to be able to measure their unknown liquid up to a line on a plastic cup. Working over plastic trays is a good idea. Students will need to be able to weigh liquids and subtract the weight of the cup using the “tare” feature on the scale or by subtracting. One way to mark a cup with a line at 100ml is to have students fill a cup by weight based on the fact that one gram of water has a volume of one milliliter. Alternatively, you can have the students measure 100ml of water in a graduated cylinder and pour this into a cup. Once they have 100ml of water in a cup, they should mark the cup with a marker so that they can now use the cup to measure 100ml of their unknown liquid.
After presenting their findings on each liquid, students will formulate a hypothesis about what will happen when the liquids are poured together. Then they will hypothesize about what will happen if the poured liquids are stirred.
To get the liquids to stratify by density as in the photo, it is best to pour them into the beaker by density, starting with the corn syrup. It helps to pour each successive liquid onto a spoon just above the surface of the previous liquid to keep it from plunging down into the layers below and mixing. It is fine to pour the liquids more vigorously to see what will happen, but some of the layers will mix.
For this activity, you will need at least 250ml each of water, isopropyl alcohol, liquid dish detergent, corn syrup, and vegetable oil. You will need scales that can measure to the nearest gram, clear plastic cups, two 600ml glass beakers (or similarly sized glass containers), and graduated cylinders. You will need two colors of food coloring to help distinguish the three clear liquids (water, alcohol, and corn syrup). In one photo the alcohol is red and the water blue, with orange dish soap. In another the dish soap is blue, the alcohol is green, and the water is red.
It is interesting to pour less dense liquids first and watch the denser liquids pass through them and layer by density. The oil will not mix with any of the other layers. After thorough mixing, the oil will come to the top, and the rest of the water-soluble liquids will form a single layer below the oil. The water and alcohol will mix easily. The other layers are miscible with water, but may need to be stirred a good deal to mix completely. If you want to keep the water and alcohol separate, pour the oil in before the alcohol, and the oil will separate the water and alcohol.