The recent round of outreach has been making density bottles, kids love them and they are very relaxing to watch. The original idea was taken from a paper in The Journal of Chemical Education. (J. Chem. Educ. 2015, 92, 9, 1503–1506, https://doi.org/10.1021/ed500830w)
The top liquid is iso-propanol and the bottom one is a saturated solution of salt. The liquids mix when you shake them but just like oil and water will separate out when you leave them to stand. The beads in the bottle are made out of two plastics, their density is greater than the iso-propanol but less then the salt water so they float in the middle of the bottle. When the bottle is shaken the density of the mixture is between the density of iso-propanol and salt water and one set of the plastic beads are less dense than the mixture and the other set is more dense so they separate out, slowly coming back together as the mixture separates back into it’s components.
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