Another group of student who is entering the same competition are doing another experiment. Their experiment is based on the different density of the solid and liquid. We really have a difficult time to find the appropriate plastic solid that has the correct density.
In the tube shown in the video, it contain 2 solutions of different density. One is acetone which has a density of 0.81g/cm3, another is saturated salt solution (normal household salt) which has a density of 1.2g/cm3. Both solution are immiscible so they will separate when mixed.
The theory is that you can use a plastic that has a lower density and another with a higher density than the solution when mixed. Example now both solution when mixed and divide by 2, it should gives a density of 1. So if you have a plastic material density that is between 1.1 - 1.9g/cm3, it should sink when mixed. Then if you have another plastic material density that is between 0.82 - 0.99g/cm3, it should float when mixed.
So when you leave it to stand and allow these 2 solutions to separate back to their normal properties. The one with the density of 1.1-1.9g/cm3 (which is the white plastic in the video) will start to float since it is less dense than salt water. Then the one with the density of 0.82-0.99g/cm3 (the green plastic in the video) will start to sink since it is denser than acetone but less dense than salt water. Then the plastic that has a density of around 1g/cm3 (the blue solid) will float in between when mixed.
End up what you can observed is the green plastic float when the solution is mixed then sink down, the white plastic will sink when mixed and then float up. The blue plastic will remain in the middle.
Honestly, I am not sure what kind of plastic I am using but I can let you know where my material come from. The green one is from the seal of a chemical bottle, the blue one is from a mineral bottle cap and the white one is from those white colour disposable spoon (see pic).
You can try it out yourself. Actually in the link, they uses rubbing alcohol (Isopropyl alcohol). I bought a 70% isopropyl alcohol from Guardian however it is miscible with salt water so both solution cannot separate when it is mixed (same for 95% ethanol). So I do not know how the kit that they sold works. If you ever tried it before please tell me the procedure for that.
Hope you will like my sharing if you really found out that iso propyl alcohol and salt water do not separate.
