Look through the plastic items in your recycling bin and identify the recycling code found on each plastic. You should see one of the following symbols, although in some cases the alphabet code might not be included:

Each of these plastics is made out of different chemicals and, because they are different, they have different properties and different uses. HDPE (#2) is a rigid polymer and is good for holding liquids, whereas LDPE (#4) is flexible and used to make trash bags. 

Materials

• Food coloring
• Isopropyl “rubbing” alcohol (70%)
• Salt (Kosher or canning salt is preferable)
• Sturdy, transparent bottle with cap
• Variety of plastics cut into small pieces that will fit into the bottle
• Water

Be Safe: Rubbing alcohol is flammable and may cause serious eye irritation. Getting alcohol on a cut or open sore may also sting. Adult supervision is recommended. 

Experiment

Half-fill the bottle with isopropyl alcohol and add a drop of food coloring. Add water until the bottle is about ¾ full. Cap and shake the bottle and observe. Now add a heaping tablespoon of salt to the bottle and shake. The salt will most likely completely dissolve and so you will want to continue adding salt and shaking the solution until the salt just barely dissolves. (Patience needed!) What happens to the solution as more salt is added? Add different pieces of plastic to the bottle, shake it, and let it rest. Observe: Where are the plastics?

What’s happening?

Initially the two liquids are miscible (dissolving completely in each other regardless of the ratio). However, when salt is added to the point of a saturated salt solution (no more salt will dissolve), the liquid separates into two layers, with the bottom liquid layer being more highly colored.

Because water is more polar than isopropyl alcohol, it will tend to surround the charged sodium and chloride ions present in salt and the resulting saturated salt solution separates from the isopropyl alcohol. The result is two liquid layers, with salt water on the bottom and isopropyl alcohol on the top.

Plastic pieces added to the bottle will appear to float (or settle) in any number of places in and between the two liquid layers. This is because one of the properties that is unique for different plastics is their density. The least dense plastics will tend to float at the interface between the salt water and isopropyl alcohol layers. The most dense plastics may settle at the bottom of the bottle. See the table below for typical densities

*https://plasticoceans.org/7-types-of-plastic/
**This is the density of pure isopropyl alcohol.
***Saturated solution—no more salt will dissolve.

When mixed plastics from a recycling bin are brought to a recycling facility, one way in which facilities separate them is by their density. This gets the plastics that are made of the same compounds together.

References

https://www.youtube.com/watch?v=wEG_JfLepU8

https://pubs.acs.org/doi/10.1021/ed500830w

To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.

- PAUL BRANDT