Yes, some types of plastic can float on water, while others sink.
The ability of plastic (or any material) to float or sink in water depends on its density compared to the density of the water it's in. As stated in the reference: "If plastic is more dense than sea water, it will sink. If it's less dense, it will float." This principle applies to both freshwater and saltwater, although saltwater is slightly denser than freshwater.
Understanding Density and Buoyancy
Density is a measure of how much mass is contained within a given volume. Water has a certain density (freshwater is approximately 1,000 kg/m³, and seawater is slightly higher, around 1,025 kg/m³).
- If a plastic object has a density less than the water's density, it is lighter for its size than the water it displaces, and the buoyant force pushes it upwards, causing it to float.
- If a plastic object has a density greater than the water's density, it is heavier for its size than the water it displaces, and gravity pulls it down, causing it to sink.
- If the density is equal, it might remain suspended within the water.
Different Plastics, Different Densities
Plastic is not a single material; it's a broad category encompassing many different polymers. Each type of plastic has its own unique properties, including density. This is why you see some plastic items floating in the ocean or a lake, while others settle on the bottom.
Here's a look at the densities of some common plastics relative to water:
Common Plastics and Their Behavior in Water
| Plastic Type | Abbreviation | Approximate Density (g/cm³) | Floats or Sinks in Freshwater (~1.0 g/cm³) | Floats or Sinks in Seawater (~1.025 g/cm³) | Common Uses |
|---|---|---|---|---|---|
| Polyethylene (Low-Density) | LDPE | 0.91 – 0.93 | Floats | Floats | Plastic bags, film, flexible bottles |
| Polyethylene (High-Density) | HDPE | 0.94 – 0.97 | Floats | Floats | Bottles (milk jugs, detergent), pipes, toys |
| Polypropylene | PP | 0.90 – 0.91 | Floats | Floats | Containers, bottle caps, fibers, car parts |
| Polystyrene (Expanded - e.g., Styrofoam) | EPS | 0.01 – 0.05 | Floats (very easily) | Floats (very easily) | Packaging foam, disposable cups |
| Polystyrene (General Purpose) | GPPS | 1.04 – 1.09 | Sinks | Sinks | Disposable cutlery, CD cases |
| Polyvinyl Chloride | PVC | 1.30 – 1.45 | Sinks | Sinks | Pipes, window frames, flooring |
| Polyethylene Terephthalate | PET | 1.38 – 1.39 | Sinks | Sinks | Beverage bottles, clothing fibers |
(Note: Densities can vary slightly depending on additives and manufacturing processes.)
Practical Observations
You can often observe this difference in your daily life:
- Most plastic bottles made of PET will sink if filled with liquid or if they lose their cap and fill with water. Empty PET bottles with trapped air will float initially.
- Plastic bags (LDPE) and milk jugs (HDPE) almost always float.
- Styrofoam (EPS) floats very high out of the water due to its extremely low density.
In summary, whether a piece of plastic floats depends directly on its specific type and thus its density relative to the density of the water it encounters.
