The straightforward answer is no; common plastics do not chemically dissolve in water under typical environmental conditions. Plastic is a collective term for a wide range of polymers, which are large molecules built from many repeating smaller units. The nature of these molecules is the reason they are so stable and resistant to water, making plastic waste a significant environmental concern.
The Chemistry of Solubility
The ability of one substance to dissolve in another depends on solubility, which is governed by molecular forces. The guiding principle in chemistry is “like dissolves like,” meaning that substances with similar molecular polarity tend to mix and form a homogeneous solution. Polarity refers to the distribution of electrical charge across a molecule. Water (H2O) is a highly polar solvent because the oxygen atom pulls electrons away from the two hydrogen atoms, creating an unequal charge distribution. This allows water molecules to form strong attractive forces, specifically hydrogen bonds, with other polar molecules or charged ions. When a substance dissolves, the solvent molecules surround the solute, pulling them apart into the solution. Polar solvents like water are excellent at dissolving other polar substances, such as salt or sugar, but not non-polar ones.
Why Plastic Polymers Resist Water
Plastic’s resistance to water is a direct consequence of its non-polar nature. Most common plastics, such as polyethylene (PE) and polypropylene (PP), are composed of extremely long polymer chains. These chains primarily consist of carbon-carbon and carbon-hydrogen bonds, which share electrons relatively equally, resulting in overwhelmingly non-polar molecules. The non-polar plastic molecules lack the partial electrical charges necessary to interact favorably with the highly polar water molecules. Water molecules prefer to bond strongly with each other through hydrogen bonds rather than engage with the non-polar plastic. This incompatibility prevents the chemical process of dissolution, keeping the plastic intact even after prolonged submersion.
The Difference Between Dissolution and Fragmentation
The misconception that plastic dissolves in water often arises because plastic debris seems to disappear or diminish over time in the environment. This disappearance is not a chemical process of dissolution, but rather a physical process called fragmentation. Dissolution is the chemical breakdown where the solute’s molecules are completely separated and mixed uniformly with the solvent to form a true solution. Fragmentation, in contrast, is the physical breaking apart of a material into smaller pieces. Environmental factors are responsible for this physical breakdown. Sunlight, specifically ultraviolet (UV) radiation, initiates a process called photodegradation, which attacks and weakens the chemical bonds within the polymer chains. Mechanical abrasion from waves, wind, and sand then physically grinds the larger plastic items into progressively smaller fragments. This process continues until the plastic is reduced to microplastics, which are pieces smaller than five millimeters, and eventually to nanoplastics. These microplastics persist in the environment, accumulating in ecosystems and posing a different kind of threat than dissolved pollutants, as they retain the original polymer structure.