When a piece of foam packaging material seems to vanish into a small pool of clear liquid, the dramatic effect appears almost magical. This rapid, complete disappearance of a solid into a liquid highlights a powerful chemical interaction that can be observed using common household materials. The underlying process is not a chemical reaction that destroys the foam, but rather a physical dissolution and subsequent collapse of a unique material structure. Understanding this phenomenon requires a closer look at the two components involved: the plastic foam and the solvent.
Understanding Polystyrene and Acetone
The foam material, commonly known as Styrofoam, is primarily made of expanded polystyrene (EPS), which is a synthetic polymer. Polystyrene is a long chain of repeating styrene monomer units, which gives the plastic its rigid structure. The foam itself is an engineered material that is incredibly light because its volume is composed of 95% to 98% trapped air within tiny, closed cells.
Acetone, on the other hand, is a simple organic compound and is the smallest of the ketone family. It is a colorless, volatile, and flammable liquid widely used as a solvent in industrial and household products, such as nail polish remover. The acetone molecule is considered polar due to its carbonyl group, but it also contains non-polar hydrocarbon sections, which makes it a highly effective and versatile solvent for many organic materials.
Why the Styrofoam Shrinks So Dramatically
The rapid, dramatic shrinking observed when the foam contacts acetone is a physical process of dissolution, not a chemical breakdown. This interaction is explained by the fundamental principle of solubility: “like dissolves like.” Polystyrene is a non-polar polymer, and acetone’s molecular structure possesses sufficient non-polar characteristics to make it an excellent solvent for polystyrene. The acetone molecules insert themselves between the long, tangled chains of the polystyrene polymer, weakening the intermolecular forces that hold the plastic structure together.
As the acetone penetrates the foam’s structure, the polystyrene chains separate and begin to dissolve, causing the rigid cell walls to soften and collapse. This physical collapse instantly releases the vast quantity of air that was trapped inside the foam’s millions of tiny bubbles. The sudden expulsion of this air, which made up the majority of the foam’s volume, is what creates the illusion that the material has instantaneously vanished. The vigorous bubbling seen during the process is simply this trapped air escaping into the atmosphere.
The foam shrinks because the polymer network can no longer maintain its expanded, low-density state without the support of the solid cell walls. The volume of the foam material decreases by a factor of 50 or more, yet the actual mass of the plastic is conserved entirely.
What Happens to the Polystyrene Mass
The common misconception is that the foam disappears entirely, but in reality, the polystyrene material is simply converted into a different physical state with a much higher density. The long polymer chains are now dissolved, or highly solvated, by the acetone, resulting in a thick, sticky, and highly concentrated solution. This residue is technically a colloidal gel, where the large polystyrene molecules are dispersed throughout the acetone solvent.
The mass of this dense, sticky residue is chemically identical to the mass of the original foam material. The resulting substance is essentially solid polystyrene plastic that has been temporarily liquefied by the solvent. If this dense, sticky mass is left exposed to the air, the volatile acetone will begin to evaporate quickly.
As the acetone evaporates, the thick residue will harden over time, leaving behind a solid, clear, or white block of non-foam polystyrene plastic. This final material is dense, brittle, and significantly smaller than the original foam object. The experiment confirms the law of conservation of mass, demonstrating that the polymer material was never destroyed, only dramatically compacted.
Real-World Uses and Safety Considerations
This property of acetone dissolving polystyrene has practical applications, particularly in recycling. The process can be used for the densification of expanded polystyrene waste, reducing the volume of the bulky foam for more efficient transport and processing. The resulting polystyrene mass can then be repurposed as a raw material for new products, such as plastic coat hangers or building materials. The concentrated solution itself is sometimes used as a powerful adhesive, often referred to as a polymer cement, for bonding other polystyrene materials.
Acetone is a highly flammable liquid with a low flashpoint, meaning it can ignite easily at room temperature. When handling it, adequate ventilation is necessary to prevent the buildup of vapors, which can be irritating or cause drowsiness if inhaled in high concentrations. Direct and prolonged skin contact should be avoided, as acetone can dry out and irritate the skin by removing natural oils. Protective equipment, such as gloves and safety goggles, should be worn to prevent splashes and exposure.