Soaking a gummy bear in water offers a surprising demonstration of a fundamental concept in science. This familiar kitchen experiment showcases how water interacts with certain materials, causing the candy to swell to several times its original volume and mass. This phenomenon is a direct result of the bear’s unique composition and a passive movement process governed by the concentration differences between the candy and the surrounding liquid. This simple act reveals a powerful mechanism that is also at work within all living cells.
The Unique Structure of the Gummy Bear
The physical properties that allow this swelling to occur are built into the candy’s structure, primarily through its main ingredient, gelatin. Gelatin is a protein derived from collagen that forms long, flexible chains known as a polymer network. This network creates a three-dimensional matrix filled with the other components of the candy.
Before it is soaked, the gummy bear is mostly composed of this dense gelatin matrix, along with a very high concentration of sugar and corn syrup solutes. The gelatin polymer strands are closely packed, but they leave microscopic spaces large enough for small molecules, like water, to pass through. This structural feature means the gelatin acts as a semi-permeable membrane, an enclosure that allows some substances to move freely while trapping others inside. This highly concentrated internal environment is the starting point for the dramatic change.
Osmosis: The Mechanism of Swelling
The movement of water responsible for the size change is called osmosis, which is the net flow of solvent molecules across a semi-permeable membrane. Water naturally moves from an area where its concentration is high to an area where its concentration is lower. In this scenario, the surrounding tap water, containing very few solutes, has a much higher concentration of water molecules than the interior of the candy.
Scientists describe the tap water as a hypotonic solution relative to the inside of the gummy bear. Because the candy’s internal environment is packed with sugar and gelatin—which are too large to escape—the water rushes inward to equalize the concentration gradient. The water molecules enter the microscopic spaces within the gelatin network, causing the polymer chains to separate and the entire structure to expand dramatically in volume. This flow continues until the concentration of water inside the bear approaches equilibrium with the surrounding liquid.
Modifying the Experiment: What Happens in Different Liquids?
The principles of osmosis explain why the swelling effect is dependent on the liquid used, not just the presence of water. If the gummy bear is placed in a liquid that is already highly concentrated with solutes, the results are significantly different. For instance, placing the candy in a strong saltwater solution creates what is known as a hypertonic environment.
In a hypertonic solution, the concentration of solutes outside the gummy bear is greater than the concentration inside. This means the concentration gradient is reversed, and the water molecules trapped within the gelatin matrix move outward into the solution. Instead of swelling, the gummy bear will shrink as it loses internal water content. The degree of shrinking is proportional to the concentration of salt in the solution; a higher salt concentration causes a more pronounced loss of water. This demonstrates that the fate of the gummy bear—whether it swells or shrinks—is determined entirely by the relative balance of water and solutes on either side of its semi-permeable gelatin structure.