Soaking a gummy bear in plain water causes it to swell considerably, often doubling or tripling its original volume. When the surrounding liquid is salty, however, the outcome shifts unexpectedly, causing the bear to shrivel. This simple kitchen experiment provides a clear, visual model for osmosis, a fundamental biological process involving how water moves across boundaries.
The Gummy Bear as a Model
A typical gummy bear is composed mainly of sugar, flavorings, and gelatin. When mixed with water and cooled, gelatin forms a complex, three-dimensional network of intertwined protein strands. This mesh-like structure provides the candy’s signature chewy, elastic texture.
The gelatin network acts as a polymer matrix that is selectively permeable, meaning it lets some substances pass through while blocking others. Small molecules, most notably water, can easily move through the tiny spaces within the mesh. Larger molecules, like the bear’s sugar and the gelatin chains, are trapped inside the structure. This quality makes the gummy bear an excellent non-living demonstration of a semi-permeable membrane, similar to those found in living cells.
The Physics of Water Movement
The movement of water across a semi-permeable boundary is governed by osmosis. Osmosis is the passive diffusion of water molecules from a region where they are highly concentrated to a region where they are less concentrated. This difference in concentration between two areas separated by a membrane is known as the concentration gradient.
Water molecules move down this concentration gradient in an attempt to reach equilibrium, where the concentration of dissolved substances is equal on both sides. This means water moves toward the side that has a higher concentration of solute, or dissolved particles. The solute concentration dictates the direction of water flow.
For example, if a membrane separates pure water and sugar water, the water will move into the sugar water because the pure water has a higher concentration of water molecules. Water movement continues until the concentration is balanced across the membrane.
The Salt Water Effect
When a gummy bear is submerged in salt water, osmosis is immediately put into action. The salt water solution is considered hypertonic, meaning it has a much higher concentration of dissolved particles, or solutes, than the inside of the gummy bear. Consequently, the water concentration is lower outside the bear than it is inside the gelatin matrix.
Because water moves from an area of higher concentration to lower concentration, it rushes out of the gummy bear and into the salt solution. This outward flow attempts to dilute the highly concentrated salt water surrounding it. As water exits the gelatin matrix, the candy loses its internal volume. This loss of water causes the gummy bear to visibly shrink and shrivel, often to a size smaller than its original form.