The simple act of placing a gummy bear in water and watching it swell is a captivating demonstration. This transformation is not magic, but a clear illustration of a fundamental biological and chemical process. The candy’s expansion offers a tangible look at how materials interact with their liquid environment at a molecular level. This common experiment helps understand the movement of water across certain barriers.
The Physical Structure of Gummy Bears
Gummy bears owe their characteristic chewiness and ability to swell to their main structural ingredient, gelatin. Gelatin is a protein derived from collagen that functions as a polymer—a large molecule made up of repeating units. During manufacturing, gelatin is dissolved in a hot mixture of water, sugar, and corn syrup.
As this mixture cools, the long protein strands of the gelatin intertwine to form a complex three-dimensional mesh or network. This mesh-like structure traps the sugar, corn syrup, and initial water content within its framework, creating a hydrocolloid gel. The resulting candy is a stable, semi-solid material that retains its shape, despite its high sugar and water content.
Crucially, the gelatin network acts as a selective barrier, or a semi-permeable membrane. This structure allows small molecules, like water, to pass through the gaps, but prevents larger molecules, such as the sugar and gelatin itself, from easily escaping.
The Mechanics of Osmosis
The dramatic swelling observed when a gummy bear is submerged is driven by osmosis. Osmosis is the net movement of solvent molecules, water, through a semi-permeable membrane. This movement occurs from an area of high solvent concentration to an area where it is lower. The goal is to equalize the concentration of solutes, or dissolved substances, on both sides of the barrier.
When a gummy bear is placed in plain water, the external water has a very low concentration of dissolved solutes. The inside of the gummy bear is a highly concentrated solution of sugar and gelatin trapped within the network. This substantial difference creates a steep concentration gradient across the candy’s gelatin barrier. To dilute the concentrated internal solution and achieve equilibrium, water molecules rush inward through the semi-permeable matrix.
This influx of water forces the flexible gelatin network to stretch and expand, causing the gummy bear to visibly grow. The process continues until the concentration of solutes inside the bear is significantly diluted. Alternatively, expansion stops when the physical resistance of the stretched gelatin network prevents further movement. The gummy bear acts as a macroscopic example of the principles that govern water absorption in biological systems.
Why Different Liquids Yield Different Results
The liquid surrounding the gummy bear determines the direction and extent of water movement by manipulating the concentration gradient. When the candy is placed in plain water, the external liquid is considered hypotonic, meaning it has a lower solute concentration than the inside of the bear. This condition results in the maximum possible concentration gradient, leading to the largest and fastest swelling as water molecules flood inward to dilute the internal sugars.
When the gummy bear is placed in a solution with a high concentration of dissolved sugar, the results are different. If the sugar water is only slightly concentrated, the difference in solute concentration is less pronounced. This reduced gradient slows the rate of water absorption and limits the final size.
If the external sugar water is highly concentrated, the environment becomes hypertonic, possessing a higher solute concentration than the candy’s interior. In a hypertonic solution, the concentration gradient is reversed entirely. The net movement of water is actually out of the gummy bear, attempting to dilute the highly concentrated external solution. This outward flow prevents swelling or can even cause the gummy bear to shrink slightly.