When submerged in liquid, a gummy bear undergoes a dramatic size change. While the candy swells in plain water, the opposite effect occurs in salt water. Soaking a gummy bear in a concentrated salt solution causes it to visibly shrink, becoming smaller and denser than its original state. This counter-intuitive result illustrates a fundamental process governing the movement of water across boundaries, as the gummy bear’s structure interacts with the high concentration of salt, forcing water out of the candy.
The Gummy Bear’s Gelatin Matrix
A gummy bear is categorized as an edible hydrogel, a material network designed to hold water. This structure is primarily composed of gelatin, a protein derived from collagen. The long strands of gelatin molecules interlace and form a three-dimensional polymer network, similar to a microscopic sponge. This intricate structure gives the candy its characteristic chewy texture and allows it to maintain its shape.
The gelatin matrix holds a significant amount of water and dissolved sugar incorporated during manufacturing. The network functions like a natural semi-permeable boundary, allowing selective permeability. Small molecules, such as water, can easily pass through the tiny spaces in the protein mesh, but the larger gelatin and sugar molecules are trapped inside.
How Water Moves Across Membranes
The process that governs the size change is called osmosis, the movement of water across a selectively permeable boundary. This movement is driven by a difference in concentration, known as a concentration gradient. Water molecules naturally migrate from an area where they are highly concentrated to an area where they are less concentrated. This tendency is a continuous effort to equalize the distribution of dissolved substances, or solutes, on both sides of the boundary.
Imagine a room separated by a screen door, where only small people (water molecules) can pass through, but large people (solute molecules) cannot. If one side of the room is packed with large people, the small people on the other side will rush over to try and spread the large people out. In the context of the gummy bear, the water moves to where the solute concentration is highest, which is where the concentration of free water molecules is lowest.
The Pulling Force of Salt Water
When a gummy bear is submerged in salt water, the surrounding solution has an extremely high solute concentration. Scientists refer to this highly concentrated solution as a hypertonic environment. This means the water outside the bear contains far more dissolved salt than the water trapped inside the gelatin matrix. Consequently, the concentration of free water molecules is lower in the salt solution than it is within the gummy bear.
The osmotic principle dictates that water must move to the area of lower water concentration, which is the hypertonic salt solution outside the bear. Water molecules trapped within the gelatin matrix are pulled out of the candy in an attempt to dilute the surrounding salt water and establish equilibrium. As the gummy bear loses its internal water content, the gelatin network collapses slightly, causing a noticeable reduction in the bear’s size and mass. The extent of this shrinkage is directly related to the concentration of salt.