Making rock candy demonstrates the transformation of table sugar into translucent crystals. This process often raises the question of whether it involves a chemical change, which rearranges molecules, or a physical change, which only shifts the substance’s form. To answer this, it is necessary to understand how scientists categorize changes in matter.
Physical Changes Versus Chemical Changes
A physical change alters a substance’s appearance or state, such as size, shape, or phase, but does not change its fundamental chemical composition. Melting an ice cube into liquid water is a physical change because the substance remains water (\(\text{H}_{2}\text{O}\)), only in a different state. These changes affect only the physical properties of the material and are often easy to reverse.
A chemical change, by contrast, is a reaction that forms a completely new substance with different properties than the original material. This change involves the breaking and forming of molecular bonds, leading to a new chemical identity. Burning wood is a classic example, as the wood is converted into ash, carbon dioxide, and water vapor.
Chemical changes are frequently accompanied by observable evidence that a reaction has occurred. This evidence includes a permanent color change, the release of light or heat, or the formation of a gas or a precipitate. For example, the rusting of iron is a slow chemical change where iron atoms combine with oxygen atoms to form iron oxide.
The Science of Supersaturation
Making rock candy begins by dissolving a large quantity of table sugar (sucrose) into heated water. Solubility, the ability of water to dissolve a solid, increases significantly when the temperature is raised. Heating the water creates more space between water molecules, allowing the solution to hold much more sugar than it could at room temperature.
When this sugar-heavy solution cools, it achieves a unique and unstable state known as supersaturation. A supersaturated solution contains more dissolved sucrose than it should theoretically hold at that lower temperature. The dissolved sugar molecules are effectively trapped in the solution, held in place by their attraction to the water molecules.
This supersaturated state is unstable because the excess sugar molecules seek to return to their solid, crystalline form. The solution remains liquid only because the molecules lack a starting point, or nucleation site, for crystallization. Introducing a string or wooden stick coated with sugar grains provides a seed, allowing the excess dissolved sugar to solidify.
Is Crystallization a Physical or Chemical Change?
The formation of the large sugar crystals is definitively a physical change. The entire process, from dissolving the sugar to its re-formation as a crystal, does not create any new chemical compound. The substance starts and ends as solid sucrose (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)), merely changing its physical state from a dissolved molecule back to a highly organized solid structure.
Crystallization involves the dissolved sugar molecules leaving the water and arranging themselves into a repeating geometric pattern. No chemical bonds within the sucrose molecule are broken or reformed during this process. The sugar molecules simply aggregate, which is why the resulting rock candy tastes exactly like the original table sugar.
This contrasts with a chemical change, which occurs if the solution were heated far beyond the boiling point of water. If heated to about \(186^\circ \text{C}\), sucrose molecules decompose, breaking internal bonds to form new molecules like caramel. Caramel is a complex mixture of compounds chemically distinct from sugar.
The rock candy process stops far short of this decomposition, utilizing only the physical properties of solubility and temperature dependence. The resulting sugar crystal is anhydrous, meaning it contains no water molecules bonded within its structure, confirming its identity as pure sucrose.