Table sugar, or sucrose (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)), is definitively a solid. Although it appears as small, uniform granules that pour easily, this physical characteristic sometimes leads to confusion about its state of matter. Sucrose is a crystalline solid, meaning its molecules are locked into a highly organized, three-dimensional structure that gives it a fixed shape and volume at room temperature.
Sugar’s Crystalline Structure
The classification of sucrose as a solid stems from the arrangement of its individual molecules. Within the sugar grain, sucrose molecules align themselves into a highly organized, repeating pattern called a crystal lattice. This structure is characteristic of a crystalline solid, possessing long-range order, much like bricks stacked precisely. Sucrose is specifically categorized as a molecular crystalline solid, where discrete molecules are held together by intermolecular forces.
Sucrose molecules are complex, composed of one glucose unit and one fructose unit chemically bonded together. The crystal lattice is held in place by relatively weak hydrogen bonds, which form between the oxygen and hydrogen atoms of adjacent molecules. This organized structure is in contrast to an amorphous solid, like glass, where the molecules are arranged randomly without a repeating pattern. While sugar can be made into a glassy state by rapidly cooling melted sugar, its stable and natural form is always crystalline.
Why Sugar Dissolves and Melts Easily
The common observation that sugar dissolves and melts easily does not negate its status as a solid, but reflects the nature of its specific molecular bonds. Sucrose is classified as a polar molecule because oxygen and hydrogen atoms share electrons unevenly, creating slight positive and negative charges across the molecule. Since water molecules are also polar, they are strongly attracted to these charged areas on the sucrose molecules.
When sugar is mixed with water, the attraction between water and sucrose molecules is strong enough to overcome the hydrogen bonds holding the crystal lattice together. The water pulls the individual sucrose molecules away from the solid grain, surrounding them completely, which is the process of dissolving. Furthermore, the weak nature of these hydrogen bonds means only a moderate amount of heat energy is required to break them. This results in a relatively low melting point—around 186°C—compared to other solids like table salt, which requires much higher temperatures.