When sugar dissolves in water, it undergoes a transformation common in everyday life, involving interactions at the molecular level. This is an example of dissolution, a physical process where one substance disperses evenly into another.
The Nature of Sugar and Water
Sugar, specifically sucrose (C12H22O11), is a disaccharide composed of glucose and fructose units. Its molecular structure features numerous hydroxyl (-OH) groups. These groups make sucrose a polar molecule, meaning it has areas with slight positive and negative charges across its structure. In its solid, crystalline form, sucrose molecules are held together by hydrogen bonds.
Water (H2O) is a polar molecule. Its oxygen atom attracts electrons more strongly than hydrogen, giving oxygen a slight negative charge and hydrogen atoms slight positive charges. This uneven charge distribution, combined with its bent molecular shape, allows water molecules to form strong hydrogen bonds.
The Molecular Process of Dissolving
When sugar crystals are added to water, polar water molecules interact with polar sugar molecules. The slightly positive hydrogen ends of water are attracted to the slightly negative oxygen areas on sugar, while the slightly negative oxygen end of water is attracted to the slightly positive hydrogen areas on sugar. This attraction causes water molecules to surround individual sugar molecules on the crystal’s surface.
As water molecules cluster around the sugar, they pull individual sucrose molecules away from the crystal lattice. This breaks the hydrogen bonds holding sugar molecules together within the solid crystal. Simultaneously, hydrogen bonds between water molecules are disrupted to make space for the incoming sugar molecules. The energy to break these bonds is compensated by energy released when new bonds form.
New hydrogen bonds form between water molecules and the separated sugar molecules. This process, known as solvation, involves each sugar molecule becoming completely surrounded by a “cage” of water molecules. Sugar molecules remain chemically unchanged; they are simply dispersed throughout the water. Dissolving sugar in water is a physical change, not a chemical reaction.
The Characteristics of a Sugar Solution
Once dissolved, the individual sugar molecules are uniformly distributed throughout the water, creating a homogeneous mixture. The solution appears clear because the sugar molecules are dispersed at a molecular level and are too small to scatter light or be seen individually. The solution is also sweet.
Since dissolution is a physical change, sugar molecules retain their chemical identity. Sugar can be recovered in solid form if water is removed, such as through evaporation. There is a limit to how much sugar can dissolve at a given temperature. Once this limit is reached, the solution becomes saturated, and no more sugar will dissolve.