Solubility describes the ability of a substance, the solute, to dissolve into another substance, the solvent, forming a homogeneous solution. Sucrose is a complex sugar molecule, while ethanol is the type of alcohol present in alcoholic beverages and various chemical applications. Understanding the solubility relationship between these two compounds requires investigating the forces that govern how molecules mix. The degree to which sucrose dissolves in ethanol, compared to its well-known solubility in water, highlights a core principle of chemical science.
The Direct Answer
Sucrose is considered to be sparingly or poorly soluble in pure ethanol at room temperature. This is in stark contrast to its behavior in water, where it dissolves extremely well, reaching concentrations of over 200 grams per 100 milliliters. To put the difference into perspective, one gram of sucrose requires about 0.5 milliliters of water to dissolve, but that same gram requires about 170 milliliters of pure alcohol. For all practical purposes, sucrose appears insoluble in ethanol to the average observer. The United States Pharmacopeia (USP) classifies substances that require 100 to 1,000 parts of solvent per part of solute as “slightly soluble.”
The Governing Principle
The vast difference in solubility comes down to the chemical principle that “like dissolves like,” specifically concerning molecular polarity. Sucrose is a disaccharide molecule, possessing eight hydroxyl (-OH) groups, which makes it an extremely polar substance. These numerous hydroxyl groups create many sites for strong hydrogen bonding, which is the primary mechanism by which it dissolves so readily in water. Water is also a highly polar solvent, capable of forming an extensive network of hydrogen bonds to surround and separate the sucrose molecules from their solid crystalline structure.
Ethanol, or ethyl alcohol, is only moderately polar. While it does contain one hydroxyl group that allows it to engage in hydrogen bonding, the majority of the molecule is a non-polar ethyl chain. This non-polar hydrocarbon tail reduces the overall polarity of the ethanol molecule compared to water. Therefore, ethanol cannot form the strong, numerous, and efficient hydrogen bond networks required to effectively pull the highly polar sucrose molecules out of their solid lattice. The result is that the attraction between the sucrose molecules themselves is much stronger than the attraction between sucrose and the moderately polar ethanol, leading to poor dissolution.
Practical Applications of Low Solubility
The low solubility of sucrose in ethanol is routinely exploited in both industrial and laboratory settings. One of the primary applications is in the purification and crystallization of sugars. Ethanol is sometimes added to sugar solutions to deliberately reduce the solubility of sucrose and encourage its controlled crystallization. This process is used to separate pure sucrose crystals from less desirable impurities, which may remain dissolved in the ethanol-water mixture.
In chemical extraction, ethanol is frequently used as a solvent to selectively pull out certain compounds from plant matter, such as essential oils or other less polar substances. In these extractions, the sugars and other highly polar carbohydrates, like sucrose, remain behind in the solid plant material because the ethanol does not dissolve them well. This differential solubility allows chemists to use ethanol to separate the target compounds from the bulk of the unwanted sugars. Furthermore, the varying solubility of different sugars in ethanol-water mixtures is used to fractionate complex sugar mixtures. By adjusting the ratio of ethanol to water, scientists can selectively precipitate out different sugar molecules based on their size and polarity.