Distillation can separate homogeneous mixtures. This physical separation process is widely used in laboratories and industry to purify substances or isolate components from a solution. The success of distillation depends entirely on the physical properties of the components within the mixture, specifically their tendencies to turn into vapor when heated. When these properties differ sufficiently, distillation provides an effective means of achieving separation.
Defining Homogeneous Mixtures and Distillation
A homogeneous mixture is a combination of two or more substances that has a uniform composition throughout, meaning the individual components are not visually distinguishable. These mixtures are often referred to as solutions, and examples include saltwater, air, and rubbing alcohol. Every part of a homogeneous mixture possesses the same properties.
Distillation is a separation method that relies on a cycle of heating, vaporization, and condensation. The process begins by heating the liquid mixture until one or more components turn into vapor. This vapor is then directed into a condenser, which is cooled, causing the gas to turn back into a liquid. This condensed liquid, called the distillate, is collected in a separate vessel, completing the physical separation.
The Governing Principle: Volatility Differences
The ability of distillation to separate a homogeneous mixture is governed by the difference in the volatility of its components. Volatility describes how easily a substance evaporates, which is inversely related to its boiling point. For a successful separation, the components must have significantly different boiling temperatures.
When a solution is heated, the component with the lower boiling point, known as the more volatile substance, will preferentially vaporize. This selective evaporation means the vapor rising from the liquid is richer in the more volatile component than the original liquid mixture. The less volatile components largely remain behind in the heating flask.
For instance, separating water from dissolved non-volatile solids, like salt, is highly effective because water boils at 100°C while salt does not vaporize. When separating two miscible liquids, such as ethanol (boiling point 78.4°C) and water (boiling point 100°C), the ethanol vaporizes first. Simple distillation is generally effective when the boiling points of the components differ by at least 25°C.
Key Limitations and Exceptions to Separation
One primary limitation occurs when the components of a homogeneous mixture possess very similar boiling points. If the difference is too small, the vapor produced will still contain a large amount of the less volatile component. In these cases, a more complex technique called fractional distillation is necessary, which utilizes a fractionating column to allow for multiple cycles of vaporization and condensation to improve purity.
The most significant exception to separation by distillation is the formation of an azeotrope. An azeotrope is a specific liquid composition that boils at a constant temperature and where the vapor has the exact same composition as the liquid. Because the vapor is not enriched in either component, distillation cannot change the mixture’s ratio once this point is reached.
A classic example is the ethanol-water mixture, which forms an azeotrope at about 95.6% ethanol by mass. This mixture boils at 78.1°C, which is lower than the boiling point of pure ethanol (78.4°C) or water (100°C). Standard distillation can concentrate the ethanol up to this 95.6% limit, but it cannot produce 100% pure ethanol. Special methods, such as adding a third component or using molecular sieves, are required to break this azeotropic barrier.