Mixtures are common combinations found throughout the world, from the air breathed every day to the saltwater in the oceans. In the field of chemistry, a mixture is defined as a material system made up of two or more different substances that are combined but not chemically bonded together. Each original substance maintains its distinct chemical identity and properties within the combination. Scientists have developed a specific classification system to categorize these combinations based on their physical characteristics.
Mixtures: The Broad Classification
A mixture is formed when two or more substances are physically blended without forming new chemical bonds. This physical blending allows the components to be separated by physical means, such as evaporation or filtration. Based on how uniformly the components are distributed, all mixtures fall into one of two main categories.
Uniform mixtures have a composition that is consistent throughout the entire sample, resulting in a blend that appears to have only a single phase. Non-uniform mixtures have visibly distinct parts where the concentration of components is not the same throughout the substance.
For example, a sweetened cup of tea is a uniform mixture because the sugar is evenly distributed and indistinguishable from the water. Conversely, a combination of sand and water is a non-uniform mixture because the distinct particles are easily visible. Solutions are a specific type of uniform mixture.
Solutions: A Homogeneous Mixture
A solution is classified as a homogeneous mixture, meaning the components are mixed at the molecular level, creating a blend that is identical throughout. A solution consists of two fundamental components. The substance present in the largest quantity, which does the dissolving, is known as the dissolving medium. The substance being dissolved, typically the minor component, is known as the dissolved substance.
The process of dissolving occurs when the attractive forces between the particles of the dissolving medium and the dissolved substance overcome the forces holding the dissolved substance together. The individual molecules or ions of the dissolved substance are then surrounded by the dissolving medium’s molecules in a process called solvation. These dissolved particles scatter, remaining evenly distributed and fully intermingled throughout the medium.
Solutions are not limited to liquids, even though liquid solutions like saltwater are the most familiar. The components can be in any state of matter. For example, air is a solution of gases, and alloys like brass are solid solutions made by dissolving one metal into another. The uniformity of a solution is defined by the fact that the dissolved particles are molecules or ions, typically less than one nanometer in diameter.
How Solutions Differ from Colloids and Suspensions
The primary difference between a true solution and other mixtures lies in the size of the dispersed particles. Colloids and suspensions can sometimes appear similar to solutions but behave differently due to their larger particle sizes.
In a true solution, the microscopic particles are so small they cannot be seen and do not scatter light. If a beam of light is passed through a solution, the path is invisible. Colloids have intermediate-sized particles, generally between one and one thousand nanometers. Colloids demonstrate the Tyndall effect, where their larger particles scatter light, making the beam visible as it passes through the mixture, such as light passing through fog.
Suspensions contain the largest particles, typically greater than one thousand nanometers. Because of their size, the particles in a suspension will settle out of the mixture over time if left undisturbed, such as mud settling in muddy water. Neither solutions nor colloids exhibit this settling behavior.
The molecular size of the particles in a solution allows them to pass completely through standard filter paper, making separation by filtration impossible, a trait shared with colloids. In contrast, the larger particles in a suspension can be separated using a simple filter.