Matter exists in two fundamental forms: pure substances and mixtures. A pure substance, like an element or a compound, has a fixed chemical composition and distinct properties. When two or more pure substances are brought together, they can combine chemically to form a new compound (e.g., hydrogen and oxygen bonding to create water). Alternatively, they can combine physically, resulting in a mixture.
Mixtures vs. Pure Substances: The Difference in Formation
A mixture is defined as a material composed of two or more substances that are physically combined, not chemically bonded. The components retain their individual chemical identities and properties. For example, when salt is mixed with water, the salt remains sodium chloride and the water remains H2O.
In contrast, compounds are formed through a chemical reaction where atoms are bonded together in a fixed ratio. This chemical combination creates a completely new substance with properties distinct from its constituent elements. Crucially, mixture formation involves only a physical change with no energy change, unlike the energy absorbed or released during the chemical formation of a compound.
The Basic Division: Homogeneous and Heterogeneous Mixtures
Mixtures are first categorized based on their uniformity, which can be observed at a macroscopic level. A homogeneous mixture, or a single-phase mixture, possesses a uniform composition throughout the entire sample. Every part of a homogeneous mixture looks and behaves identically, and the components are indistinguishable from one another.
Saltwater is an example of a homogeneous mixture, where the dissolved salt is distributed uniformly. Conversely, a heterogeneous mixture has a non-uniform composition, and its components are easily distinguishable, often existing in two or more distinct phases. Examples include cereal with milk or sand mixed in water, where the different parts are visibly separate.
Classification by Particle Size: Solutions, Colloids, and Suspensions
Mixtures can be further classified based on the diameter of the dispersed particles, leading to the categories of solutions, colloids, and suspensions.
Solutions
Solutions have the smallest dispersed particles, typically less than one nanometer (nm) in diameter. These particles are individual molecules or ions, making them invisible even under a powerful microscope, and they pass completely through any filter paper. Because of their minuscule size, the particles do not scatter light, meaning a beam of light passed through the solution is invisible. The components will never settle out over time. Alloys, like brass, and sugar dissolved in water are common examples.
Colloids
Colloids occupy the intermediate range of particle sizes, ranging from 1 nm to 100 nm in diameter. While a colloid may appear homogeneous, it is technically a heterogeneous mixture because the particles are not molecularly dissolved. These medium-sized particles are large enough to scatter a beam of light, a phenomenon known as the Tyndall effect, which makes the light path visible.
Particles in a colloid, such as milk or fog, remain suspended and do not settle out due to gravity. They cannot be separated by standard filtration but require specialized techniques like centrifugation.
Suspensions
Suspensions contain the largest particles, greater than 100 nm in diameter, which are often visible to the naked eye. These are clearly heterogeneous mixtures where the dispersed particles are so large they will settle out of the mixture upon standing. Muddy water is a primary example, where the large solid particles gradually fall to the bottom over time. The particle size allows the components of a suspension to be readily separated by simple filtration.
Reversing the Process: Physical Separation Techniques
The physical nature of mixtures allows them to be easily separated. Separation techniques exploit differences in physical properties, such as particle size, boiling point, or density. Filtration utilizes differences in particle size to separate an insoluble solid from a liquid in a suspension.
For homogeneous mixtures, techniques that rely on changes of state are employed. Evaporation involves heating the mixture until the liquid solvent turns into a gas, leaving the solid solute behind. Distillation is used when separating two miscible liquids with different boiling points, such as water and alcohol. This process involves heating the mixture to vaporize the more volatile component and then condensing it back into a separate container.