A mixture is created whenever two or more substances are combined physically without forming new chemical bonds. Each original substance retains its individual chemical identity within the mixture. The uniformity of the components determines the mixture’s classification. This distinction allows us to categorize mixtures into solutions, which are completely uniform, and other types that are not, fundamentally affecting their appearance and how they can be separated.
Solutions: The Homogeneous Standard
A solution represents the ultimate degree of mixing, defined as a homogeneous mixture where the composition is uniform throughout. This means that no matter where you sample the mixture, the ratio of its components remains identical. Solutions are composed of a solute, the substance being dissolved, and a solvent, the substance doing the dissolving. For instance, in saltwater, salt is the solute and water is the solvent.
The particles in a solution (atoms, ions, or molecules) are incredibly small, typically less than one nanometer (10⁻⁹ meters) in diameter. Because of this microscopic size, the solute particles fully integrate with the solvent, making them invisible even under high magnification. A solution is therefore optically clear and transparent, like filtered air or brewed tea.
The components of a solution will never separate or settle out, even if left undisturbed for an extended period. Common examples include metal alloys, like bronze, where copper and tin atoms are uniformly mixed in a solid state. The uniformity of a solution is its defining feature, setting the standard for a completely dissolved and blended system.
Heterogeneous Mixtures: Colloids and Suspensions
Mixtures that are not solutions fall under the category of heterogeneous mixtures, where the components are not uniformly distributed and remain visibly distinct. Within this category, two primary types exist: colloids and suspensions, distinguished mainly by the size of their internal particles. Suspensions contain the largest dispersed particles.
Suspensions
In a suspension, the particles are large enough to be seen with the naked eye, measuring greater than 1,000 nanometers. These large components do not dissolve but are temporarily dispersed throughout the medium, such as sand mixed into water. A definitive characteristic of a suspension is its instability; the force of gravity will eventually cause the particles to settle out of the liquid over time. For this reason, many liquid medicines are suspensions and require shaking before use to redistribute the settled active ingredients.
Colloids
Colloids occupy the intermediate range of particle size, falling between the microscopic particles of a solution and the large particles of a suspension. Colloidal particles range from about 1 to 1,000 nanometers in diameter. Although these particles are too small to settle out due to gravity, they are large enough to interact with light.
When a beam of light passes through a colloid, the particles scatter the light, an effect known as the Tyndall effect, which makes the beam visible. This property is why fog, an example of a colloid, makes car headlights visible at night. Milk is another common example, where tiny droplets of fat and protein are evenly dispersed but not truly dissolved in water. Despite the uniform appearance of many colloids, their particle size technically classifies them as heterogeneous, unlike true solutions.
Defining Differences: Particle Size and Separation
Particle size is the fundamental property that dictates the difference between solutions, colloids, and suspensions. Solutions contain the smallest particles (individual molecules or ions). Colloids have particles hundreds of times larger, while suspensions contain particles thousands of times larger. This size difference directly impacts the stability and appearance of the mixture.
The particle size also determines the method required to separate the components of the mixture. Because the components of a solution are fully dissolved at the molecular level, they cannot be separated by physical means like filtration. Separating a solvent from a solute, such as obtaining pure water from saltwater, requires processes that change the state of matter, like distillation or evaporation.
Suspensions, with their large particles, are the easiest to separate. The components can be physically isolated using simple filtration, as the particles are too large to pass through filter paper. Colloids present a challenge because their intermediate-sized particles pass through a standard filter, preventing separation by simple filtration. Instead, colloids often require specialized techniques like centrifugation, which uses rapid spinning to force the dispersed phase to separate from the continuous phase.