A mixture is a combination of two or more different substances that are physically blended together but are not chemically bonded. Each substance retains its distinct chemical properties, meaning they can typically be separated by physical means. The composition of a mixture can vary, unlike a pure chemical compound which has a fixed ratio of elements. Understanding how these combinations are organized forms the basis for their scientific classification.
Classification by Uniformity
The simplest way to categorize mixtures is by looking at how uniformly the components are distributed throughout the whole sample. This initial division separates all mixtures into two broad categories: homogeneous and heterogeneous.
Homogeneous mixtures display a uniform appearance and composition throughout. When observing a homogeneous mixture, such as sugar dissolved in water or clean air, you cannot visually distinguish the individual components, even under a microscope. The dissolved particles are distributed at a molecular level, resulting in a single, consistent phase.
In contrast, heterogeneous mixtures are non-uniform. You can readily see the individual components or separate phases in a heterogeneous mixture, like a salad or sand mixed with water. These mixtures often exhibit distinct boundaries between the substances, and the components may settle out over time if left undisturbed.
Classification by Particle Size
A more precise classification method uses the size of the dispersed particles as a defining characteristic, separating mixtures into solutions, colloids, and suspensions. Solutions represent one end of the spectrum, consisting of particles less than 1 nanometer (nm) in diameter. These tiny particles, like dissolved salt ions, are completely surrounded by the solvent and never settle out, resulting in a perfectly transparent mixture.
Colloids occupy the intermediate range of particle sizes, between 1 nm and 1000 nm (or 1 micrometer). These medium-sized particles remain permanently suspended without settling due to gravity. Milk and fog are examples of colloids, and while they may appear uniform to the naked eye, they possess unique light-scattering properties.
Suspensions contain particles larger than 1000 nm, which are visible to the unaided eye. The large mass of these particles means they are not permanently supported by the medium and will settle out over time, a process called sedimentation. Examples include muddy water or flour mixed in water, which can be separated by simple filtration.
The Tyndall effect is used to distinguish colloids from true solutions. This effect describes the scattering of a beam of light as it passes through a colloidal dispersion. Because a colloid’s dispersed particles are large enough to deflect light, the path of the beam becomes visible.
A true solution cannot scatter the light, so a beam of light passes through it invisibly. The Tyndall effect makes a flashlight beam visible when shining through fog (a colloid), but invisible when shining through clean air (a solution). Suspensions can also scatter light, but they are easily identified because their particles separate upon standing.
Identifying Mixtures in Daily Life
Observational tests can help classify mixtures based on the scientific categories. If the mixture separates into layers, it is likely a suspension, such as a bottle of orange juice with pulp that requires shaking before drinking. The separation is caused by the large, heavy particles falling out of the liquid medium.
If the mixture does not separate, the next step is to test for the Tyndall effect using a focused light source. If the light beam becomes distinctly visible inside the liquid, the mixture is a colloid, such as milk or mayonnaise, where the dispersed fat droplets scatter the light.
If the light beam passes through the mixture invisibly, it is a true solution. Examples include clear drinking water (a blend of dissolved minerals and water) and vinegar (acetic acid dissolved in water). These simple observations connect the scientific definitions of particle size and uniformity to the mixtures encountered every day.