A mixture forms when two or more substances are combined physically without chemical reaction, meaning their chemical identities remain unchanged. Mixtures are broadly classified based on the distribution of their components, falling into the categories of either homogeneous or heterogeneous. A heterogeneous mixture is the focus of this discussion.
Defining the Non-Uniform Structure
A heterogeneous mixture is fundamentally characterized by its non-uniform composition, where components are not evenly distributed throughout the substance. Taking a sample from one part of the mixture will likely yield a different ratio of components than a sample taken from another area. Visibly, a heterogeneous mixture consists of two or more distinct phases, which are physical regions with their own uniform properties.
These separate phases are often distinguishable by the naked eye or with the aid of a microscope. For instance, a mixture of sand and water clearly shows a solid phase and a liquid phase that do not dissolve into one another. Another common example is oil and vinegar salad dressing, where two immiscible liquids form distinct layers.
How They Differ from Homogeneous Mixtures
The primary distinction between heterogeneous and homogeneous mixtures lies in the concept of uniformity. Homogeneous mixtures, also known as solutions, have a composition that is uniform down to the molecular or atomic level. Conversely, heterogeneous mixtures feature a non-uniform composition where components are physically separated into distinct phases.
Particle size is a key factor separating the two types of mixtures. The particles in a homogeneous mixture, like sugar dissolved in water, are atomic or molecular in size (typically 0.1 to 1 nanometer) and are therefore invisible. In a heterogeneous mixture, the component particles are much larger, often visible to the eye, and are not dispersed at a molecular level. Heterogeneous mixtures are also generally less stable than solutions, as the larger particles may settle out or separate upon standing.
Subcategories: Suspensions and Colloids
Heterogeneous mixtures are further categorized into suspensions and colloids, a classification largely based on the size of the dispersed particles. Suspensions are defined by having the largest dispersed particles, typically with diameters greater than 1,000 nanometers. These particles are large enough that they will settle out of the dispersion medium over time when the mixture is left undisturbed, due to gravity.
A classic example of a suspension is muddy water, where the solid soil particles will eventually fall to the bottom of the container. Suspensions are opaque because their large particles block the passage of light. Colloids, however, contain intermediate-sized particles, generally ranging from 1 to 1,000 nanometers in diameter.
While these particles are too small to settle out by gravity, they are large enough to scatter visible light, a phenomenon known as the Tyndall effect. The Tyndall effect is observable when a beam of light is passed through a colloid, making the path of the beam visible, as seen with milk or fog.
The dispersed particles in a colloid remain suspended indefinitely due to their small size and the constant, random motion of the particles, known as Brownian motion. Although colloids appear uniform to the naked eye, they are still considered heterogeneous mixtures because they possess distinct phases.
Practical Methods for Component Separation
One practical consequence of a heterogeneous mixture’s non-uniformity is that its components can often be separated using simple physical methods. The large size and distinct physical properties of the components make separation straightforward. Common physical methods used for separation include:
- Filtration is a common technique used to separate an insoluble solid from a liquid in a suspension by trapping larger solid particles in a filter medium.
- Decanting is used when a solid has settled at the bottom of a liquid; the liquid is carefully poured off, leaving the solid behind.
- Sieving uses a screen with uniformly sized holes to separate solid components of different sizes.
- Magnetic separation can be employed for mixtures containing a magnetic substance, such as iron filings mixed with sand.