A mixture is a material created by combining two or more different substances physically, but not chemically. Unlike compounds, the components of a mixture retain their individual chemical identities and properties. The proportions of the substances can vary, and they can typically be separated using physical methods such as filtration or evaporation. Mixtures are classified based on the uniformity of their composition and the size of the particles involved, distinguishing between homogeneous mixtures, heterogeneous mixtures, and sub-categories defined by particle size.
Homogeneous Mixtures and Solutions
A homogeneous mixture appears completely uniform throughout its entire volume, meaning the composition is the same no matter where a sample is taken. The component substances are perfectly intermingled at a microscopic level, making it impossible to distinguish them, even with magnification. These mixtures exist in a single phase (solid, liquid, or gas) and are perfectly stable, with the components never separating over time.
The most common example of a homogeneous mixture is a solution, which consists of a solute and a solvent. The solute is the substance present in the smaller amount that is dissolved. The solvent is the substance present in the larger amount that does the dissolving.
A familiar example is salt water, where table salt acts as the solute and water is the solvent. Solutions are not limited to liquids; air is a gaseous solution where oxygen and other gases are solutes dissolved in nitrogen. Alloys, such as brass (copper and zinc), are solid solutions formed by mixing metals while they are molten, resulting in a uniform solid mixture.
Heterogeneous Mixtures
In contrast to homogeneous mixtures, heterogeneous mixtures are those where the component substances are not evenly distributed throughout the volume. The composition varies from one region to another, and the individual parts often remain visibly distinct, sometimes with the naked eye.
A defining feature of a heterogeneous mixture is that it consists of two or more distinct phases, which can be different states of matter or just separate layers. For example, a mixture of sand and water clearly shows a solid phase and a liquid phase that do not blend. Similarly, a common vinaigrette salad dressing is a heterogeneous liquid mixture where the oil and vinegar separate into two visible layers, or phases.
Because the components retain their physical boundaries and individual properties, they can be separated relatively easily using simple physical techniques. Allowing the components of a mixture to settle and then pouring off the liquid (decanting), or pouring the mixture through a filter, are standard ways to separate these mixtures. Granite rock, composed of visibly distinct crystals of quartz, feldspar, and mica, is a solid example of a naturally occurring heterogeneous mixture.
Sub-Classifications Based on Particle Size
Heterogeneous mixtures are further categorized based on the size of the dispersed particles, leading to the classifications of colloids and suspensions. This distinction relates directly to the mixture’s stability and how it interacts with light. The particles in true solutions are extremely small, typically less than one nanometer in diameter, making them invisible and unable to settle.
Suspensions are heterogeneous mixtures containing particles larger than 1,000 nanometers, which makes them the largest of the three categories. Due to their size, these particles are affected by gravity and will eventually settle out of the mixture upon standing, which is why a jar of muddy water will eventually clarify. The particles in a suspension are large enough to be separated by standard filtration methods.
Colloids represent an intermediate category, with particles ranging in size from roughly one to 1,000 nanometers. These particles are small enough that they do not settle out over time, giving the mixture a deceptively uniform appearance, even though it is technically heterogeneous. Common examples include milk, fog, and whipped cream.
A colloid can be definitively distinguished from a true solution using the Tyndall effect, which is the scattering of a beam of light as it passes through the mixture. While the tiny particles in a true solution are too small to scatter light, the intermediate-sized particles in a colloid are large enough to deflect the light beam, making its path visible. This effect can be observed when a car’s headlights are used in fog, which is a colloid of water droplets in air.