A mixture is a combination of two or more substances that are physically blended but not chemically bonded. Mixtures are classified into three main categories—solutions, suspensions, and colloids—based on the physical properties of the dispersed substance, particularly the size of its individual particles. Understanding these distinctions helps explain why some mixtures are transparent and stable, while others are cloudy and prone to separation.
Defining Solutions and Suspensions
Solutions represent the smallest end of the mixture spectrum, where the dissolved particles (the solute) are individual atoms, ions, or molecules. These particles are tiny, typically having a diameter less than 1 nanometer (nm). Because the particles are so small, they integrate completely with the solvent, making the mixture appear uniform throughout, which is why solutions are classified as homogeneous.
A true solution, such as salt dissolved in water, is completely transparent and stable; the particles will never settle out, even when left undisturbed. Furthermore, the solute particles cannot be separated from the solvent using ordinary filtration methods, as they pass easily through the pores of standard filter paper.
Suspensions represent the opposite extreme, characterized by particle sizes significantly larger than those in a solution, generally exceeding 1,000 nanometers (1 micrometer). These particles are large enough to be visible to the unaided eye and do not dissolve into the dispersing medium. Because the particles are not evenly distributed, suspensions are classified as heterogeneous mixtures.
Gravity exerts a noticeable effect on these larger particles, causing them to settle out of the mixture upon standing. This instability means that if a suspension, like muddy water, is left alone, the solid particles will eventually collect at the bottom. Due to their size, the dispersed particles can be separated from the liquid medium using simple filtration.
The Intermediate State of Colloids
Colloids occupy the middle ground between solutions and suspensions, possessing unique properties distinct from both. The dispersed particles have an intermediate size, ranging from 1 nanometer to 1,000 nanometers. This size range is too large to fully dissolve like a solution, but small enough to resist settling out due to gravity, unlike a suspension.
While a colloid may appear uniform and clear, similar to a solution, it is technically considered a heterogeneous mixture because the particles are larger than single molecules. The stability of a colloid is maintained by the continuous, random motion of the dispersed particles, a phenomenon known as Brownian motion. This constant movement, caused by collisions with molecules of the dispersing medium, keeps the particles from settling.
Practical Methods for Differentiation
Distinguishing a true solution from a colloid can be difficult because both appear stable and often transparent. The most definitive test is the Tyndall effect, an optical phenomenon that involves shining a beam of light through the mixture.
In a true solution, the individual particles are too small to interact with light waves, so the beam passes through invisibly. However, when light passes through a colloid, the intermediate-sized particles are large enough to scatter the light in all directions, making the path of the beam clearly visible. This scattering confirms the presence of colloidal particles.
Another practical method involves examining filterability. Both solutions and colloids will pass through standard filter paper because their particles are too small to be trapped. This characteristic distinguishes them from suspensions, which are easily separated by filtration. The Tyndall effect remains the singular test to distinguish between the two stable, non-filterable mixtures.
Everyday Examples of Mixtures
Mixtures encountered in daily life fall into these three categories based on their particle behavior. True solutions are those where the components are completely integrated, such as sweetened tea (sugar dissolved in water). Air is also a solution, composed of various gases like nitrogen and oxygen that are thoroughly mixed.
Colloids include many common household items that appear uniform but exhibit the Tyndall effect. Examples include milk, an emulsion of fat globules dispersed in water. Other common colloids are fog, where liquid water droplets are suspended in air, and gelatin, which is a gel formed by protein fibers dispersed in water.
Examples of suspensions are mixtures that will eventually separate if left undisturbed. These include muddy water, where soil particles settle at the bottom, and some types of oil-and-vinegar salad dressings. The need to shake a container before using certain liquid medications or paints is often an indication that the contents are a suspension.