A mixture is a physical combination of two or more substances that are not chemically bonded. The components in a mixture retain their individual chemical identities and properties. Mixtures are classified based on uniformity: homogeneous mixtures have a uniform composition throughout, while heterogeneous mixtures have visibly separate substances and a non-uniform composition. The primary factor used to further classify these mixtures is the physical size of the particles dispersed within the main substance.
Suspensions and Their Defining Characteristics
The heterogeneous mixture with the largest particles is called a Suspension. These dispersed particles are larger than 1000 nanometers (nm) in diameter, making them generally visible to the naked eye or under low magnification. Because the particles are so large, they are significantly affected by gravity.
Suspensions are inherently unstable. When the mixture is left undisturbed, the large, heavy particles will eventually settle out of the dispersing medium, a process known as sedimentation. This settling means that suspensions are temporary and require mechanical agitation, such as shaking, to temporarily redistribute the components. Common examples include muddy water, sand mixed into water, and many liquid medications that instruct the user to “shake well before use.”
Once the particles have settled, they can be easily separated from the liquid. This separation is achieved using simple physical methods, such as filtration. The filter paper traps the solid particles while allowing the liquid to pass through.
The Unique Properties of Colloids
Colloids represent the intermediate class of mixtures, sitting between large-particle suspensions and small-particle solutions. Colloidal particles range from 1 nm to 1000 nm in diameter, making them much smaller than those in suspensions. Although colloids appear homogeneous to the casual observer, they are scientifically classified as heterogeneous mixtures.
Colloids are significantly more stable than suspensions because their particles do not settle out over time. This stability is often maintained by Brownian motion, which is the constant, random movement of the particles, and sometimes by electrostatic repulsion. The most distinguishing feature of a colloid is its ability to scatter light, a phenomenon called the Tyndall effect.
When a beam of light passes through a colloid, the intermediate-sized particles deflect the light, making the path of the beam visible. This effect is why a car’s headlight beam is visible in fog or why sunlight is visible when passing through a dusty room. Examples of colloids include milk, fog, smoke, and blood.
Differentiating Mixture Types
The three main types of mixtures—solutions, colloids, and suspensions—are differentiated primarily by particle size, stability, and separation method. Solutions have the smallest particles, generally less than 1 nm, which are individual atoms, molecules, or ions. This tiny size makes solutions perfectly transparent and entirely homogeneous.
In terms of stability, solutions are the most stable, with particles remaining permanently dispersed. Colloids also exhibit high stability, remaining dispersed indefinitely because their particles resist the pull of gravity. In contrast, suspensions are the least stable, with their larger particles succumbing to gravity and settling over time.
Separation methods also distinguish these mixtures. The microscopic particles in solutions cannot be separated by ordinary filtration or by standing. Colloidal particles will pass through standard filter paper, but the large particles in a suspension are easily separated using common filtration techniques.