What type of mixture is a smoke flare?

Smoke flares, courtesy of the U.S. Air Force.

Imagine you are sailing on a yacht.  The engine suddenly breaks down and you are stranded in the middle of the ocean.  You call the Coast Guard on your radio, but cannot give them an exact location because your GPS isn’t working.  Fortunately, you have a smoke flare, which you fire off.  The dense colored smoke shows the Coast Guard where you are so they can rescue you.  In using the flare, you are taking advantage of a type of mixture called a colloid.

Colloids

A colloid is a heterogeneous mixture whose particle size is intermediate between those of a solution and a suspension.  The dispersed particles are spread evenly throughout the dispersion medium, which can be a solid, liquid, or gas.  Because the dispersed particles of a colloid are not as large as those of a suspension, they do not settle out upon standing.  The Table below summarizes the properties and distinctions between solutions, colloids, and suspensions.

Colloids are unlike solutions because their dispersed particles are much larger than those of a solution.  The dispersed particles of a colloid cannot be separated by filtration, but they scatter light, a phenomenon called the Tyndall effect .

Tyndall Effect

When light is passed through a true solution, the dissolved particles are too small to deflect the light.  However, the dispersed particles of a colloid, being larger, do deflect light.  The Tyndall effect is the scattering of visible light by colloidal particles.  You have undoubtedly “seen” a light beam as it passes through fog, smoke, or a scattering of dust particles suspended in air.  All three are examples of colloids.  Suspensions may scatter light, but if the number of suspended particles is sufficiently large, the suspension may simply be opaque and the light scattering will not occur.

Figure 2. Figure 15.12 The Tyndall effect allows sunlight to be seen as it passes through a fine mist. Image by Dave Stokes, CC-BY.

 

Examples of Colloids

The table below lists examples of colloidal systems, most of which are very familiar.  The dispersed phase describes the particles, while the dispersion medium is the material in which the particles are distributed.

Another property of a colloidal system is the observed when the colloids are studied under a light microscope.  The colloids scintillate, reflecting brief flashes of light because the colloidal particles move in a rapid and random fashion.  This phenomenon, called Brownian motion, is caused by collisions between the small colloidal particles and the molecules of the dispersion medium.

Emulsions

Butter and mayonnaise are examples of a class of colloids called emulsions.  An emulsion is a colloidal dispersion of a liquid in either a liquid or a solid.  A stable emulsion requires an emulsifying agent to be present.  Mayonnaise is made in part of oil and vinegar.  Since oil is nonpolar and vinegar is an aqueous solution and polar, the two do not mix and would quickly separate into layers.  However, the addition of egg yolk causes the mixture to become stable and not separate.  Egg yolk is capable of interacting with both the polar vinegar and the nonpolar oil.  The egg yolk is called the emulsifying agent.  Soap acts as an emulsifying agent between grease and water.  Grease cannot be simply rinsed off your hands or another surface because it is insoluble.  However, the soap stabilizes a grease-water mixture because one end of a soap molecule is polar and the other end is nonpolar.  This allows the grease to be removed from your hands or your clothing by washing with soapy water.

Glossary