Why is the sea clear on some days, but not on others?

Waves at Cable Beach, courtesy of the U.S. Navy.

On a calm day, the Caribbean Sea is clear and smooth.  You can see deep into the water, deep enough to see fish and underwater plant life without obstruction.  The situation changes markedly when a storm blows up.  The sand on the bottom of the sea is stirred up, the water becomes turbid, and you can’t see anything below the surface.  The sand forms a suspension in the water that slowly clears up again after the storm blows over.

Suspensions

Take a glass of water and throw in a handful of sand or dirt.  Stir it and stir it and stir it. The water may become turbid , or unclear.  Have you made a solution?  Sand and dirt do not dissolve in water and though it may look homogeneous for a few moments, the sand or dirt gradually sinks to the bottom of the glass.

Figure 1. A suspension of dirt in water. Image from the CK-12 Foundation – Joy Sheng

 

A suspension is a heterogeneous mixture in which some of the particles settle out of the mixture upon standing.  The particles in a suspension are far larger than those of a solution and thus gravity is able to pull them down out of the dispersion medium (water).  The typical diameter for the dispersed particles (the sand) of a suspension is about 1000 times greater than those of a solution (less than approximately two nanometers for particles in solution, compared to greater than 1000 nanometers for particles in suspension).  Unlike in a solution, the dispersed particles can be separated from the dispersion medium by filtering.  Suspensions are heterogeneous because at least two different substances in the mixture can be identified.

Some Typical Aqueous Suspensions

Milk

Milk is a complex mixture of water, proteins, fats, carbohydrates, and minerals.  While the minerals and carbohydrates are water-soluble, the fats and some of the proteins do not dissolve but are held in suspension.

Paint

Paint can either be water-based or use an organic solvent.  For our discussion, we will limit ourselves to water-based paints.  The materials will contain binders (organic polymers that help hold the paint to the surface of the material being painted).  In addition, pigments are included to provide the desired color.  Both the binders and the pigments are not water-soluble, so the paint must be stirred every time it is used.  Paint that is allowed to sit for a long period of time will slowly begin to separate as the suspension of binders and pigments settles out.

Summary

• Suspensions are heterogeneous mixtures.
• Some of the material in a suspension will settle out on standing.
• Solid material in a suspension can be removed by filtration.

Glossary