CONTINUUM OF WATER MIXTURES

SAT Subject Test Chemistry

PART 2 REVIEW OF MAJOR TOPICS

CHAPTER 7 Liquids, Solids, and Phase Changes

Figure 32 shows the general sizes of the particles found in a water mixture.

Figure 32. Size of Particles in Water Mixture

The basic difference between a colloid and a suspension is the diameter of the particles dispersed. All the boundaries marked in Figure 32 indicate only the general ranges in which the distinctions between solutions, colloids, and suspensions are usually made.

The characteristics of water mixtures are as follows:

Solutions	Colloids	Suspensions

… … … … … … . . 1 nm … … … … … … …1,000 nm … … … … … … … … … … … … ….
Clear; may have color Particles do not settle. Particles pass through ordinary filter paper.		Cloudy; opaque color Settle on standing Do not pass through ordinary filter paper
Particles pass through membranes.	Do not pass through semipermeable membranes such as animal bladders, cellophane, and parchment, which have very small pores*
Particles are not visible.	Visible in ultramicroscope	Visible with microscope or naked eye
	Show Brownian movement	No Brownian movement

* Separation of a solution from a colloidal dispersion through a semipermeable membrane is called dialysis.

When a bright light is directed at right angles to the stage of an ultramicroscope, the individual reflections of colloidal particles can be observed to be following a random zigzag path. This is explained as follows: The molecules in the dispersing medium are in motion and continuously bumping into the colloidal particles, causing them to change direction in a random fashion. This motion is called Brownian movement after the Scottish botanist Robert Brown, who first observed it.