Illustrated Guide to Home Chemistry Experiments

Chapter 18 Laboratory: Colloids and Suspensions 319

photograph well, even though it was clearly visible to the eye.
The bright green line that crosses the beaker is the actual laser
beam, reflected by the colloidal dispersion. The green laser
pointer is bright enough that the scattered light illuminates the
rest of the contents of the beaker as well.

Table 18-2 summarizes the physical characteristics of solutions,
colloids, and suspensions. It’s important to understand that there
are no hard-and-fast boundaries between solutions, colloids,
and suspensions. Whether a particular mixture is a colloid or a
suspension, for example, depends not just on the particle size,
but the nature of the continuous phase and the dispersed phase.
For example, note that the particle size of colloids may range from
about 1 nm to about 200 nm, and the particle size of suspensions
may be anything greater than 100 nm. Furthermore, particle
sizes are seldom uniform, and may include a wide range in any
particular mixture.

So, is a particular mixture with a mean particle size of 100 nm a
colloid or a suspension? It depends on the nature of the particles
and the continuous phase. Solutions, colloids, and suspensions are
each separated by a large gray area. Near the boundaries between
types, it’s reasonable to argue that a substance is both a solution
and a colloid, or both a colloid and a suspension. As George S.
Kaufman said, “One man’s Mede is another man’s Persian.”

FIGURE 18-1: The Tyndall Effect

TABLE 18-2: Physical characteristics of solutions, colloids, and suspensions

Characteristic Solution Colloid Suspension

Type of particle Individual molecules

or ions

Very large individual molecules

or aggregates of tens to thousands

of smaller molecules

Very large aggregates

of molecules

particle size < 1 nm ~1 nm to ~200 nm > 100 nm

Separation by

gravity?

No No (usually; otherwise, very slowly) Yes

Separation by

centrifugation?

No Yes, for more massive

dispersed particles

Yes

Captured by filter

paper? No No Yes

Captured by

membrane? No Yes (usually) Yes

precipitatable by

flocculation? No Yes Yes

Exhibits Tyndall

Effect?

No Yes Yes

Affects colligative

properties?

Yes No No

In this chapter, we’ll prepare various colloids and suspensions

and examine their properties.