Chapter 15: Solutions - Weironline.net

Objectives
Section 15.4 Heterogeneous Mixtures
• Identify the properties of
suspensions and colloids.
• Describe different types of
colloids.
• Explain the electrostatic
forces in colloids.
Vocabulary
suspension
colloid
Brownian motion
Tyndall effect
Figure 15-19
A suspension is a type of heterogeneous
mixture. Suspension
particles settle out over time a ,
and can be separated from the
mixture by filtration b .
a b
476 Chapter 15 Solutions
As you learned in Chapter 3, most of the forms of matter that you encounter
are mixtures. A mixture is a combination of two or more substances that keep
their basic identity. Components of a mixture come in contact with each other
but do not undergo chemical change. You have been studying homogeneous
mixtures called solutions so far in this chapter. Not all mixtures are solutions,
however. Heterogeneous mixtures contain substances that exist in distinct
phases. Two types of heterogeneous mixtures are suspensions and colloids.
Suspensions
Look at the mixture shown in Figure 15-19a. Although it resembles milk,
it is actually a freshly-made mixture of cornstarch stirred into water. If you
let this mixture stand undisturbed for a while, it separates into two distinct
layers: a thick, white, pastelike substance on the bottom and the water on
top. Cornstarch in water is a suspension, a mixture containing particles that
settle out if left undisturbed. As shown in Figure 15-19b, pouring a liquid
suspension through a filter also separates out the suspended particles. Other
examples of suspensions include fine sand in water and muddy water.
Suspended particles are large compared to solvated particles, with diameters
greater than 1000 nm (10 �6 m) compared to diameters less than 1 nm
(1 �9 m) for solvated particles. Gravity acts on suspended particles in a short
time, causing them to settle out of the mixture. Interestingly, the settled out
cornstarch particles form a solidlike state on the bottom of the container.
However, when stirred, the solidlike state quickly begins flowing like a liquid.
Substances that behave this way are called thixotropic. One of the most
common applications of a thixotropic mixture is house paint. The paint flows
rather easily when applied with a brush, but quickly thickens to a solidlike
state. The quickly thickening paint helps it stick to the house and prevents
runs in the paint.