# 15.3: Colloids and Suspensions

Difficulty Level: At Grade Created by: CK-12

## Lesson Objectives

• Describe the properties of a suspension.
• Describe the properties of a colloid and distinguish from a solution or a suspension.

## Lesson Vocabulary

• colloid
• emulsion
• suspension
• Tyndall effect

## Check Your Understanding

### Recalling Prior Knowledge

• What are the identifying characteristics of a heterogeneous mixture?
• What prevents oil and water from being miscible?

Solutions are homogeneous mixtures, meaning that a solution is completely uniform throughout. There are two other types of mixtures that are related to a solution, but which are heterogeneous. In this lesson, you will learn the distinguishing characteristics of suspensions and colloids.

## Suspensions

Take a glass of water and throw in a handful of sand or dirt. Stir it and stir it and stir it. 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 below).

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, so gravity is able to pull them down out of the dispersion medium (water). The diameter for the dispersed particles in a suspension, such as the sand in the suspension described above, is typically at least 1000 times greater than those in a solution. Unlike a solution, the dispersed particles can be separated from the dispersion medium by filtering. Suspensions are still considered heterogeneous because the different substances in the mixture will not remain uniformly dispersed if they are not actively being mixed.

## Colloids

A colloid is a heterogeneous mixture in which the dispersed particles are intermediate in size between those of a solution and a suspension. The 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. Table below summarizes the properties and distinctions between solutions, colloids, and suspensions.

Properties of Solutions, Colloids, and Suspensions
Solutions Colloids Suspensions
Homogeneous Heterogeneous Heterogeneous
Particle size: 0.01-1 nm; atoms, ions, or molecules Particle size: 1-1000 nm, dispersed; large molecules or aggregates Particle size: over 1000 nm, suspended; large particles or aggregates
Do not separate on standing Do not separate on standing Particles settle out
Cannot be separated by filtration Cannot be separated by filtration Can be separated by filtration
Do not scatter light Scatter light (Tyndall effect) May either scatter light or be opaque

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

Colloids are often confused with true homogeneous solutions because the individual dispersed particles of a colloid cannot be seen. 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 (Figure below). 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.

(A) Light passes first through a colorless solution and is not scattered. When it passes through a diluted milk solution, the light is scattered by colloidal particles, an observation called the Tyndall effect. (B) The Tyndall effect allows sunlight to be seen as passes through a fine mist.

### Examples of Colloids

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

Classes of Colloids
Class of Colloid Dispersed Phase Dispersion Medium Examples
Sol and gel solid liquid paint, jellies, blood, gelatin, mud
Solid aerosol solid gas smoke, dust in air
Solid emulsion liquid solid cheese, butter
Liquid emulsion liquid liquid milk, mayonnaise
Liquid aerosol liquid gas fog, mist, clouds, aerosol spray
Foam gas solid marshmallow
Foam gas liquid whipped cream, shaving cream

Some common colloids: (A) gelatin dessert (gel); (B) smoke (solid aerosol); (C) butter (solid emulsion); (D) mayonnaise (liquid emulsion); (E) fog (liquid aerosol); (F) marshmallows (foam); (G) whipped cream (foam).

Another property of a colloidal system is 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 a polar aqueous solution, 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 in water. 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.

## Lesson Summary

• Suspensions and colloids are heterogeneous mixtures. A suspension is identifiable because its particles are large and settle out of the dispersing medium due to the effects of gravity.
• The dispersed particles of a colloid are intermediate in size between those of a solution and a suspension. Colloids are distinguishable from solutions because of the light scattering phenomenon called the Tyndall effect. Gels, aerosols, foams, and emulsions are some classes of colloids.

## Lesson Review Questions

### Reviewing Concepts

1. How can you distinguish between a suspension and a solution?
2. How big are the particles in a colloid compared to those of a suspension and a solution?
3. What is the Tyndall effect? Why don’t solutions demonstrate the Tyndall effect?
4. Explain the difference between the dispersed phase and the dispersing medium of a colloid.
5. Explain the difference between an emulsion and an emulsifying agent.
6. If you add a large spoonful of salt to a glass of standing water, the salt sinks to the bottom. Is this a suspension? Explain.

### Problems

1. Identify each of the following descriptions or examples as being representative of a solution, suspension, or colloid. More than one answer may apply.
1. dispersed particles can be filtered out
2. heterogeneous
3. particles are not visible to the unaided eye
4. paint
5. lemonade with no pulp
6. particle size larger than 1 nm
7. milk
8. particles do not settle upon standing
9. fog

## Points to Consider

Solutions have a wide array of properties and uses. While some solutes are very soluble in certain solvents, some other solutes are not.

• How is solubility measured?
• What factors affect solubility?

### Notes/Highlights Having trouble? Report an issue.

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