Chapter 18. Laboratory: Colloids and Suspensions
What About Gels?
Many reference sources incorrectly list gel as a type of colloid, describing a gel as a liquid dispersed phase in a solid continuous phase, which is properly called a solid emulsion. In fact, a gel is a type of sol in an intermediate physical phase. The density of a gel is similar to the density of the dispersing liquid phase, but a gel is physically closer to solid form than liquid form. Prepared gelatin is a good example of a typical gel. Dr. Mary Chervenak adds, “I think toothpastes are defined as colloidal gels with viscoelastic properties.”
What differentiates a colloid from a solution or a suspension is the size of the dispersed particles. In a solution, the dispersed particles are individual molecules, if the solute is molecular, or ions, if the solute is ionic. Particles in solution are no larger than one nanometer (nm), and usually much smaller. In a colloid, the dispersed particles are much larger, with at least one dimension on the close order of 1 nm to 200 nm (= 0.2 micrometer, µm). In some colloids, the dispersed particles are individual molecules of extremely large size, such as some proteins, or tightly bound aggregates of smaller molecules. In a suspension, the dispersed particles are larger than 100 nm.
These differing particle sizes affect the physical characteristics of solutions, colloids, and suspensions, as follows:
Solutions and (usually) colloids, do not separate under the influence of gravity; suspensions eventually settle out. In a colloid, the interactions among the tiny particles of the dispersed phase with each other and/or with the continuous phase are sufficient to overcome the force exerted by gravity on the tiny particles of the dispersed phase. In a suspension, the force of gravity on the more massive particles of the dispersed phase is sufficient to cause them to settle out eventually, although it may take a long time for that to occur. (If the particles of the dispersed phase are less dense than those of the continuous phase, as in a mixture of oil dispersed in water, for example, the dispersed phase “settles” out on top of the continuous phase, but the concept is the same.)
Solutions do not separate when centrifuged; neither do colloids, except those that contain the largest (and most massive) dispersed particles, which may sometimes be separated in an ultracentrifuge.
The particles in solutions and colloids cannot be separated with filter paper, but suspensions can be separated by filtering.
Solutions pass unchanged through semipermeable membranes—which are, in effect, filters with extremely tiny pores—while suspensions and all colloids except those with the very smallest particle sizes can be separated by membrane filtration.
Flocculants are chemicals that encourage particulate aggregation by physical means. Adding a flocculant to a solution has no effect on the dispersed particles (unless the flocculent reacts chemically with the solute) but adding a flocculant to a colloid or suspension causes precipitation by encouraging the dispersed particles to aggregate into larger groups and precipitate out.
The particles in a solution affect the colligative properties of the solution, and the particles in a colloid or suspension have no effect on colligative properties.
Solutions do not exhibit the Tyndall Effect, while colloids and suspensions do. The Tyndall Effect describes the scattering effect of dispersed particles on a beam of light. Particles in solution are too small relative to the wavelength of the light to cause scattering, but the particles in colloids and suspensions are large enough to cause the light beam to scatter, making it visible as it passes through the colloid or suspension.
Figure 18-1 shows the Tyndall Effect in a beaker of water to which a few drops of milk had been added. I used a green laser pointer for this image, because the much dimmer red laser pointer that I used when I actually did the lab session proved impossible to 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.”
In this chapter, we’ll prepare various colloids and suspensions and examine their properties.
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 |
Caution
Be careful with the laser pointer. Although standard 1 mW Class 2 laser pointers are reasonably safe to use, you should never look directly into the beam. (And be cautious about specular reflections, too. A beam accidentally reflected off something shiny can be as hazardous as a direct exposure.) Although none of the chemicals used in this laboratory session are particularly hazardous, it’s always good practice to wear splash goggles, heavy-duty gloves, and protective clothing. Discard all food items when you are finished; do not consume them.
Laboratory 18.1 Observe Some Properties of Colloids and Suspensions
In this laboratory session, we’ll use gravitational separation and the Tyndall Effect to test various samples to determine whether they are solutions, colloids, or suspensions.
Procedure
If you have not already done so, put on your splash goggles, gloves, and protective clothing.
Light the incense or joss stick and blow it out. When it starts to produce smoke, place the 250 mL beaker inverted over the incense and allow the beaker to fill with smoke. Use the watch glass to cover the beaker.
Direct the beam from the laser pointer into the beaker and note whether the Tyndall Effect is evident. Allow the beaker to sit undisturbed for at least a minute or two, and then note whether the smoke/air sample separates on standing. Based on your observations, decide whether the smoke/air sample is a solution, colloid, or mixture. Record your observations by circling the appropriate items on line A of Table 18-3.
Rinse the beaker thoroughly. Add about a quarter teaspoon of table salt to about 200 mL of water in the beaker and stir until the salt dissolves. Repeat the procedures in step 3 and record your observations on line B of Table 18-3.
Rinse the beaker thoroughly. Add about 200 mL of club soda to the beaker. Repeat the procedures in step 3 and record your observations on line C of Table 18-3.
Rinse the beaker thoroughly. Add about 200 mL of water to the beaker and then about 20 drops of homogenized milk. Stir until the contents of the beaker are thoroughly mixed. Repeat the procedures in step 3 and record your observations on line D of Table 18-3.
Rinse the beaker thoroughly. Add about 200 mL of water to the beaker and then about 20 drops of vegetable oil. Stir until the contents of the beaker are thoroughly mixed. Repeat the procedures in step 3 and record your observations on line E of Table 18-3.
Rinse the beaker thoroughly. Add about 200 mL of starch water to the beaker. Repeat the procedures in step 3 and record your observations on Line F of Table 18-3.
Rinse the beaker thoroughly. Add about a quarter teaspoon of talcum powder to about 200 mL of water in the beaker and stir until the contents of the beaker are thoroughly mixed. Repeat the procedures in step 3 and record your observations on line G of Table 18-3.
goggles, gloves, and protective clothing | |
eye dropper or Beral pipette | |
beaker, 250 mL (1 or more) | |
watch glass | |
stirring rod | |
matches or lighter | |
laser pointer, red | |
incense or joss stick (1) | |
table salt (~1/4 teaspoon) | |
club soda (~200 mL) | |
homogenized milk (~20 drops) | |
vegetable oil (~20 drops) | |
starch water (~200 mL) | |
talcum powder (~1/4 teaspoon) |
Sample | Tyndall Effect? | Separates on standing? | Classification |
A. Smoke in air | yes / no | yes / no | solution / colloid / suspension |
B. Salt in water | yes / no | yes / no | solution / colloid / suspension |
C. Carbon dioxide in water | yes / no | yes / no | solution / colloid / suspension |
D. Milk in water | yes / no | yes / no | solution / colloid / suspension |
E. Oil in water | yes / no | yes / no | solution / colloid / suspension |
F. Starch in water | yes / no | yes / no | solution / colloid / suspension |
G. Talcum powder in water | yes / no | yes / no | solution / colloid / suspension |
Review Questions
Q: | Q1: What observable physical characteristic allows you to discriminate a colloid from a suspension? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ |
Q: | Q2: What observable physical characteristic allows you to discriminate a solution from a colloid? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ |
Q: | Q3: Lunar gravity is about one-sixth Earth’s gravity. Might a sample that exhibits the characteristics of a suspension on the moon exhibit the characteristics of a colloid on Earth? Why? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ |
Q: | Q4: Some samples are difficult to classify, because their physical properties are intermediate or mixed between the characteristics of solutions, colloids, and suspensions listed in Table 18-2 (for example, they may separate under the force of gravity, but very, very slowly). Why do some samples display such intermediate/mixed properties? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ |
Q: | Q5: Consider a mixture of a solid material in water that clearly exhibits the properties of a suspension. If you created a similar mixture, but using a different continuous medium (such as vegetable oil), might that mixture behave as a colloid? Why? __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ |
Laboratory 18.2: Produce Firefighting Foam
A foam is a colloidal gas phase dispersed in a liquid continuous phase. Foams are commonplace in everyday life. The lather produced by shampoo is a foam, as are sea foam, shaving cream, marshmallows, the meringue in a lemon-meringue pie, and the head on a glass of beer.
Foams are an example of an unstable colloidal system. In the ordinary course of things, colloidal gas bubbles dispersed in a liquid quickly coalesce into larger and larger gas bubbles until the gas bubbles are large enough to be displaced by the liquid phase. If it is to persist longer than momentarily, a foam must be stabilized by the addition of a detergent, soap, protein, or other stabilizer to the mixture. Even when stabilized, a foam inevitably collapses into its component liquid and gas, so in that respect a foam can be thought of as a suspension that takes on the characteristics of a colloid for a short time.
Firefighters use foams made up of a carbon dioxide gas phase dispersed in a liquid water phase. Such foams suppress fires in three ways. First, the carbon dioxide dispersed in the foam does not support combustion and is heavier than air. When a foam layer covers a fire, the carbon dioxide covers and smothers the fire as the water cools it. Second, the foam itself presents a physical barrier that prevents air (and oxygen) from reaching the flame. Third, because the foam is elastic and has very low density, it covers and floats upon any burning solid or liquid. These characteristics mean that foam is effective in fighting nearly any type of fire, including burning oils and fats, for which liquid water simply spreads the fire.
In this lab, we’ll produce a foam of carbon dioxide gas in water. We’ll produce the carbon dioxide by reacting vinegar (acetic acid) and an aqueous solution of sodium hydrogen carbonate (sodium bicarbonate or baking soda), which is represented by the following equation:
CH3COOH(aq) + NaHCO3(aq) → H2O(l) + CH3COONa(aq) + CO2(g)
The carbon dioxide produced by this reaction constitutes the gas phase of the colloidal foam, and the water the liquid phase. We’ll use ordinary liquid dishwashing detergent as the stabilizing agent. And, just to make our foam more attractive, we’ll use food coloring for a festive appearance.
Procedure
If you have not already done so, put on your splash goggles, gloves, and protective clothing.
Transfer about 100 mL of vinegar to the 250 mL beaker.
Add about 1 mL (20 drops) of liquid dishwashing detergent to the vinegar and stir gently until it is thoroughly mixed. (You don’t want to produce a lather, just to mix the vinegar and detergent.)
Transfer about 100 mL of tap water to the 150 mL beaker, and add about 7.5 g of sodium hydrogen carbonate to the water. Stir until the solid dissolves.
Dump the contents of the 150 mL beaker into the larger beaker, and watch what happens (preview in Figure 18-2).
Review Questions
Laboratory 18.3: Prepare a Gelled Sol
A sol is a solid phase dispersed in a liquid continuous phase. Ordinarily a sol is a liquid, but it can be converted to a semi-solid gel by adding a gelling agent. In some cases, the solid phase itself may also serve as the gelling agent.
In this lab, we’ll prepare a flammable gelled sol that comprises ordinary gasoline as the liquid continuous phase with polystyrene plastic serving as both the dispersed solid phase and the gelling agent. This gelled sol is a slightly modified version of the material the military calls Super Napalm B. (Actual Super Napalm B uses low-octane gasoline rather than standard gasoline, and includes a small percentage of benzene, which we’ll leave out because it’s difficult to obtain.)
Procedure
If you have not already done so, put on your splash goggles, gloves, and protective clothing. Verify that there are no open flames or other potential ignition sources nearby, and make sure that you have a fire extinguisher handy.
Use the graduated cylinder to measure 50.0 mL of gasoline and transfer it to the 250 mL beaker.
Weigh 15.0 g of polystyrene. The density of polystyrene foam (Styrofoam or similar) is so low that you need a large weighing boat to contain a reasonable mass. I used a 1-quart plastic kitchen container that comfortably held 15 g of rigid Styrofoam packing material broken into small chunks.
Add a small (thumb-size) chunk of polystyrene to the beaker and observe the reaction. The foam fizzes and appears to dissolve in the gasoline, leaving a small amount of undissolved residue. In fact, what appears to be undissolved residue is the first appearance of the gelled sol.
Continue adding the first 15 g of polystyrene in small chunks, using the stirring rod to press the polystyrene down into the liquid. Note that the gelled sol continues to grow in volume. After you’ve added the first 15 g, the beaker appears to contain mostly gelled sol, but with a significant amount of liquid gasoline remaining.
Weigh out another 15 g of polystyrene, and continue adding it in small chunks to the beaker, with stirring. When you’ve added a total of 30 g of polystyrene, the sol appears to have “soaked up” nearly all of the liquid gasoline, as shown in Figure 18-3.
Caution
To state the obvious, we’re making napalm, or at least something very close to napalm. Napalm burns furiously, sticks to anything it touches, and is very difficult to extinguish. Use extreme caution when preparing the sol. Have a fire extinguisher ready, and make absolutely sure that there are no open flames, sparks, or other potential ignition sources nearby. Work outdoors if possible, or at least in an area with excellent ventilation. Do not ignite the product indoors. Wear splash goggles, heavy-duty gloves, and protective clothing.
Weigh out a final 5 g of polystyrene foam, and continue adding it in chunks to the beaker, using the stirring rod to make sure that the foam you add is incorporated in the gelled sol. After a total of 35 g of polystyrene foam has been added, the contents of the beaker will appear to be completely gelled, with no liquid visible. At this point, the gelled sol is semi-rigid, enough so that it resists the force of gravity if the beaker is inverted, as shown in Figure 18-4. (Don’t invert the beaker indoors, as the gelled sol might fall out unexpectedly.)
If you are not already outdoors, take the beaker to a safe outdoor location with a nonflammable surface such as dirt, gravel, or concrete. (Remember that asphalt, made from tar, is flammable.) Invert the beaker, and wait a few moments to see whether the gelled sol separates from the beaker. If not, tap the beaker gently to release the gelled sol. When the gelled sol separates, note that it retains its form, as shown in Figure 18-5. There should be little or no liquid gasoline remaining. (The stain visible to the right of the gelled sol in Figure 18-5 is a small amount, probably less than 1 mL, of liquid gasoline that was not incorporated in the sol.)
Make sure that you have a fire extinguisher handy, and verify that there are no children or pets in the vicinity (or indeed, any other life forms who are unaware of what’s going on). After you have verified that it is safe to do so, note the time and use the lighter or match to ignite the napalm. While the napalm burns, as shown in Figure 18-6, note your observations, including the appearance and intensity of the flame, whether the flame spreads or stays in one place, how long the flame continues, any unusual odor, and so on.
After the napalm has finished burning and you have allowed the surface to cool, repeat the burning test using 50 mL of liquid gasoline. (Use extreme care when igniting liquid gasoline, and don’t do it in your glass beaker.) Once again, make sure that you have a fire extinguisher handy, and verify that there are no children or pets in the vicinity. After you have verified that it is safe to do so, note the time and use the lighter or match to ignite the gasoline. While the gasoline burns, note your observations, including the appearance and intensity of the flame, whether the flame spreads or stays in one place, how long the flame continues, any unusual odor, and so on.
