Professor Dave here, I wanna tell you about colloids. We’ve learned quite a bit about different
kinds of mixtures. For example, we’ve learned about homogeneous
mixtures involving ionic or covalent solids that disperse evenly in water or some other
solvent to form a solution. We have also seen certain heterogeneous mixtures
where the components do not mix whatsoever, like oil and water. More often than not, a discussion of mixtures
will end there. But the reality is that there are other types
of mixtures that do not fall strictly into one of these extremes, but rather sit in between,
so let’s find out what these could be. One such type of mixture is called a suspension. This is a heterogeneous mixture composed of
large particles that are visible to the naked eye. An example of this can be generated by mixing
mud and water. They will not remain completely separate,
like oil and water do, nor will things disperse perfectly evenly. Instead, there will simply be a cloudy mix
of water and suspended mud particles. We can also create a mixture that is in between
a suspension and a homogeneous solution. This will usually form because the particle
size is larger than ions or small molecules, but smaller than the particles in a suspension. Such a solution is called a colloid, or a
colloidal dispersion. To illustrate the difference between a solution,
a colloid, and a suspension, let’s look at saltwater, milk, and mud. The difference in particle size is what determines
the different characteristics of these mixtures. The particles in a colloid are large enough
to scatter light, which is a phenomenon called the Tyndall effect. This is why colloidal mixtures will appear
cloudy or opaque, the way that particles in the air are visible inside the beams of bright
lights, which is what illuminates big event spotlights. Colloids like fog and smoke are what make
these beams so bright, due to their scattering ability. Some colloidal particles consist of hundreds
or thousands of molecules, but some are just one large molecule, like a protein. And when we discuss colloids, instead of solute
and solvent, we will call the particulate component present in small amounts the dispersed
phase, and the substance the particulate is dispersed in will be the dispersion medium. The dispersed phase and the dispersion medium
can each be of any phase. Smoke is solid dispersed in gas. Clouds, fogs, and mist involve liquid dispersed
in gas. Milk, mayonnaise, and butter involve liquid
dispersed in liquid. Jellies and gels are liquids dispersed in solids. Foams and whipped cream involve gas dispersed
in liquid. So we can see that there are quite a wide
variety of colloids. Now that we know what colloids are, let’s
discuss how they can be prepared. In order to prepare a colloid, we must first
produce particles of colloidal dimensions, which means they must be above a size that
would generate a solution but below a size that would generate a suspension, and then
we must distribute these particles throughout a dispersion medium. There are two ways we can prepare these particles. The first is by breaking down larger particles. This is how many paint pigments are produced. The other way is by combining smaller molecules. This is how clouds form. Water molecules high up in the atmosphere
where it gets very cold, are able to condense enough to form tiny droplets that then aggregate. If the water droplets become too large to
remain in the cloud, we get precipitation, which is what causes rain, sleet, and snow. There are also some solid substances that
spontaneously form colloidal systems when mixed with water, like gelatin, or starch. These substances contain particles that are
already of colloidal size, the water simply disperses them. Another type of colloid is called an emulsion. This occurs when two immiscible liquids are
forcibly blended. One liquid will break into colloidal droplets
and disperse throughout the other liquid. This is why oil spills in the ocean are hard
to clean up, the action from waves causes emulsions to form. But sometimes, emulsions will coalesce and
separate from the other liquid. To combat this tendency, an emulsifying agent
can be used. This is a substance that inhibits the coalescence
of the dispersed liquid. Milk and mayonnaise are examples of emulsions. Milk is an emulsion of butterfat in water,
with a protein called casein acting as the emulsifying agent, and mayonnaise is an emulsion
of oil in vinegar with egg yolk components acting as the emulsifying agents. Sometimes dispersed colloidal particles will
bear an electrical charge. In any particular system, all the colloidal
particles will have the same charge, which helps keep the particles dispersed, since
like charges repel one another. We take advantage of the formal charges on
colloidal particles to remove them from a variety of mixtures. We can place charged electrodes in a colloidal
dispersion and the charged colloidal particles will migrate to one of the electrodes. Once this occurs, the colloidal particles
lose their charge and consequently will coagulate as a precipitate. This is how we remove the carbon and dust
particles in smoke. If smoke passes through something called an
electrostatic precipitator, the charged particles in the colloid will be attracted to electrodes
where they will be neutralized and then deposited as dust. This method is used in large-scale industrial
processes, as well as simple air filters designed for home use. So now we know the difference between solutions,
colloids, and suspensions, as well as quite a few things about different types of colloids
and their properties.