What Are Colloids?
Colloids are a type of substance. The word "colloid" stems from
the Greek word kolla, which means “glue.” English chemist
Thomas Graham first coined the term "colloid" when working
with the chemical properties of glue. Graham introduced a
whole new field of chemistry that is known as colloid chemistry.
Since Graham’s time, many other colloids have been
recognized. Butter, milk, fog, aerosol sprays, smoke,
shaving cream, soaps, egg whites, gels, and jellies
are all examples of colloids.
Colloid Components
Unlike elements and compounds, colloids are not
pure substances; they are actually mixtures
containing particles of various sizes. Mixtures can
either be homogeneous or heterogeneous. In a
homogeneous mixture, the particles are all similar
in size and composition. The particles are spread
evenly throughout the mixture, making it difficult
to see them individually. Solutions such as
saltwater and air are examples of homogenous
mixtures. On the other hand, heterogeneous
mixtures consist of particles of different sizes and
composition that are not spread evenly throughout
the mixture.
Since colloids are made up of particles with varying
sizes, they are considered heterogeneous mixtures.
Colloids are distinguished from solutions and other Fog, whipped cream,
mayonnaise, and smoke are
types of heterogeneous mixtures, such as
all forms of colloids.
suspensions, by the size of their particles. A
colloid’s particles range in size from 1 to 1,000 nanometers. A
nanometer is one billionth of a meter, which is an incredibly
small length. A piece of paper is about 100,000 nanometers
thick, and a very fine strand of hair is 10,000 nanometers thick.
Other items that are measured on the nanoscale are DNA and
viruses. The particles of a colloid are larger than those in a
solution but smaller than those in a suspension.
The particles of a colloid indicate its properties. Since the size of
colloid particles is between those of solutions and suspensions,
colloids share some of the properties of both of these types of
mixtures. Many colloids are not transparent, or "see-through."
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What Are Colloids?
Many colloids look hazy or milky when they are concentrated.
Unlike many solutions, the appearance of colloids is similar to
the appearance of suspensions. However when a colloid is
diluted, the colloid can be nearly translucent, which is more
similar to a solution. Also, like a solution, colloid particles are
too small to be filtered out with filter paper.
Due to their hazy appearance, colloids scatter light. This ability
is known as the Tyndall effect, named for its discoverer, 19thcentury British physicist John Tyndall. Normally the sun’s rays
cannot be seen in the sky, but if the sun’s rays pass through a
cloud, a spray of mist, or dust in the air then the light can be
seen. This is known as the Tyndall effect. If a beam of light
passes through a colloid, the particles in the mixture scatter the
light in different directions and the light can be seen. Although
the particles in a colloid are tiny, they are still large enough to
exhibit the Tyndall effect. Suspensions can
also scatter light, but the smaller particles of
solutions cannot.
Colloids are always made up of particles
dispersed in some medium. There are many
different combinations of particles and
mediums. For example, whipped cream is
made up of gas particles dispersed in a
liquid. Marshmallows are similar; they
consist of gas particles dispersed in a solid.
Fog and aerosol sprays are made up of tiny
liquid particles spread through a gas (air).
Gelatin, hair gel, and egg whites combine
solid particles in liquid mediums.
A beam of light scatters in the fog. Fog is a
colloid so it exhibits the Tyndall effect.
Emulsions
A special type of colloid exists when liquid particles are
dispersed in a liquid medium. This is called an emulsion.
Emulsions are not naturally occurring. They are made from
liquids that do not normally mix, such as oil and water. For this
reason, there must be an emulsifier present to hold them
together. Emulsifiers include egg yolks, soaps, mustard, and
several types of proteins. Milk, butter, and salad dressings are
also examples of emulsions. The most common one, however, is
mayonnaise. The main ingredients in mayonnaise are oil,
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What Are Colloids?
vinegar, and eggs. Oil and vinegar are two immiscible liquids,
which means that oil and vinegar do not mix together. If oil and
vinegar are shaken, these ingredients will mix temporarily but
will eventually separate into layers again. However, when the
emulsifying agent in eggs is added to the mixture, the vinegar
and oil stay combined. The result is the formation of a colloid
called mayonnaise.
How does an emulsifier do this? The reason that oil and vinegar
cannot mix is because oil is nonpolar while vinegar is polar.
Polar and nonpolar substances cannot mix together. They repel
one another and form separate layers when they are combined.
An emulsifier, like egg yolk, changes this when it is added to the
mixture. One end of the egg yolk molecule is polar. The other
end is nonpolar. The polar end mixes with the vinegar, while the
nonpolar end dissolves in the oil. The emulsifier holds the entire
mixture together.
Concerns About Colloids
The discovery of colloids is just the beginning of this exciting
field. More and more research is going into discovering these
unique mixtures. But there are concerns. Some people have
used a colloidal form of silver to help fight disease, which
resulted in serious side effects. Colloidal silver consists of tiny
particles of silver dispersed in liquid. There is no evidence that
silver fights disease or serves any purpose in the human body.
However, the side effects of taking colloidal silver are quite
serious. The U.S. Food and Drug Administration (FDA) does
“not consider colloidal silver to be safe or effective for treating
any disease or condition" and has issued a warning regarding its
safety. Colloidal silver can cause serious side effects, such as
argyria. Argyria is a condition in which a person’s skin turns
blue.
Scientists need to continue their study on colloids. Colloids are
unique substances and may have many uses in space, the oil
industry, engineering, and medicine, if used safely.
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