Video Transcript
In this video, we will learn about
colloids and suspensions and explore some of their properties. The starting point for
understanding suspensions and colloids is understanding mixtures. A mixture is simply the combination
of two or more different chemical substances that aren’t reacted together. In a chemical mixture, the
substances generally retain their original chemical behavior. Mixtures can be homogeneous or
homogeneous with a composition that’s even throughout. Or they can be heterogeneous with a
composition that varies depending on where you look.
Another term we use when talking
about mixtures is the word “stable.” Mixtures can be stable with a
composition that’s consistent over time or unstable, where the composition changes
with time. Suspension and colloid will get
more words we use to describe certain types of mixture. But before we look at these
categories, let’s recap what a solution is.
A solution is a mixture of one or
more substances dissolved in another substance. The dissolved substances are known
as the solutes, and the substance that they’re dissolved in is called the
solvent. If we have two or more substances
that are mixed together on the atomic scale, we have a solution. We’re familiar with solutions like
soda, ocean water, and nail varnish remover. But solutions don’t need to be
liquids, but we’ll look at some more examples later.
Colloid and suspension become
important terms when we go beyond the scale of a few atoms, and the lumps of
substance get a bit bigger. Colloid is the name we give to a
mixture where we have slightly larger lumps of substance in a continuous medium of
another. Typically, the diameter of the
particles in a colloid will be between one and 1000 nanometers. By definition, colloids are
homogeneous; they have a consistent composition throughout. When we’re talking about colloids
and suspensions, we do need to zoom out a little when we’re talking about
composition. We’ll take an average over a
slightly larger area than when we’re talking about solutions. And by definition, colloids are
also stable. They do not on their own separate
out. We can destabilize a colloid in
some ways, but we’ll come to that later.
The substance or substances that
are spread in microscopic lumps throughout the medium are called the dispersed
phase. And the continuous medium that
surrounds them is called the dispersion medium. So the dispersed phase is dispersed
throughout the dispersion medium. We’ll look at some examples in a
moment. The terminology for a suspension is
very similar. We still have a dispersed phase and
a dispersion medium, but the particles tend to be a bit bigger. The key difference between a
suspension and a colloid is that suspensions are unstable. The lumps of substance eventually
clump together and form areas of high concentration, leaving areas of low
concentration behind. Therefore, suspensions are also by
definition heterogeneous. The composition of the mixture is
not consistent when we look in different places.
Next, we’ll look at some examples
of solutions, colloids, and suspensions. When we make mixtures, we generally
mix a solid, a liquid, or a gas with another solid, liquid, or gas. As we populate this table,
solutions will go into the bottom, colloids will go in the middle, and suspensions
will go in the top. So along the top, we either have
the solute or the dispersed phase. And along the left-hand side, we
have the state of this solvent or dispersion medium. Solids tend to be stable over long
periods, so we don’t tend to think about solid–solid mixtures being suspensions but
they can be colloids. For instance, colored glasses can
be made by introducing very small particles of other materials into glass. And it is possible to have
solid–solid solutions. For instance, alloys will be made
of different types of metal blended together on the atomic scale.
Next, we’ll look at liquid–liquid
mixtures. Oil and water are two liquids that
don’t naturally mix together. However, if you shake them up, you
can get tiny droplets of one phase in the other. Milk is a colloid and various fats
and proteins suspended in water. We can call milk a colloid because
it actually undergoes chemical changes when it goes sour. The stuff we start with can be
considered stable on its own. And our example of a liquid–liquid
solution is vinegar, which is a mixture of ethanoic acid or acetic acid and
water. There’re a few varieties of gas–gas
mixture. By definition, a gas is made of
separated particles, so we can only form a solution and not a colloid or a
suspension.
We can describe air as a few
different gases dissolved in nitrogen. We consider nitrogen the solvent
simply because it makes up the majority of the air. What about the other
combinations? Blood is a good example of a solid
dispersed in a liquid. Various types of cell are suspended
in water, but they will separate out over time. We can see this when we centrifuge
blood, separating it into various layers. Meanwhile, inks are good examples
of solid–liquid colloids. They are often made of solid
particles dispersed in water. And for our solution, we have sugar
water, sugar which is normally a solid dissolved in water. But we can, of course, have
liquid–solid mixtures.
Suspensions of liquids in solids
aren’t that common, but there are examples of colloids like jello, water dispersed
in gelatin. And we can also have liquid–solid
solutions like mercury mixed in with solid gold. Next, we have gases in liquids. We can get suspensions of liquid in
gases by using a spray bottle, but we can also form liquid–gas colloids like
mist. Under the right conditions, very
small droplets of water are stable in the air. And the only way of making a
liquid–gas solution would be to evaporate the liquid, turning into a gas, so that
doesn’t count.
Instead, let’s look at gas–liquid
mixtures. In bubble bath, we get foam, which
is thin films of soap and water surrounded by air. But those bubbles quickly
collapse. If we want a stable foam, we need
smaller bubbles like those in shaving foam or whipped egg whites. And for our gas–liquid solution, we
have carbonated water, carbon dioxide dissolved in water. And finally, we have the slightly
less common mixtures of solids and gases. Dust in the air is a solid–gas
mixture that’s unstable. It’s a suspension. But finer particles like those in
smoke can form colloids, which are stable. But just like with liquid–gas
mixtures, we can’t have a solid–gas solution.
We don’t tend to think of gas–solid
suspensions. Instead, the solid is considered a
stable structure. So we can form colloids, a good
example of which is plastic foam. And it is possible to form
gas–solid solutions. For instance, palladium is a heavy
metal that will absorb hydrogen gas. These are just some examples, and
some people may disagree about which ones are actually colloids or suspensions. When we have a solid suspended in a
gas, it can have a special name, a solid aerosol. And mist is an example of a liquid
aerosol. Collectively, these are known as
aerosols.
Next, we’re going to look at some
of the key properties of colloids and suspensions. Water is colorless, and fats and
oils tend to have very little color, maybe a little yellow. So why is milk, which is a mixture
mainly of fat and water, white? Milk does contain other substances
besides fat and water, but none of these are strongly colored. Milk is a colloid with globules of
fat suspended in a continuous medium of water. The water is the dispersion medium,
and the fat and other chemicals are the dispersed phase. When light passes through milk, the
light is scattered. This means all the colors of light
hitting the milk are averaged out, and we get something that looks white.
Now, the Tyndall effect is a
special phenomenon we see with colloids and some very fine suspensions, but this is
not the Tyndall effect. Instead, the Tyndall effect is
about how different wavelengths of light interact slightly differently with a
colloid or suspension. The Tyndall effect is something we
see if we have particles in the range of 40 to 900 nanometers, fairly close to the
wavelengths of visible light. Shorter wavelengths, like blue
light, scatter more when they interact with these colloids or suspensions.
If we light milk with a white light
from the side, a greater proportion of the red light will go straight through. And if we look from the side, we’ll
see that the milk appears slightly blue because a greater proportion of the blue
light is scattered out the side. This is evidence of the Tyndall
effect. Different wavelengths of light are
being scattered to different degrees, producing a different color than pure
white.
So if you see a blue tinge in your
milk in the fridge, it’s probably not because it’s off. It’s just the Tyndall effect. We can use the Tyndall effect to do
a quick check of whether a liquid is a solution, a suspension, or a colloid. If light isn’t absorbed by the
solution, it will pass straight through. On the other hand, if it’s a
colloid, the light will be scattered and the liquid will appear white with a blue
tinge if we’re viewing it off-axis. And if we have a suspension with
large enough particles, the light will still be scattered, but we won’t get the
Tyndall effect, so we won’t get a blue tinge.
The next effect we’re going to look
at is called thixotropy, which is the property of becoming less viscous the longer
stress is applied. This is an apparently strange
property that happens when colloids and suspensions are stirred. To start off with, stirring can be
difficult. But after a bit more stirring, the
mixture becomes easier to stir as it gets less viscous. The initial thickness is caused by
the fact that particles in the suspension or colloid will stick together a little
and form regular structures. As you stir, you start to break up
these structures, making it easy for the particles to flow in and around each
other. This is why shaking a ketchup
bottle makes it easier to get the ketchup out. It helps it flow better.
Now, the last general property of
colloids and suspensions we’re going to look at is stability. We’ve already mentioned that
suspensions are unstable and colloids are stable. However, we haven’t explored
why. Firstly, unstable means that a
mixture will eventually separate. It doesn’t tell us how long it will
take. Technically, any particle
dispersion that eventually separates is a suspension. The stability of a particle
dispersion will depend on many things, some of which are more subtle than others:
the particle–particle interactions, the particle–solvent interactions, the
solvent–solvent interactions, and so on. But here we’re going to look at the
two main features.
If we take a particle dispersion
and we make the particles larger, we’re going to make it easier for those particles
to stick together because it’s harder to keep them suspended. This will make the suspension more
unstable. And if we have exactly the same
type of particle but we change their surfaces so they attract one another more
strongly, they’ll stick together and fall out of solution. The mixture will be less
stable. If we want to make a dispersion
more stable, we can charge up the particles, making them repel each other. If they have the same type of
charge, the more charge they have, the more they’ll repel each other. We can affect the stability of
particles by changing the temperature, the concentration of salt in solution, or
changing the pH, for instance.
The last thing we’ll look at are
the two types of method for preparing colloids and suspensions. Dispersion methods involve taking
larger lumps of substance and breaking them into smaller particles, which can then
be dispersed. And condensation methods involve
taking smaller particles or dissolved substances and making them grow into larger
particles. We can start with a solution or a
colloid and produce a colloid or a suspension. We can apply the dispersion method
to ground up starch, adding it to water to produce a suspension or a colloid. And we can see the condensation
method in action when we bubble carbon dioxide gas through a solution of calcium
hydroxide, otherwise known as limewater. A fine suspension of calcium
carbonate in solid form is produced. This process also produces water,
but I’ve left it out of the equation.
Now we’re going to look at a
practice question.
Which of the following substances
or phenomena is an example of a solid aerosol, a solid dispersed through a gas? (A) Mist, (B) fog, (C) paint, (D)
air, or (E) smoke.
The key phrase in the question is
solid aerosol, which, thankfully, has already been defined. It’s a solid dispersed through a
gas. Dispersed here means spread
out. Here we have a box that contains a
lump of solid surrounded by gas. The solid is concentrated in one
lump. It’s not dispersed. Instead, we’re looking for tiny
particles of solid spread throughout the gas.
Now, let’s look at our five options
and see which one matches. Mist consists of fine droplets of
water suspended in air, while fog is exactly the same thing but much denser, so you
can’t see as far. Water under these conditions will
be a liquid not a solid. So we’re dealing with a liquid
aerosol and not a solid aerosol, so these are not correct answers. Meanwhile, paints usually consist
of solid particles of pigment suspended in oil or water. So paints are solid–liquid
mixtures, and they may be colloids or suspensions. Either way, the dispersion medium
for paint is not a gas. It’s a liquid, so paint is not a
correct answer.
The fourth option, air, is a
mixture of many different gases. So we consider this a gas mixture,
although you could call it a gas solution because there’s a lot more nitrogen than
the other gases. Air is not a correct answer because
there’s no solid dispersed through it. We’d have to have dusty air or
smoky air for it to work. And lastly, we have smoke, which
consists of fine particles produced from combustion dispersed in air. So out of these five options, the
only one that’s an example of a solid aerosol, which is a solid dispersed through a
gas, is smoke.
Now, let’s look at the key
points. Suspensions and colloids are
mixtures consisting of a dispersed phase in a dispersion medium. The dispersed phase consists of
individual bubbles or particles spread out in a continuous solid, liquid, or gas
phase. Any mixture like this that’s
unstable and therefore heterogeneous is considered a suspension. Unstable means the mixture
separates over time, and heterogeneous means that the composition is uneven. And this type of mixture that’s
also stable and homogeneous is a colloid.
Suspensions and colloids are
thixotropic, meaning as we stir them, they get thinner and thinner. Colloids and some fine suspensions
exhibit the Tyndall effect, where they scatter some frequencies of light more than
others. It’s this effect that can give milk
a blue tinge. And the two methods to produce
these substances are the dispersion method, where we first make the fine particles
and then spread them throughout the dispersion medium by mixing. And then there’s the condensation
method, where we use chemical reactions to make particles from solution or from
smaller particles.
