In this explainer, we will learn how to describe suspensions and colloids and explain their properties.
Let’s imagine we have two glasses of water. Into one, we add a spoonful of sugar, and into the other, we add a spoonful of sand. What is the difference between these two mixtures? The spoonful of sugar will dissolve in water to form a type of mixture called a solution. The molecules of sugar are separated by the water molecules and distributed evenly throughout the solution.
On the other hand, the sand does not dissolve in water. We can stir the sand mixture to spread the sand out, but over time, the sand will settle at the bottom. We call this kind of mixture a suspension.
A third type of mixture is called a colloid. One example of a colloid is milk, where globules of fat and protein are spread evenly throughout water. These particles are bigger than an individual sugar molecule, but smaller than a grain of sand. Let’s compare the compositions, particle sizes, and stability of these kinds of mixtures.
A suspension is a type of heterogeneous mixture with particles large enough to settle out of the solvent.
A colloid is a type of mixture where small particles of one substance are suspended and distributed throughout a continuous medium.
A solution is a homogeneous mixture of one or more substances dissolved in another substance.
|Mixture Type||Composition||Particle Size||Stability||Visibility|
|Solution||Homogeneous||0.01–1 nm||Stable||Neither naked eye |
|Colloid||Homogenous by appearance,|
heterogeneous by composition
|1–1 000 nm||Stable||Microscope|
|Suspension||Heterogeneous||nm||Unstable: particles will |
settle out of solution
Suspensions, like sand in water, are by definition heterogeneous, meaning they have an uneven composition. While we can stir the mixture to give it the appearance of homogeneity, over time, the sand will settle out into the bottom of the container. Suspensions have large particles of approximately 1 000 nm or greater in size.
Solutions like sugar dissolved in water are by definition homogeneous, meaning they have an even composition. Since the sugar molecules are separated and evenly dispersed across the water, any region of the solution we look at will have the same composition as any other. Unless we change the temperature, concentration, or other characteristics of the solution, the sugar will remain dissolved in the water. The particles in solutions are typically individual atoms, ions, or molecules. These particles are much smaller than the clumps or grains found in suspensions.
Depending on how we look at them, colloids may be heterogeneous or homogeneous. At first glance, a glass of milk might seem homogeneous, as the substance has a consistent color and composition throughout. However, at the microscopic level they are often heterogeneous in composition.
If we look at milk on a microscopic scale, we will see certain regions containing droplets of fat and protein, and other regions containing only water. Unless the milk undergoes a physical change, like heating, or a chemical change, like spoiling, the colloid will remain stable. The particles in a colloid are typically between 1–1 000 nm and are made up of large molecules or aggregations of molecules.
Example 1: Comparing Particle Size in Suspensions, Colloids, and Solutions
Order the following mixtures according to the size of the particles found in them, from smallest to largest: suspensions, colloids, solutions.
This question is asking us to determine the relative particle sizes of these three types of mixtures. Knowing the definitions of these mixture types will help us determine the particle size.
The solute of a solution is dissolved on an atomic scale. Typically, individual atoms, molecules, or ions of the solute are separated by the solvent. These particles are less than 1 nm in size.
The dispersed phase of a colloid typically forms particles consisting of macromolecules or aggregations of molecules. They are evenly distributed throughout the dispersion medium. These particles are typically between 1–1 000 nm in size.
The particles of a suspension are typically grains or chunks of a substance. These particles are often large enough to be distinguished with the naked eye. These particles are over 1 000 nm in size, the largest of the three choices.
Therefore, from the smallest particle to the largest, the correct order is: solution, colloid, suspension.
In colloids, the particles that are suspended are known as the “dispersed phase.” The phase or medium that the particles are distributed through is called the “dispersion medium.”
For milk, both the dispersed phase and the dispersion medium are liquids, but the dispersed phase or dispersion medium could each be a solid, liquid, or gas.
Definition: Dispersed Phase
The dispersed phase of a colloid is the substance that is evenly distributed throughout a continuous, or dispersion, medium.
Definition: Dispersion Medium
The dispersion medium of a colloid is the phase or medium that a substance, known as the dispersed phrase, is evenly distributed into.
As we mentioned before, both the dispersed phase and the dispersion medium of a colloid can be solid, liquid, or gaseous. The table below shows the names and examples of colloids with different phases of the dispersion medium and the dispersed phase.
|Name and Example of Colloid||Dispersed Phase|
|Dispersion Medium||Solid||Solid sol|
Dust in air
With a solid dispersion medium, we can form a colloid with dispersed gas to create a solid foam, which includes lightweight material such as Styrofoam. A liquid dispersed through a solid can create gels such as gelatin or agar jelly. A solid dispersed through a solid is known as a solid sol and includes colored glass.
A solid dispersed through a liquid is known as a sol and can be used to produce paints and inks. An emulsion is a liquid dispersed through a liquid and includes milk or mayonnaise. The dispersion of gas through a liquid can form foams like shaving foam and low-density creams.
With a gaseous dispersion medium, we can have dispersed solids in the form of solid aerosols, such as smoke, or dispersed liquids in the form of liquid aerosols, such as mists or fog. No gas–gas colloids exist, as any two gases will mix together at the atomic or molecular level and form solutions, not colloids.
Example 2: Identifying the Solid Foam Among Colloids
Which of the following substances or phenomena is an example of a solid foam: a gas dispersed through a solid?
- Whipped cream
This question is asking us to identify the colloid among the choices that is a gas dispersed through a solid. More specifically, we are looking for the answer choice that has a gaseous dispersed phase that is evenly distributed throughout a solid dispersion medium. By identifying the dispersed phase and dispersion medium of each choice, we can pick the correct answer. We can construct a table to note the dispersed phase and dispersion medium for each of the options in the question.
|Substance||Dispersed Phase/Solute||Dispersion Medium/Solvent|
|Whipped cream||Gas (air)||Liquid (cream)|
|Smoke||Solid (particulates)||Gas (air)|
|Air||Gas (many gases)||Gas (nitrogen)|
|Marshmallow||Gas (air)||Solid (sugar)|
The one answer choice that has a gaseous dispersed phase distributed throughout a solid dispersion medium is a marshmallow. The name “solid foam” applies to this type of colloid.
Note that since the particles in air are dissolved on an atomic or molecular level, it is technically a solution of gases and not a colloid. Because of this distinction, we refer to the gases in air as the solute and solvent of the solution.
The correct answer is option D, marshmallow.
One important property of solutions, colloids, and suspensions is their stability. While colloids and solutions will remain mixed together, suspensions will eventually separate into their constituent parts. Consider the suspension of water and sand from earlier. We can stir the sand to temporarily disperse it throughout the water, but over time, the sand will settle out at the bottom of the container. In contrast, a colloid, like paint, will remain stable.
Colloids are by definition stable mixtures. However, we can change the stability of the colloid by changing some of its properties. While there are many factors that contribute to stability, the most significant one is particle size. Larger particles are less stable and more likely to separate. For example, since dairy companies want their milk to last a long time, they sometimes use a process called “homogenization,” which breaks the fat droplets down into smaller sizes. Smaller particles result in a colloid that is more stable and make the fat less likely to separate out as cream rising to the top.
Another key contributor to colloid stability is the particle–particle interactions of the dispersed medium. Namely, strong particle–particle interactions can disrupt the stability by causing the dispersed phase to aggregate and separate out. For example, the process of making cheese from milk involves using an enzyme to allow fats and proteins to bind together with a strong attraction. When these fat and protein particles combine, they form larger particles, which then separate out from the colloid in the form of cheese curds.
The strengths of the particle–particle interactions in a colloid might change as we alter the temperature, pH, or concentration of the solution. For this reason, colloids are often manufactured and stored in particular ways to preserve their stability.
Many colloids are thixotropic mixtures. A thixotropic mixture is a substance whose viscosity reduces as we stir or shake it. A good example is ketchup, which can often stick in the bottom of the bottle as a viscous fluid. If we shake or tap the bottle, the shear force applied to the ketchup can reduce its viscosity, causing it to liquify and flow out of the container.
Definition: Thixotropic Mixture
A thixotropic mixture is a mixture that is thick or viscous at rest but becomes fluid when shaken or otherwise agitated.
Example 3: Identifying the Phases of a Colloid
Colloids consist of two phases: a particulate phase and the state of matter these particles are spread between. What names are given to these two phases?
- System phase and surrounding phase
- Low-concentration phase and high-concentration phase
- Solute phase and solvent phase
- Dispersed phase and dispersion medium
- Solid phase and liquid phase
This question is asking us to identify the general names for the two components of a colloid.
Option C may be familiar, but it is incorrect. Solute and solvent are the names given to the components of a solution.
Options B and E are incorrect as well. Either component of the colloid can be a liquid, a solid, or a gas and can be of high or low concentration, so we cannot distinguish the two components in these ways.
The terms in option A are not relevant to colloids.
The correct answer is option D, dispersed phase and dispersion medium. The dispersed phase is evenly distributed throughout the dispersion medium.
If we come across a stable mixture, we might wonder how we can determine if it is a solution or a colloid. One approach relies on something called the Tyndall effect. The Tyndall effect describes the scattering of light that occurs when it passes through a colloid or a fine suspension.
The Tyndall effect relies on the fact that the particles in colloids are of approximately the same size as the wavelengths of visible light, approximately around hundreds of nanometers. This size similarity means that light with shorter wavelengths will collide with particles more frequently and scatter more intensely than light with longer wavelengths. In solutions, the particles are too small to deflect light.
The scattering of light in the Tyndall effect is visible in two main ways. First, since the colloid scatters light in all directions, the light beam is visible in the colloid itself. Passing a beam of light through a solution is only visible on surfaces the light beam strikes. The second visible pattern of the Tyndall effect is the tinting of light. Since light with a shorter wavelength such as blue light is scattered more intensely, scattered light to the side of the beam will have a bluish tinge. Conversely, since light with a longer wavelength such as red light is scattered less intensely, light that passes directly through the colloid will have a reddish tinge.
An everyday example of the Tyndall effect is the visible beam of car headlights in fog. While the coloration of this light is not clearly visible, we can see that the colloid of water suspended in air scatters light to make the beam visible. When passed through the solution of air, the beam is only visible directly from the bulbs or on the surfaces it strikes.
Example 4: Identifying a Solution from the Behavior of Light Passing through It
Which of the following diagrams shows a beaker containing a solution?
This question is asking us to identify the picture that represents light passing through a solution. The difference between the three pictures is in the visibility of the light beam in the beaker. To answer this question, we need to understand the Tyndall effect.
The Tyndall effect is the scattering of light that occurs when it passes through a colloid or a fine suspension. The scattering of light makes the beam of light visible within the colloid. When light passes through a solution, the Tyndall effect does not occur, so the beam of light remains invisible in the solution.
The picture with an invisible beam of light passing through the beaker is choice A, so the correct answer is A.
There are various ways to produce colloids. These methods largely fall into two categories: dispersion and condensation.
Dispersion methods involve taking larger particles, breaking them down into smaller particles between 1 and 1 000 nm in diameter, and forming a colloid. Burning wood to make smoke, grinding corn to make cornstarch and adding it to water, and grinding pigments to mix into paint are all examples of forming colloids by dispersion.
Condensation methods, on the other hand, rely on taking small particles and making them bigger, typically with particle sizes less than 1 000 nm. An example of condensation is the formation of fog or clouds. The small water droplets in the air combine to form larger particles that form a visible colloid.
In a laboratory or factory, there are a variety of ways to form colloids. To form a colloid through dispersion, an industrial process might use an electric current or a peptizing agent to split up and distribute large particles.
To form a colloid through condensation, an industrial process might change the state of a substance with a carefully controlled method. They might also carry out a chemical reaction that results in the formation of aggregations of molecules.
One common example is the formation of colloidal sulfur through the condensation method. The chemical equation for this process is the following:
- Solutions, colloids, and suspensions are all types of mixtures.
- Solutions, colloids, and suspensions differ in their particle size, homogeneity, and stability.
- The components of a colloid are the dispersed phase and the dispersion medium.
- The dispersed phase is the name for the particles that are evenly distributed throughout the dispersion medium.
- The dispersed phase and dispersion medium can each be a solid, a liquid, or a gas.
- Aerosols, sols, gels, foams, and emulsions are names for specific combinations of phases that make up colloids. For example, a gel is a liquid dispersed through a solid.
- The Tyndall effect describes the scattering of light that occurs when light passes through a colloid. It results in a visible cone of light as well as colored tinges of light in different directions.
- Colloids can be prepared through condensation (making small particles bigger) or dispersion (making big particles smaller).