What Are Colloids?

The Tyndall Effect in Colloids

Core Concepts

In this article, you will learn about colloids – one of the major kinds of mixtures. Beyond defining colloid mixtures, this article will cover unique colloidal properties, how to identify a colloid mixture, and applications of colloids in everyday life. Learn more about an essential mixture to lifeforms – colloids!

Topics Covered in Other Articles


A colloid, or a colloidal solution, is a mixture consisting of molecules or particles dispersed in solution. Unlike the other two primary types of mixture, solutions and suspensions, colloids contain particles that are evenly distributed throughout the solution. Moreover, the particles suspended/dispersed belong to a different substance than the solution they are suspended in. Because colloids consist of particles dispersed in another substance, the particles are called the dispersed phase while the substance the particles are dispersed in are called the continuous phase.

Dispersed Medium vs. Dispersed Phase in Colloids
Dispersed Medium vs. Dispersed Phase in Colloids

These particles are microscopic and insoluble, and their sizes range from 1 to 100 nanometers. The mixture is a suspension when the particles are larger than this range. If the particles are smaller than this range, the mixture is a solution.

Colloidal Properties

Brownian Motion Phenomenon in Colloids
Brownian Motion in Colloids

Chemists examine various properties to determine whether a mixture can be classified as a colloid.

  • The dispersed particles should never settle at the bottom of solution and must remain suspended in solution.
  • Centrifugation can separate colloid particles from the solvent.
  • Filtration methods cannot separate colloid particles, due to their small size.
  • All colloids are heterogenous mixtures, as colloid solutions consists of different substances dispersed in each other.
  • Colloidal particles display Brownian Movement – the phenomena of random particle movements in solution.

Tyndall Effect

Tyndall Effect in Colloids
The Tyndall Effect in Solution vs. Colloid

In addition to Brownian Motion and heterogeneity, one major property of colloids is displaying the Tyndall Effect. When someone shines a beam of light through a colloidal solution, the light scatters and reflects in all directions. For example, think of shining a flashlight through dust or fog – that same phenomenon occurs with a true colloid. In contrast, shining light through a solution will pass cleanly through, and not refract in different directions. The suspended particles characteristic of colloids create the Tyndall Effect, and many chemists will test whether a mixture is a colloid by shining light through it.

Colloid Preparation

When creating a colloid mixture, there are a wide variety of preparation methods for colloids. Some major methods include chemical reaction, peptization, mechanical dispersion, and electrical dispersion.

  • Preparing colloids via chemical reaction can involve double replacement, oxidation, reduction and hydrolysis. The products of these reactions aggregate to form colloids. Some examples of chemical reactions that form colloid solutions are below.

    \begin{align*} {As_{2}O_{3} + 3H_{2}S \rightarrow As_{2}S_{3 (sol)} + 3H_{2}O} \end{align*}


    \begin{align*} {SO_{2} + H_{2}S \rightarrow 3S_{(sol)} + 2H_{2}O} \end{align*}


    \begin{align*} {2AuCl_{3} + 3HCHO +3H_{2}O \rightarrow 2Au + 3HCOOH + 6HCl} \end{align*}

  • Peptization is when an electrolyte (or a peptizing agent) is slowly added in small increments to a precipitate. The precipitate then absorbs the electrolyte ions and becomes a colloid. Some peptide examples include gelatin or glucose.
  • Mechanical dispersion – solutes ground into colloidal particle size and then mixed into a dispersion medium. By using mechanical grinders at high speeds and revolutions, the particles forcefully disperses into colloidal suspension. This industrial method is used in the production of paint, black ink, and more.
  • Electrical dispersion – when an electrical current runs between two metal electrodes immersed in a dispersion solution. From the heat generated with this method, a colloidal solution forms. Stabilisation processes mainly use this method.
Electrical Dispersion - A Colloid Preparation Method
Electrical Dispersion: A Colloid Preparation Method

Different Types of Colloids

In addition to preparation method, colloids are often classified into 6 categories based on their state of matter: aerosol, solid solution, foam, gels, hydrocolloids and emulsion.

  1. Solid solution – a colloidal suspension with solid particles in liquid.
  2. Aerosol – small solid or liquid particles dispersed in a gas.
  3. Foam – gas particles dispersed in a solid or liquid solution.
  4. Gels – liquid particles dispersed within a solid.
  5. Hydrocolloids – any colloid where the dispersion medium is water.
  6. Emulsions – a colloid consisting of liquid particles dispersed in liquid solution.

Colloid Examples

Whether you’ve heard of them or not, colloids are all around us in all shapes and forms! Whether it’s the grey fog you wake up to in the morning or the milk you drink for breakfast, colloidal solutions are essential to daily life, as well as the medical/biochemistry field. Therefore, some examples of colloids include paint, car exhaust, lava, cheese, and all sorts of starches. Within medical applications, colloids are important as major bodily fluids are often colloids, such as bile or blood.

Aerogel, an example of a colloid.
Aerogel, example of a colloid.
Milk, an example of a colloid.
Milk, example of a colloid.

Practice Problems


  1. What kind of colloid is a mixture containing solid particles dispersed in a gas?
  2. What phenomenon occurs when you shine light through a colloid mixture?
  3. Which colloid preparation method involves running an electrical current between two electrodes?
  4. Are colloids homogenous or heterogenous mixtures?
  5. What are the particles in a colloid called?


  1. Aerosols
  2. The Tyndall Effect
  3. Electrical Dispersion
  4. Heterogenous Mixtures
  5. The Dispersed Phase