Dispersion
Definition and meaning of Dispersion in chemistry.
A dispersion is a physical mixture where small particles of one substance are scattered throughout a second continuous substance. Unlike a true chemical solution, the scattered particles in a dispersion do not dissolve down to the molecular level.
In more detail
You see dispersions every day in your kitchen and outdoors. Chemists identify dispersions based on the size of the scattered pieces, known as the dispersed phase. The material holding these pieces is called the dispersion medium.
If the scattered pieces are incredibly tiny, the mixture is called a true solution. In a solution, salt or sugar dissolves completely into individual invisible molecules. If the pieces are very large and chunky, the mixture is called a suspension.
Sand shaken up in a glass of water is a suspension. The heavy sand particles will quickly fall to the bottom when you stop shaking. Colloids sit perfectly in the middle of these two extremes.
The particles in a colloid are larger than single molecules but small enough to stay afloat indefinitely. The constant bumping of the surrounding liquid molecules keeps the colloidal particles suspended. Because these particles are slightly bulky, they bounce light rays around.
This scattering of light is called the Tyndall effect. You can see this effect when a car headlight shines through thick fog. Students often confuse a dispersion with a simple compound. A dispersion is purely a physical mixture, meaning no chemical bonds join the two mixed substances together.
Key facts
| Field | General Chemistry |
|---|---|
| Dispersed phase | The physical particles that are scattered |
| Dispersion medium | The continuous substance holding the scattered particles |
| Colloid particle size | Roughly 1 to 1000 nanometers |
| Diagnostic test | Tyndall effect (scattering of light) |
| Nature of mixture | Physical mixture with no chemical bonding |
Homogenized milk is a great example of a colloidal dispersion called an emulsion. The dispersed phase consists of millions of microscopic liquid fat droplets. The dispersion medium is liquid water. Even though oil and water do not mix, the fat droplets are small enough that they stay permanently suspended in the water. If you shine a laser pointer through a glass of milk, the fat droplets will scatter the beam. This proves it is a dispersion and not a true solution.
Frequently asked questions
How is a dispersion different from a normal chemical solution?
In a solution, the solute breaks apart into single invisible molecules or ions. In a dispersion, the particles remain in larger clumps that do not fully dissolve.
Why do colloidal dispersions never settle at the bottom of the jar?
The particles are small enough that the constant random movement of the surrounding liquid molecules keeps them bouncing around and afloat forever.
What is the Tyndall effect used for in chemistry?
It is a quick visual test. If a liquid scatters a beam of light so you can see the beam's clear path, the liquid is a dispersion, not a true solution.