Lower secondary · 10 min

The Tyndall effect: is it a solution or a colloid?

Shine a narrow beam through water and through a colloidal sol: only the colloid shows a glowing path — the classic test for particle sizes between 1 nm and 1 μm.

Goal

Distinguish true solution, colloid and suspension by their scattering behaviour and link the cone width to particle size.

Apparatus and reagents

Laser pointer or narrow-beam lamp, glasses of distilled water, starch or gelatin sol, milk diluted in water, a dark background.

Procedure

Start with the smallest particle setting: the beam is nearly invisible — this is a true solution.
Raise the particle size into the colloidal range and watch a glowing cone appear from the side.
Push toward suspension sizes: the cone widens further and the medium looks turbid.
Classify fog, milk, salt water and paint on this solution–colloid–suspension scale.

What to observe

In a colloid the beam path glows when viewed sideways because particles scatter light in all directions; in a true solution the path stays dark.
Larger particles scatter more strongly: the cone widens and brightens until the sample looks milky.

Explanation

Particles much smaller than the wavelength barely scatter (Rayleigh, ~ $1/\lambda^4$ ), so dissolved molecules leave the beam invisible. Colloidal particles of 1 nm–1 μm scatter enough to reveal the path — the Tyndall effect — while still small enough to stay dispersed by Brownian motion instead of settling.

History of the experiment

John Tyndall described the effect in the 1860s while studying why the sky is blue; Richard Zsigmondy later counted colloidal particles with the ultramicroscope he built on the same scattering principle (Nobel 1925).

Chemists behind it

Richard Adolf Zsigmondy Theodor Svedberg