Colloidal particles typically measure between one and one thousand nanometers across. This puts them right in the middle between true solutions and heavy suspensions. In a true solution like salt water, the salt breaks into individual tiny ions.
In a suspension like muddy water, the heavy dirt eventually sinks to the bottom. Colloid particles never sink because they constantly crash into the surrounding fluid molecules. These tiny collisions keep the larger chunks bouncing around permanently.
A classic way to spot a colloid is by shining a flashlight through it. The suspended chunks are just large enough to bounce the light waves around. This scatters the light beam and makes it clearly visible from the side.
Scientists call this unique light scattering behavior the Tyndall effect. Students often confuse cloudy colloids with completely uniform solutions. A colloid will look cloudy or milky, while a true solution looks completely clear.
Colloids take many forms depending on what is mixed together. A foam traps gas bubbles in a liquid, while an aerosol traps liquid in a gas.