A colloid is a mixture composed of particles in a dispersing medium. A colloid is defined by the size of the particles involved. If the particles in a mixture are on the scale of individual molecules, around 1 nanometer, it is defined as a solution. If the particles are larger than 1,000 nanometers, it is a suspension. Anything in between is a colloid. The unique characteristics of colloids are due to this intermediate size of the dispersed particles.
Types of Colloid
A colloid may consist of particles suspended in a gas, liquid or solid, although many colloidal properties are most pronounced in liquid colloids. Gas colloids consist of particles suspended in the air or a gas medium, and include fog, smoke and atmospheric dust. Liquid colloids can consist of liquid or solid particles suspended in a liquid medium, such as milk, or incorporate gas bubbles, such as whipped cream. Solid colloids include solid foams, such as plaster, liquid-bearing solids, such as butter or cheese, and firm substances, such as paper.
Persistence of Suspension
A key characteristic that separates colloids and suspensions is the tendency for the particles in a suspension to settle out over time. If left undisturbed, a well-mixed suspension will separate out into two distinct layers with the particles sinking to the bottom of a container, and the dispersing medium remaining at the top. The particles in a colloid resist settling out over time.
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The particles in a colloid exhibit Brownian movement. No matter how long a colloid is left undisturbed, the particles in it never fully rest. Instead, they exhibit constant zigzagging movement at the microscopic scale. This is caused by the constant collisions between the particles and molecules in the dispersing medium. The particles in a suspension are too large to be strongly affected by Brownian movement.
Colloids can be readily distinguished from solutions by the Tyndall effect. When a beam of light shines through a colloid, the suspended particles scatter the light, making it visible as a distinct column of illumination. The molecule-sized particles in a solution are too small to scatter light in this way, and do not make a beam of light visible. This is especially striking in colloids that appear transparent, since shining a beam of light through them makes them appear suddenly cloudy.