A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture (although a narrower sense of the word suspension is distinguished from colloids by larger particle size). A colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.
Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.
Colloidal suspensions are the subject of interface and colloid science. This field of study began in 1845 by Francesco Selmi and expanded by Michael Faraday and Thomas Graham, who coined the term colloid in 1861.
Colloids can be classified as follows:
Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal suspensions, colloidal foams, colloidal dispersions, or hydrosols.
File:Aerogel hand.jpg|Aerogel
File:Jello Cubes.jpg|Jello cubes
File:Opaleszens Kolloid SiO2.jpg|Colloidal [[silica gel]] with light [[opalescence]]
File:Crème Chantilly.jpg|Whipped cream
File:Dollop of hair gel.jpg|A dollop of hair gel
File:Cream in round container.jpg|[[Creams]] are semi-solid emulsions of oil and water. Oil-in-water creams are used for cosmetic purpose while water-in-oil creams for medicinal purpose
File:Why is the sky blue.jpg|[[Tyndall effect]] in an [[opalite]]:it scatters blue light making it appear blue from the side, but orange light shines through.[[opal]] is a gel in which water is dispersed in silica [[Colloidal crystal|crystals]]
File:Milk and straw.
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