A third kind of mixture, a colloidal system, has particles with sizes in the intermediate range. These systems transmit light, and sometimes appear transparent, but are often translucent if the suspended particles are larger.
Examples of Colloids
The word, ‘colloid’, comes from the Greek word for ‘glue’. Indeed glues and gums, when suspended in a solvent, are colloidal in nature.
Paints and jellies consist of solid colloidal particles suspended in liquid. An emulsion, like milk, has liquid particles suspended in a liquid. When colloidal sized gas bubbles are dispersed through a liquid, a foam is produced, like whipped cream. Colloidal gas bubbles in a solid, on the other hand, produce a meringue or pumice. Pearls and coloured glasses contain solid particles in a solid medium. In fogs, mists and clouds, liquid particles are suspended in a gas, while smoke is an example of an aerosol, in which the colloid is a solid.
Brownian Movement
Sometimes, colloidal particles can be detected under a microscope, because of their ability to scatter light sideways. If smoke is observed through a microscope, while illuminated by a bright light, tiny pinpoints can be seen jumping about continuously. This ceaseless motion is known as Brownian movement, and is caused by the smoke particles being bombarded by tiny, invisible, but rapidly moving molecules of air.
Clouds are made of colloidal water droplets and ice crystals. If the temperature falls, water vapour from the surrounding air condenses onto these, increasing their masses until they can no longer remain suspended, and fall as rain.
Colloids and Electricity
Sometimes, colloid particles gather electric charges onto their surfaces. Repulsion between these charges keeps the particles separated. If the charges can be neutralised, the particles coagulate and drop out of suspension.
Colloidal clay in river water is precipitated by the charged ions in salt, as the river enters the sea, so that estuaries are usually surrounded by extensive mud flats. Similarly, a salt like aluminium sulphate is added to domestic water to precipitate colloidal materials and so clarify the water. Adding salt to a wound helps blood to clot.
Scattering of Light
Light scattering is an important property of colloidal systems. Light passes unhindered through a solution, and is blocked completely by a coarse suspension. If the particles in suspension are colloidal, then the path of a light beam can be seen when viewed from the side, a phenomenon known as the Tyndall effect. The appearance of sunbeams is an example of this, in which light is scattered by particles of mist or dust suspended in the atmosphere.
White light consists of radiation of different wavelengths. The shorter wavelengths give rise to blue light and the longer to red light. Colloidal particles scatter blue light more effectively than red. Smoke from a burning object thus appears blue. When the particles are in the larger colloidal range, like water droplets in a cloud, all wavelengths of light are scattered, so the cloud appears white.
Suspended in air are particles of dust, ice crystals and water droplets, in the lower range of colloidal size. These allow the passage of light of longer wavelengths, but scatter blue light very effectively.
As sunlight passes through the atmosphere, this blue light is scattered towards the ground, causing the sky to appear blue. As the sun sinks, the sideways scattering of blue light leaves the red to continue, giving us the fiery spectacle of a glorious sunset.
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