Colloids Around Us

Colloids are everywhere in daily life — our food, clothes, wooden furniture, houses and newspapers are largely colloidal.

Colloids in nature

• Blue colour of the sky: dust and water particles suspended in air scatter blue light toward our eyes.
• Fog, mist and rain: when moist air cools below its dew point, water condenses on dust particles into colloidal droplets that float as fog or mist; clouds are aerosols of water droplets that grow until they fall as rain. Rain can even be induced artificially by spraying electrified sand or an oppositely charged sol from an aeroplane onto clouds.
• Food articles: milk, butter, halwa, ice cream and fruit juices are all colloids.
• Blood: a colloidal solution of an albuminoid; alum and ferric chloride clot blood (styptic action) by coagulating it.
• Soils: fertile soils are colloidal, with humus as a protective colloid, letting them adsorb moisture and nutrients.
• Formation of delta: river water is a colloidal solution of clay; when it meets the electrolyte-rich sea water, the clay coagulates and deposits, building a delta.

Applications of colloids

• Electrical precipitation of smoke: smoke (colloidal carbon, dust, etc. in air) is passed through a chamber with plates charged opposite to the smoke particles; they lose their charge and settle. This is the Cottrell precipitator (Fig. 5.16), using a high-voltage electrode (30,000 V or more).
• Purification of drinking water: alum coagulates suspended impurities, making water fit to drink.
• Medicines: many are colloidal — argyrol (silver sol eye lotion), colloidal antimony for kala-azar, colloidal gold for injection, milk of magnesia for stomach disorders — and are effective because their large surface area aids assimilation.
• Tanning: positively charged animal hide soaked in negatively charged tannin (or chromium salts) undergoes mutual coagulation, hardening it into leather.
• Cleansing action of soaps and detergents (see Section 5.4.3). …