`=>` These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid.

`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
An emulsion is a colloidal dispersion in which both the dispersed phase and dispersion medium are liquids.

`=>` If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid in the other is obtained which is called emulsion.

`=>` Generally, one of the two liquids is water. There are two types of emulsions :

(i) Oil dispersed in water (O/W type) : In this, water acts as dispersion medium. Examples of this type of emulsion are milk and vanishing cream. In milk, liquid fat is dispersed in water. Emulsions of oil in water are unstable and sometimes they separate into two layers on standing.

(ii) Water dispersed in oil (W/O type) : In this, oil acts as dispersion medium. Common examples of this type are butter and cream.

`text(Emulsifying Agent :)` For stabilisation of an emulsion, a third component called emulsifying agent is usually added. The emulsifying agent forms an interfacial film between suspended particles and the medium.

● The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc.

● For W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc.

● Emulsions can be diluted with any amount of the dispersion medium.

● On the other hand, the dispersed liquid when mixed, forms a separate layer. The droplets in emulsions are often negatively charged and can be precipitated by electrolytes.

● They also show Brownian movement and Tyndall effect.

● Emulsions can be broken into constituent liquids by heating, freezing, centrifuging, etc.

`text(Properties of emulsions)`:
•They exhibit all properties like tyndall effect, Brownian movement.
•Coagulation on addition of electrolyte.
•Can be separated into their constituents liquids by boiling, freezing, centrifuging, electrostatic precipitation etc. The separation of cream from milk is a well known example of centrifuging.

`=>` Most of the substances, we come across in our daily life, are colloids.

`=>` The meals we eat, the clothes we wear, the wooden furniture we use, the houses we live in, the newspapers we read, are largely composed of colloids.

`=>` Following are the interesting and noteworthy examples of colloids :

(i) Blue colour of the sky : Dust particles along with water suspended in air scatter blue light which reaches our eyes and the sky looks blue to us.

(ii) Fog, mist and rain : ● When a large mass of air containing dust particles, is cooled below its dewpoint, the moisture from the air condenses on the surfaces of these particles forming fine droplets.

● These droplets being colloidal in nature continue to float in air in the form of mist or fog.

● Clouds are aerosols having small droplets of water suspended in air.

● On account of condensation in the upper atmosphere, the colloidal droplets of water grow bigger and bigger in size, till they come down in the form of rain.

● Sometimes, the rainfall occurs when two oppositely charged clouds meet.

● It is possible to cause artificial rain by throwing electrified sand or spraying a sol carrying charge opposite to the one on clouds from an aeroplane.

(iii) Food articles : Milk, butter, halwa, ice creams, fruit juices, etc., are all colloids in one form or the other.

(iv) Blood : It is a colloidal solution of an albuminoid substance. The styptic action of alum and ferric chloride solution is due to coagulation of blood forming a clot which stops further bleeding.

(v) Soils : Fertile soils are colloidal in nature in which humus acts as a protective colloid. On account of colloidal nature, soils adsorb moisture and nourishing materials.

(vi) Formation of delta : River water is a colloidal solution of clay. Sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal solution of clay resulting in its deposition with the formation of delta.

Colloids are widely used in the industry. Following are some examples :

(i) Electrical precipitation of smoke : Smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc., in air. The smoke, before it comes out from the chimney, is led through a chamber containing plates having a charge opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge and get precipitated. The particles thus settle down on the floor of the chamber. The precipitator is called Cottrell precipitator (Fig.5.15).

(ii) Purification of drinking water: The water obtained from natural sources often contains suspended impurities. Alum is added to such water to coagulate the suspended impurities and make water fit for drinking purposes.

(iii) Medicines: Most of the medicines are colloidal in nature. For example, argyrol is a silver sol used as an eye lotion. Colloidal antimony is used in curing kalaazar. Colloidal gold is used for intramuscular injection. Milk of magnesia, an emulsion, is used for stomach disorders. Colloidal medicines are more effective because they have large surface area and are therefore easily assimilated.

(iv) Tanning : Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin, which contains negatively charged colloidal particles, mutual coagulation takes place. This results in the hardening of leather. This process is termed as tanning. Chromium salts are also used in place of tannin.

(vi) Photographic plates and films: Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.

(vii) Rubber industry: Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of
latex.

(viii) Industrial Products : Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc. are all colloidal solutions.