The following tests can be employed to distinguish between the two types of emulsions.

(i) Dye test : An oil soluble dye is added to the given emulsion. If the background becomes coloured, the emulsion is water in oil type and if the tiny droplets become coloured, the emulsion is oil in water type.

(ii) Dilution test : If the emulsion can be diluted with water, this indicates that the emulsion is oil in water type. In case the emulsion cannot be diluted with water, it is water in oil type.

Emulsion can be prepared by mixing oil and water and shaking them vigorously in the presence of small quantity of a substance known as emulsifying agent or emulsifier. It is essential to add emulsifier in order to prepare stable emulsions. Such emulsions have properties, which resemble somewhat those of lyophilic sols e.g. high viscosity, relatively high concentrations and stability to electrolytes. An excess of electrolyte may salt out the emulsifier and so cause instability. Emulsifying agents fall roughly into three categories.

(i) These are either, the long chain compounds with polar groups, such as the soaps and long chain sulphonic acids and sulphates, all of which produce a considerable decrease in the oil-water interfacial tension. When olive oil and water are shaken together, very little emulsification occurs, but the addition of a small amount of `NaOH` results in the formation of a stable emulsion. The sodium soap formed by hydrolysis acts as the emulsifier. It appears that certain optimum concentration of emulsifier is required to obtain stable emulsions. Any concentration of the emulsifier more or less than this does not cause effective stabilisation.

(ii) There are substances of a lyophilic nature such as proteins, e.g. casein in milk and gums.

(iii) Insoluble powders for example, basic sulphates of iron, copper or nickel, finely divided lead sulphate and ferric oxide and lamp black stabilise a number of emulsions. The soaps of alkali metals favour the formation of oil in water emulsions, but those of the alkaline earth metals and of zinc, iron and aluminium give water in oil emulsions. Similarly, the basic sulphates stabilise oil in water emulsions whereas opposite type is formed when finely divided carbon is the emulsifying agent.

Some of the important applications of emulsions are as given below:

(i) In medicines : A number of pharmaceutical preparations are emulsions. These are oil in water type emulsion, which are easily acted upon by the digestive juices in the stomach.

(ii) In Metallurgical Operations : The sulphide ores are concentrated by froth flotation process, which involves the treatment of crushed ore with emulsion of pine oil.

Soaps and detergents form true solutions in water when their concentration is low. However, at high concentration, they form colloidal solutions, in which colloidal particles are formed by agglomeration of large number of soap or detergent molecules. These type of colloids are called associated colloids and the agglomerated colloidal particles are called `text(micelles)`. The formation of micelles takes place above a certain concentration called critical micellization concentration (CMC). Each micelle system has a specific value of CMC. The soap and detergent molecules have a hydrophobic group at one end and a hydrophilic group at the other end. Such type of molecules are called surfactant molecules. Sodium oleate (`C_(17)H_(33)COO^(-) Na^+`) is a typical example of soap. The long hydrocarbon part of oleate anion is the hydrophobic end while `-COO^-` part is the hydrophilic end. The CMC value of sodium oleate is `3xx10^(-3)` `M`. When the concentration exceeds CMC, the hydrophobic parts of `C_(17)H_(33)COO^(-)` anions are repelled by solvent molecules and are compelled tocome close to each other. The lyophilic part i.e. `COO^-` ions spreads out due to attraction by the solvent molecules forming micelles, which are of colloidal dimensions.

`text(Cleansing action of soap)` : A soap molecule contains a non polar hydrophobic group and a polar hydrophilic group. The dirt is held on to the surface of clothes by the oil or grease, which is present there. Since grease or oil is not soluble in water, the dirt particles cannot be removed simply by washing the clothes with water. When soap is applied, the non polar hydrophobic end dissolves the oil or grease. While the polar `-COO^-` groups remain dissolved in water. In this way each oil droplet is surrounded by negatively charged `-COO^-` groups and hence cannot coalesce. These oil droplets containing dirt can be washed away with water.

Certain colloidal systems have the property of setting to a semi solid, jelly - like form when they are present at high concentration. Such a process is called gellation and the colloidal systems with jelly-like form are called gels. Gelatin, gum Arabic, agar and processed cheese are the examples of gels. Gels possess rigid structure formed due to interlocking of particles of disperse phase and create a loose framework. The particles of dispersion medium are trapped into the loose framework. The degree of rigidity varies from substance to substance. When gels are allowed to stand for a long time, they give out small quantity of trapped liquid, which accumulates on the surface. This is called syneresis or weeping.