A colloidal system in which both the dispersed phase as well as dispersion medium are immiscible or partially miscible liquids is called an emulsion. e.g. Milk, cod liver oil, oil paints, vanishing cream, cold creams, etc. are emulsions.

Generally, one of the two liquids is water and the other which is immiscible with water is designated as oil. The liquid in excess is the dispersion medium and the liquid which forms droplets or globules is the dispersed phase.

Emulsifying Agent or Emulsifier:

Emulsions of oil and water are unstable and sometimes they separate into two layers on standing. To obtain a stable emulsion a small quantity of third substance is added along with two immiscible liquids. This third substance is called Emulsifying agent or Emulsifier. Emulsifier forms a protective layer around disperse phase droplets and prevent coagulation. e.g. Soaps, Detergents, Lyophilic substances like gelatin, gum etc.

Types of Emulsions and Their Preparation:

Oil in Water Type (O/W):

In this type of emulsion, oil is the dispersed phase and water is the dispersion medium. e.g. Milk is (fats) oil in water type of emulsion, vanishing creams.

Preparation: 

The emulsifier is dissolved in water and oil is added to it drop by drop with continuous agitation.

Characteristics of Oil in Water Emulsions:

• Oil is the dispersed phase and water is a dispersion medium.
• If water is added it is miscible with the emulsion.
• If oil is added it is not miscible with the emulsion.
• Addition of small amount of electrolyte makes emulsion conducting.
• Water is a continuous phase.
• Basic metal sulphates, water-soluble alkali metal soaps are used as emulsifiers.

Water in Oil Type (W/O):

In this type of emulsions, water is the dispersed phase and oil is the dispersion medium. e.g. Cod liver oil in which particles of water are dispersed in oil, Cold creams.

Preparation: 

The emulsifier is dissolved in oil and water is added to it drop by drop with continuous agitation.

Characteristics of Water in Oil Emulsions:

• Water is the dispersed phase and oil is the dispersion medium.
• If oil is added it is miscible with the emulsion.
• If water is added it is not miscible with the emulsion.
• Addition of small amount of electrolyte does not make emulsion conducting.
• Oil is a continuous phase.
• Water-insoluble soaps such as those of Zn, Al, Fe, alkaline earth metals are used as emulsifiers.

Methods of Identification of Type of Emulsions:

Dye Solubility Test:

In this test, an emulsion is mixed with a water-soluble dye (amaranth) and observed under the microscope. If the continuous phase appears red, it means that the emulsion is o/w type as water is in the external phase and the dye will dissolve in it to give colour. If the scattered globules appear red and continuous phase colourless, then it is w/o type. Similarly, if an oil-soluble dye (Scarlet red C or Sudan III) is added to an emulsion and the continuous phase appears red, then it is w/o emulsion.

Viscosity Test:

The viscosity of water in oil type emulsion is more than the viscosity of oil in water type emulsion.

Electrical Conductive Test:

The basic principle of this test is that water is a good conductor of electricity. In the case of o/w emulsion, this test will be positive as water is the continuous phase. If electrolyte like NaCl is added to oil in water type emulsion, its conductivity increases greatly.

Spreading Test:

Water in oil type emulsion spread on the surface of oil but not on the surface of the water.

Dilution Test:

In this test, the emulsion is diluted either with oil or water. If the emulsion is o/w type and it is diluted with water, it will remain stable as water is the dispersion medium. For example, milk which is oil in water emulsion can be diluted with water, but butter which is water in oil emulsion cannot be diluted with water.

If emulsion o/w type is diluted with oil, the emulsion will break as oil and water are not miscible with each other. Oil in water emulsion can easily be diluted with an aqueous solvent whereas water in oil emulsion can be diluted with an oily liquid.

Cobalt Chloride Test:

When a filter paper soaked in cobalt chloride solution is dipped into an emulsion and dried, it turns from blue to pink, indicating that the emulsion is o/w type.

Fluorescence Test:

If an emulsion on exposure to ultra-violet radiations shows continuous fluorescence under a microscope, then it is w/o type and if it shows only spotty fluorescence, then it is o/w type.

Properties of Emulsions:

Precipitation:

In many emulsions, the size of the dispersed droplets is larger than the particles found in sols. These droplets can be precipitated by adding suitable electrolytes.

De-emulsification:

Separation of two liquid phases (two distinct layers) from emulsions due to the removal of emulsifier by any means is called as de-emulsification or de-emulsification. It can be done by heating, freezing, centrifuging or by addition of electrolytes.

Dilution:

Emulsions can be diluted with any amount of dispersion medium. But it is to be noted that any increase in the concentration of dispersed phase results in demulsification.

Coalescence:

It is the phenomenon of disappearance of the boundary between two particles in contact (as in case of droplets and bubbles). The formation of aggregates may be followed by coalescence. In excessive conditions, the coalescence leads to breaking of the emulsion. Coalescence of solid particles is called sintering.

Note:

Emulsions show similar properties as that of sols. When the droplets are very small they show Tyndall effect, Brownian movement and electrophoresis.

Characteristics of Emulsions:

• It is the colloidal system in which the dispersed phase and the dispersion medium both are liquid.
• It is a mobile liquid.
• Droplets of one liquid dispersed in another liquid.
• No tendency to absorb a liquid or to swell.
• The emulsifying agent is needed.
• It is classified as oil in water and water in oil.
• They show Tyndall effect, Brownian movement and electrophoresis.

Uses of Emulsions:

• Phenyl, when poured in water, gives oil in water emulsion.  It is used as a disinfectant.
• Cleansing action of soap or detergent is due to emulsion formation of oil with water.
• Emulsions are used in medicines such as emulsions of cod liver oil, malt and yeast.
• They are used in oil paints, plastic emulsion paints.
• In the concentration of sulphide ore by forth floatation method, oil in water type of emulsion is formed.
• Milk is an important household emulsion.
• Asphalt emulsified in water is used for the construction of roads.

Method of Breaking of Emulsions:

• The two liquids in the emulsion can be separated by heating, freezing or centrifuging.
• The addition of a large quantity of electrolyte causes coagulation of the dispersed phase.
• The chemical destruction of the emulsifier causes the separation of the two liquids.

Disadvantages of Emulsion:

• To refine the emulsion of petroleum with water is expensive.
• The emulsion of oil in water may be coming as water supply, which is unfit for drinking.

Science > Chemistry > Colloids > Emulsions

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In this article, we shall study the concept of solubility, solubility product, and its applications.

Some ionic solids are highly soluble in water while others are almost insoluble in it. The solubility of ionic solid depends on lattice enthalpy of the salt and hydration enthalpy of ions in solution. The lattice enthalpy of salt is defined as the energy required to overcome the attractive forces between the ions. It is always positive. The hydration enthalpy or solvation enthalpy is the energy released during the interaction between the ions and solvent molecules. It is always negative. If salt is to be dissolved then its solvation enthalpy should be greater than its lattice enthalpy.

Solubility:

The concentration of a substance in its saturated solution is called as its solubility at a given temperature. It is denoted by letter ‘S’ It is expressed as grams per litre or as moles per litre at a given temperature.

Classification of Solids on the Basis of Solubility:

• The solids having a solubility greater than 0.1 M are classified as soluble solids e.g. NaCl, Sugar, etc.
• The solids having a solubility between 0.01 M and 0.1 M are classified as slightly soluble solids e.g. calcium phosphate.
• The solids having a solubility less than 0.01 M are classified as sparingly soluble solids e.g. barium sulphate, silver chloride, etc.

Sparingly Soluble Salt:

A certain substance like AgCl, PbSO₄, BaSO₄ etc. have negligible solubility in water at ordinary tempera­ture.  Such substances which are practically insoluble in water are called as sparingly soluble electrolytes. The amount of such salts getting dissolved is so small that their saturated solution may be regarded as extremely dilute and hence dissolved part can be considered as completely ionized.

Solubility Product:

In a saturated solution of a sparingly soluble elec­trolyte, the product of molar concentration of ions is constant at a given temperature. This constant ‘ K_sp ’ is called a solubility product.

Explanation:

Suppose ‘BA’ is a sparingly soluble electrolyte.  In aqueous solution, it dissociates to a very little extent there exist two equilibria.

BA_(s) → BA_(aq) ⇌ B⁺_(aq) + A^–_(aq)

The mass law equation of the equilibrium is

But [BA] = constant

∴  K . constant = [B⁺][A^–]

∴  K_sp = [B⁺][A^–]

Where K_sp is solubility product. If ‘S’ moles/dm³ is solubility of electrolyte ‘BA’ then [B⁺] = S and [A^–] = S

K_sp = [S] [S]

K_sp = S²

Effect of pH on Solubility:

The solubility of salt of weak acids increases in more acidic solutions e.g. ZnS, CuS, NiS, etc. Marble (CaCO₃) statues and monuments corrode by the effect of acid rain.

Saturation, Unsaturation and Precipitation:

• In a solution, when the ionic product is equal to the solubility product.  Then the solution is just saturated and precipitation doesn’t occur.
• In a solution, when the ionic product is less than the solubility product then the solution is unsaturated and precipitation doesn’t occur.
• In a solution, when the ionic product exceeds the solubility product then the solution is supersaturated and precipitation of electrolyte takes place.
• Thus precipitation is possible only when the ionic product is greater than the solubility product.

Applications of Solubility Product:

In the precipitation of II group cations:

Group II cations like Cu⁺⁺, Cd⁺⁺, Pb⁺⁺ etc. are precipitated as their sulphides.

Precipitation is carried out by adding dil. HCI followed by passage of H₂S gas through the solution.  Being weak acid, H₂S ionizes as,

H₂S_(aq) ⇌  2 H⁺ (aq.) + S^– –_(aq)

HCI being strong acid dissociate almost completely as,

HCl_(aq) → H⁺_(aq)  + Cl^–_(aq)

The concentration of H⁺ is increased. Since H⁺ ions are common ions, due to common ion effect dissociation of H₂S is suppressed so that S^– – ion concentration is decreased to such an extent that only group II cations get precipitated. Ionic product of sulphides of II group cations exceeds solubility product, so only II group cations form a precipitate and other cations belonging to further groups remain as it is in the solution.

In the precipitation of III A group cations:

III A group cations like AI³⁺, Fe³⁺, Cr³⁺, etc. are precipitated as their hydroxides by adding NH₄CI followed by adding NH₄OH.

Being weak base NH₄OH dissociate as

NH₄OH_(aq) ⇌ NH₄⁺_(aq) + OH^–_(aq)

Being strong electrolyte NH₄CI dissociate as,

NH₄Cl_(aq) ⇌ NH₄⁺_(aq) + Cl^–_(aq)

Since NH₄⁺ ions are common, their concentration increases and due to common ion effect dissociation of NH₄OH is suppressed so that concen­tration of OH^– ion decreases to such an extent that only IIIA group cations are precipitated. Ionic product of hydroxides of IIIA group cation exceeds solubility product while the ionic product of hydroxides of IIIB group is lower than solubility product hence only IIIA group cations are precipitated.

In Prediction of Precipitation:

Solubility product is the highest limit of ionic product at a particular temperature. When ionic product exceeds the solubility product, excess ions combine with each other to form the precipitate of the salt. Hence to find whether a precipitation can take place or not, ionic product of salt is calculated and it is then compared with the solubility product of the salt at the same temperature.

By knowing the molar concentration of ions in a solution and solubility product, it can be predicted whether precipitation would occur or not.  Precipitation is an ionic reaction.  According to the solubility product concept, precipitation occurs only when the ionic product exceeds solubility product. If K_sp = ionic product or K_sp > ionic product, then precipitation doesn’t occur.

In general

• Ionic product = K_sp, the solution is saturated (No precipitation)
• Ionic product < K_sp, the solution is unsaturated(No precipitation)
• Ionic product > K_sp, the solution is supersaturated (precipitation)

Example:

K_sp of BaSO₄ at 298 K is 1 x 10^-10 then for the precipita­tion of BaSO₄ in the solution,

ionic product [Ba⁺⁺ ] [SO4^– –]   >  K_sp of BaSO₄

[Ba⁺⁺ ] [SO₄^– –]   >  1 x 10^-10

Science > Chemistry > Physical Chemistry > Ionic Equilibria > Solubility Product

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