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Classification of Colloids
Classification of colloids based on Physical state
Gold sol, Mud, Fe(OH)3 sol,
Gems, Ruby glass, Minerals
Smoke (Carbon in air) Volcanic dust
Curd, Cheese, Jellies
Milk, water in benzene, cream
Liquid aero sol
Clouds, fog (water in air) mist
Froth on beer , whipped cream
A colloidal system in which the dispersion medium is a liquid or gas are called sols. They are called hydrosols or aqual sols. If the dispersion medium is water. When the dispersion medium is alcohol or benzene, they are accordingly called alcohols or benzosol.
Colloidal syems in which the dispersion medium is air are called aerosols.
Colloids in which the dispersion medium is a solid are called gels, e.g. chese etc. They have a more rigid structure. Some colloids, such as gelatin, can behave both as a sol and a gel. At high temperature and low concentration of gelatin, the colloids is hydrosol. But at low temperature and high gelatin concentration, the hydrosol can change into a gel.
The important points of difference between the lyophilic sols and lyophobic sols are summarized in the table given below:
Ease of preparation
Lyophilic sols can be prepared easily by simply shaking the lyophilic colloids with the dispersion medium
Lyophobic sols cannot be obtained simply by shaking the lyophobic colloids with the medium. They can be obtained only by using special techniques.
They are heavily hydrated.
They are not much hydrated.
Due to hydration, they are quite stable and are not easily coagulated.
They are less stable and get coagulated by heating, by agitating or on addition of small amount of an electrolyte.
The dispersed particles are neither visible nor detected easily even under ultra-microscope.
The dispersed particles, through not visible can be detected easily under ultra microscope.
The viscosity is much higher than that of the dispersion medium.
The viscosity is almost the same as that of the dispersion medium.
The surface tension is usually lower than that of the dispersion medium
The surface tension is nearly the same as that of the dispersion medium.
Charge on particles
The dispersed particles have little or no charge on them.
The dispersed particles carry a definite charge which is either positive or negative.
Migration of particles in an electric field
The dispersed particles may migrate in either direction or may not migrate at all.
Depending upon the nature of charge present, the dispersed particles migrate in a particular direction.
We have already seen that the colloidal particles present in a colloidal system have size lying in the range 1nm-100nm. Depending upon how different substances forming colloidal solution acquire the size of particles in this range, colloidal solutions may be classified into the following three categories.
Multimolecular Colloids: When a large number of atoms or small molecules (having diameters of less than 1nm) of a substance combine together in a dispersion medium to form aggregates having size in the colloidal range, the colloidal solutions thus formed are called multimolecular colloids. The species (atoms or molecules) constituting the dispersed particles in multimolecular colloids are held together by Vander Waals’ forces.
The gold sol, sulphur sol etc. are some examples of multimolecular colloids. A gold sol may contain particles of various size composed of several atoms of gold. Similarly, sulphur sol consists of particles containing about a thousand of S8molecules.
Macromolecular Colloids: Certain substances form large molecules whose dimensions are comparable to those of colloidal particles. Such molecules have very high molecular masses and are termed as macromolecules. When such substances are dispersed in suitable dispersion medium, the resulting colloidal solutions are known as macromolecular colloids. Thus, in macromolecular colloids, the dispersed particles are themselves large molecules having very high molecular masses.
Most of the lyophilic sols are macromolecular colloids. For example colloidal dispersion of naturally occurring macromolecules such as starch, proteins, gelatin, cellulose, nucleic acids etc. are macromolecular colloids. Synthetic polymers such as polyethylene, polypropylene, synthetic rubber etc. also form macromolecular colloids when dispersed in suitable solvents.
Associated Colloids (Micelles): Associated colloids are those colloids which behave as normal strong electrolytes at low concentrations but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles. The aggregated particles thus formed are called micelles.
Colloids which behave as normal electrolytes at low concentrations, but exhibit colloidal properties at higher concentrations due to formation of aggregated particles called micelles are referred to as associated colloids. The micelles are formed by the association of dispersed particles above a certain concentration and certain minimum concentration is required for the process of aggregation to take place. The minimum concentration required for micelle formation is called micellisation concentration (CMC) and its value depends upon the nature of the dispersed phase. For soaps CMC is 10 -3 mol L-1
The associated colloids are usually formed by surfactants (surface active agents) like soaps and synthetic detergents. These agents form micelles when present in solution at a concentration greater than critical micellization concentration (CMC). The formation of a micelle can be understood by taking the example of soap solution as described below.
The commonly used soaps are sodium or potassium salts of higher fatty acids such as palmitic acid (C15H31COOH), stearic acid (C17H35COOH) etc. The most commonly used washing soap is sodium stearate, C17H35COONa. In general, soap can be represented as RCOONa, where R represents a long chain alky1 group. When dissolved in water, soap ionizes to give RCOO– and Na+ ions.
The RCOO– ion consists of two parts – long hydrocarbon chain R and the polar group -COO– as shown diagrammatically. The hydrocarbon residue R is hydrophobic, i.e. water repelling, whereas the -COO- group is hydrophilic, i.e. water loving. Therefore, RCOO– ion orients itself in such a way that -COO- end dips in water and the group R stays away from water.
The -COO- groups of different RCOO- ions tend to stay away from one another due to like charges,whereas the R-groups try to approach each other to form other to form a bunch. This leads to the formation of a micelle as shown in the figure below.
Thus a soap micelle is a negatively charged colloids particle in which the negatively charged -COO– groups are arranged in a spherical manner at the surface, while the hydrocarbon chains point towards the centre. The -COO– groups present at the surface of the micelle get surrounded by Na+ ions which tend to drag the micelle in the bulk of the solution. A micelle may contain as many as 100 molecules or more.
The cleansing action of soaps: Soaps are used for cleaning dirty clothes. Clothes become dirty due to the deposition of dust and oily or greasy substances. Water is not capable of wetting oily or greasy substances. However, the hydrocarbon residue R of the soap anion (RCOO–) can do so. When a dirty cloth is dipped into a soap solution, the hydrocarbon residue R of RCOO– ion dissolves the oily or greasy dirt and encapsulates it to form a micelle. When the cloth is rinsed with water, the micelles carrying the oily or greasy dirt get washed away.
The cleaning action of detergents such as sodium laury sulphate, CH3 (CH2)11SO4Na+ or sulphonates of long chain hydrocarbons is similar to that of soaps. In case of detergents, the polar groups are sulphate (–SO4) or sulphonate (-SO3) groups.
Question 1: Cheese is a
Question 2: Which of the following colloid is an example of solid foam?
Question 3: Colloidal syems in which the dispersion medium is air are called
Question 4: Certain substances form large molecules whose dimensions are comparable to those of colloidal particles. Such molecules have very high molecular masses and are termed as
a. macromolecular colloids
b. micromolecular colloids
c. associated colloids
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