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Colloidal Suspension

All substances can be classified as elements, compounds and mixtures. An element consists of only one type of atoms and cannot be broken down into a simple part by either physical or chemical means. A molecule is composed two or more atoms of same or different elements. In a molecule atoms are bonded through chemical bonds. Compounds are formed by the combination of atoms of two or more different elements. They can be decomposed to simple elements by chemical methods but it does not mean that they are unstable. Compounds are quite stable as they cannot break by any physical method. The properties of compounds are quite different from the properties of elements they composed of. A mixture is composed of two or more different elements or compounds which can be can be separated into its components with the help of physical methods. In mixture, the components retain their properties. On the basis of uniformity, mixtures can be classified as homogenous and heterogeneous mixtures. A homogeneous mixture has the same uniform appearance throughout the mixture. Such mixtures are usually known as solutions. Unlike homogenous, a heterogeneous mixture has different phases.

On the basis of particles size, mixtures can be classified as true solution, colloids and suspensions. True solutions are homogenous mixtures with particles size of less than 10-9 m or 1 nm.  A true solution mainly contains two components; solute and solvent. Solute is the component which is present in less amount whereas solvent is present in large amount in a solution. Due to small size of particles, they are not visible by naked eyes and even they cannot be filtered through filter paper. Sugar in water or salt in water are good examples of true solutions. 

Unlike true solutions, suspensions are heterogeneous mixture in which particle size is greater than $10^{3}$ nm. Mud or sand in water is a common example of suspension. Because of big particle size, they can settle down due to influence of gravity.  Colloidal solutions are also heterogeneous mixtures with particles size between 1nm to $10^{3}$ nm. 

Such particles are small enough therefore cannot be seen through naked eye as well as easily pass through filter paper and can be blocked by animal membrane.


Colloidal Suspension Definition

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We have discussed the classification of substances as true solutions, colloids and suspensions. Colloids are close to suspensions as their particle size is close to each other. In colloidal solutions the particles remain suspended in the medium. Some common examples are smoke, dust and milk. The colloidal suspensions can be defined as solution in which a solid permanently suspended in a liquid medium. The Brownian movement of particles can be checked to determine the property of solution as colloidal. 

The continuous movement of particles resists the settlement of them to the bottom. So we can say that they are heterogeneous mixtures which have the intermediate size of particles between true solutions and a suspension.  Due to this size only, they can pass through a filter paper but not through a parchment paper. At the same time such solutions exhibit the Tyndall effect which is the scattering of light within a solution.

Colloidal Suspension Meaning

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Out of three types of mixture, colloidal suspension exhibits the intermediate properties of true solution and suspensions. The particle size of colloidal suspension is ranging between 1 and 1000 nanometers in diameter. Because of this intermediate size, they remain distributed throughout the solution. There are two main components of a colloidal suspension; dispersion phase and dispersed medium. The dispersion phase remains dispersed in the dispersion medium and do not settle at the bottle like true suspension. Dispersion medium is a continuous phase in which the particles remain dispersed. 

Properties of Colloidal Suspension

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In colloidal suspensions, the particle size is most important. It must be larger than the size of a molecule but smaller that can be seen with the naked eye. Because of this particle size, colloidal suspensions exhibit many unique properties such as Tyndall effect, Brownian movement etc.  Let’s discuss few properties of colloidal solutions. Because of particle size, colloidal suspensions can scatter the light which is due to dispersed particles in medium. When a beam of light is passed through the solution, the dispersed particles scatter light and we can observe the path of reflected light. If same light is passed through true solution little scattering of the light takes place, and then we cannot see the path of the light. The scattering of light due to colloidal particles is called as Tyndall effect as it was first explained by John Tyndall; a British physicist. So we can say that Tyndall effect is the property of colloidal solutions. 

Another important property for colloidal solutions is Brownian movement. When we see any colloidal suspension with the help of an ultra-microscope, the particles appear as tiny points of light with constant motion. This random motion of particles is called as Brownian movement. Due to this movement, colloidal suspensions exhibit the resistance for settlement of particles in the influence of gravity. Let’s discuss another property of colloidal solution, known as absorption.

Because of small size of colloidal particles, they have large surface area to be exposed. That is the reason; colloidal particles impart little effect on the colligative properties such as boiling point, freezing point, etc.

Because of absorption capacity, colloidal particles can absorb ions that make them charged entity. Since all the particles of a colloidal suspension have same charge so they repel each other and remain disperse in solution. In the presence of an electric potential, these charged particles move toward the oppositely charged electrode. The migration of particles towards electrode in applied electric field is called as electrophoresis. 

Example of Colloidal Suspension

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On the basis of dispersed phase and dispersion medium, colloidal suspensions can be different types. Some common examples of colloidal suspensions are sol, emulsion, foam, and aerosol. In sol, solid particles are dispersed in a liquid dispersion medium whereas in emulsion both dispersed phase and dispersion medium are in liquid state. Foams are colloidal suspensions in which gaseous particles remain disperse in a liquid or solid medium. Similarly in aerosol, liquid or solid particles dispersed in a gaseous medium. Some common examples of these colloidal suspensions are listed below. 

 Colloidal suspension   Example 
 Sol  Ruby glass,  Paints, cell fluids 
 Gel  Pearl, cheese
 Solid foam  Lava, pumice
 Emulsion  Milk
 Foam  Soap suds, Whipped cream
 Aerosol  Smoke,  Fog, mist

Colloidal Suspension in Water

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In some of the colloidal suspensions, water is present as dispersion medium. Such solutions are called as hydrocolloid. In such colloids, the dispersed phase can be in all three physical states; solid, liquid or gas. Jello powder in with water is one of the examples of hydrocolloid. Usually such colloids are used for medical dressings.
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