Classification of Colloids

Before we get into the specifics of how we classify colloids, it’s important to first define what a colloid is. A colloid is a material made up of big molecules mixed with another substance in chemistry. This encompasses a wide range of items, many of which you may already have in your house, which is why it’s critical that we break down their classes. The various approaches are employed depending on the applications or to think about colloids in various ways.

Colloids can be categorised based on the physical states of the dispersion medium and dispersed phase, the kind of interaction between the two, or the particle types in the dispersed phase.

• Dispersion Medium: It is the medium (or phase) in the colloidal solution in which the particles are dispersed uniformly. e.g.
  • In gold sol, water is the dispersion medium since the gold particles are distributed in it.
  • In fog, the air is the dispersion medium since the liquid is distributed in it.
• Dispersed phase: It is the phase that is dispersed uniformly throughout the colloidal solution. e.g.
  • In gold sol, the particles of gold is the dispersed phase.
  • In clouds, the particles of dust and water vapour are the dispersed phase.

Classification of Colloids

The colloids are classified into different categories based on different criteria:

1. Classification of colloids based on the Physical states of the dispersion phase and the dispersion medium.
2. Classification of colloids on the basis of interaction between the molecules
3. Classification of colloids on the basis of the size of the particles.

Classification of Colloids on the Basis of the Physical States of the Dispersion Phase and the Dispersion Medium

Colloids can be classified into seven categories on the basis of dispersion phase and the dispersion medium. They are given in the below table:

Dispersed Phase	Dispersion Medium	Name of Colloid	Examples
Solid	Solid	Solid sol	Coloured Glass, Gemstones, Rock Salt, some alloys.
Solid	Liquid	Sol	Paints, inks, gold sol, silver sol, muddy water, starch.
Solid	Gas	Solid aerosol	Smoke, dust storms, exhaust from industries and automobiles.
Liquid	Solid	Gels	Jellies, Cheese, curd, shoe-polish.
Liquid	Liquid	Emulsion	Milk, butter, cod liver oil
Gas	Solid	Solid Foam	Pumice stone, rubber, cake.
Gas	Liquid	Foam	Soap lather, soda (CO2 is dissolved in water) water, whipped cream

The terms used in the above table are defined as below:

• Emulsion: A fine dispersion of minute droplets of one liquid into another immiscible liquid is called Emulsion. The diameter of the droplets ranges from 10^-4  to 10^-6 cm.
• Solid Aerosols – A solution in which very minute fine particles of solid particles are dispersed into a gas is called solid aerosols.
• Solid Sols – A colloidal system in which the particles of one solid substance is dispersed into the particles of another solid substance.
• Gels – A colloidal solution in which minute droplets of liquid is dispersed into a solid dispersion medium.
• Solid Foam – A colloidal solution in which minute particles of gas is dissolved into a solid dispersion medium.

Classification of Colloids on the basis of Interaction between molecules

On the basis of the interaction between the molecules the colloids can be classified into two types:

1. Lyophilic Colloids (solvent-loving) – These types of colloidal solutions have a strong affinity between the particles of the dispersed phase and the particles of the dispersed medium. They are very stable. They cannot be coagulated easily and require a strong electrolyte for coagulation. Some examples of Lyophilic Colloids are Gum, Starch, gelatin, proteins. If water is used as a dispersion medium in lyophilic colloids, then it is called hydrophilic colloids.
2. Lyophobic Colloids (solvent-hating) – These types of colloidal solutions have a weak affinity between the particles of the dispersed phase and the particles of the dispersion medium. They are less stable than the lyophilic colloids. They can be easily coagulated by adding a little electrolyte. Some examples of Lyophobic colloids are Silver Sol, Ferric hydroxide sol and arsenious sulfide sol. If water is used as a dispersion medium in lyophilic colloids, then it is called hydrophobic colloids.

Classification of Colloids on the basis of the Size of the colloidal particles

Colloids are divided into three different categories based on the size of the colloidal particles,

1. Multimolecular Colloids – These colloids consist of particles that are made from large aggregates of atoms or molecules. The atoms and molecules of the particles are held together by van der Waals force. For example, particles are present in the gold and Sulphur sols.
2. Macromolecular Colloids – The colloids which are formed by dissolving macromolecules in suitable solvents are called macromolecular colloids. For example, Natural polymers like proteins, starch etc.
3. Associated colloids – There are certain substances whose molecules are diphilic in nature which means that the molecules of these particles contain a non-polar hydrophilic part and a polar hydrophobic part. At low concentration, these particles act as electrolytes but at high concentration, these particles act as colloids and the colloidal solution formed from these particles is called Associated colloidal solutions. These colloidal particles are called micelles. Micelles are formed above a certain concentration of the solution. For example, Soap and detergents form associated colloids.

Questions

Question 1. The particles of the colloidal solution do not settle down when left undisturbed, but the particles of the suspension do. Explain.

Answer: 

The particles of the colloidal solution is in the range of 10-7 – 10-5 cm (1-100 nm) while the particles of the suspension are in the range of >10-5 (>100nm). Thus, it is very clear the particles of the colloidal solution are much smaller than the particles of the suspension. Thus, due to the smaller size of the particles, there is greater molecular interaction between the particles which holds them and thus,  does not settle down when left undisturbed.

Question 2: How will you distinguish between a colloidal solution and a suspension with the help of filter paper?

Answer: 

We can easily distinguish between a colloidal solution and a suspension with the help of filter paper by filtering both the solution through the filter paper. You will find that the particles of the colloidal solution can easily pass through the filter paper but the particles of the suspension cannot pass through the filter paper.

Question 3: Smoke and clouds both are aerosols. Are they the same? Explain.

Answer:

No, they are not the same. This is because smoke is a solid aerosol where the dispersion medium is gas and the dispersed phase is solid while clouds are liquid aerosol where the dispersion medium is liquid but the dispersed phase is liquid. Thus, they are different.

Question 4: List the main four properties of colloids.

Answer:

The four main properties of colloids are:  

1. They are a heterogeneous mixture.
2. The size of the colloidal particles is too small to be seen by naked eyes.
3. The particles of the colloids are large enough to scatter light and thus shows the Tyndall effect.
4. They cannot be separated by using filter paper as they can pass through filter paper.

Question 5: State the dispersion medium and dispersed phase of the following colloids:

(a) Paints

(b) Rock Salt

(c) Whipped Cream

Answer:  

The following table shows the dispersion medium along with the dispersed phase of the given colloids as,

	Colloid	Dispersion Medium	Dispersed Phase
(a)	Paints	Liquid	Solid
(b)	Rock Salt	Solid	Solid
(c)	Whipped Cream	Liquid	Gas

Question 6: What is the Tyndall effect?  

Answer:

When a beam of light is allowed to pass through a colloidal solution placed in a dark room, we can see the illuminated path of light from a direction of right angles to the direction of incident light due to the scattering of the light due to the colloidal particles. This phenomenon of scattering of light by the particles of the colloids is known as the Tyndall effect.

Question 7: What are the main differences between the Lyophilic Colloids and Lyophobic Colloids?

Answer: 

Following are the differences between the Lyophilic Colloids and Lyophobic Colloids: 

	Lyophilic Colloids	Lyophobic Colloids
1.	These types of colloidal solutions have a strong affinity between the particles of the dispersed phase and the particles of the dispersed medium.	These types of colloidal solutions have a weak affinity between the particles of the dispersed phase and the particles of the dispersion medium.
2.	These colloids are more stable and are reversible in nature	These colloids are less stable and are irreversible in nature.
3.	They can be easily prepared.	These colloids required special methods of preparation and cannot be easily prepared.
4.	They do not coagulate easily and require strong electrolytes to coagulate.	They can be coagulated easily by using weak electrolytes or just by boiling.
5.	They are mainly organic compounds.	They are mainly inorganic substances.