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Colloids or Colloidal solutions or Colloidal systems are a mixture in which a component made up of insoluble particles that are scattered at a microscopic scale is suspended within another component. In chemistry, the mixture of two substances, in which one substance is separated into minute particles (or colloidal particles) and dispersed over another substance is called the Colloid. Colloids can be of various types like Sol, Emulsion, Foam, Aerosol, Lyophobic Sols, Lyophilic Sols, etc. Hence, this article on Colloids helps to understand various concepts of colloids like Classification, Preparation, Characteristics, Applications, Examples, and FAQs!

What are Colloids?

A solution in which the size of solute particles is intermediate between those in true solution and suspension is called Colloids. Simply, colloids are the mixtures where one substance is split into minute particles which are dispersed throughout a second substance. e.g. Soap solution, milk, blood, etc are some of the examples of colloids.

Colloids (also colloidal solutions or colloidal systems) are mixes of microscopically distributed insoluble particles of one material floating in another. A colloid’s suspended particles can range in size from 1 to 1000 nanometres (or 10-9 meters).

The suspended particles in a combination must not settle in order for them to be classed as a colloid (in the manner that the particles of suspensions settle at the bottom of the container if left undisturbed). Colloidal solutions are known to show the Tyndall Effect, a phenomenon in which light beams impacting on colloids are dispersed as a result of interactions between the light and the Colloidal component.

Examples of Colloids


Classification of Colloids

Colloids are classified into different classes based on different conditions:

Classification of Colloids on the Basis of Physical States

  • Solid Sols – A colloidal system in which the particles of one solid substance is dispersed into the particles of another solid substance. e.g. Coloured Glass, Gemstones, Rock Salt, some alloys, etc.
  • Sol – When the dispersion medium is solid and the dispersion phase is liquid in a colloidal system, then it is termed as Sol. e.g. Paints, inks, gold sol, silver sol, muddy water, starch, etc.
  • 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. Milk, butter, cod liver oil, etc. 
  • Solid Aerosols – A solution in which very minute fine particles of solid particles are dispersed into a gas is called solid aerosol. e.g. Smoke, dust storms, exhaust from industries and automobiles, etc.
  • Gels – A colloidal solution in which minute droplets of liquid are dispersed into a solid dispersion medium. e.g. jellies, Cheese, curd, shoe polish, etc.
  • Solid Foam – A colloidal solution in which minute particles of gas are dissolved into a solid dispersion medium. e.g. Pumice stone, rubber, cake, etc.

Classification of Colloids on the Basis of Interaction

  • Lyophilic Colloids: These types of colloidal solutions have a strong affinity between the particles of the dispersed phase and the particles of the dispersed medium. e.g. Gum, Starch, gelatin, and proteins. etc.
  • Lyophobic Colloids: These types of colloidal solutions have a weak affinity between the particles of the dispersed phase and the particles of the dispersion medium. e.g. Silver Sol, Ferric hydroxide sol, and arsenic sulfide sol. 

Classification of Colloids on Properties of Sol Particles

  • Multimolecular Colloids: When smaller molecules of substance or many atoms get dissolution and combine to form a species whose size is in the range of colloidal size is known as multi-molecular colloids. e.g. sulfur solution with thousands of S8 particles.
  • Macromolecular Colloids: The biomolecular such as enzymes or proteins which are bigger in size when immersed in a proper dispersion is known as macromolecular colloids. e.g. rubber, cellulose, starch, etc.
  • 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.

Classification Based on Dispersion Medium

  • Hydrosol: As the name suggests, Hydrosol, Hydro means water therefore, in these colloids the dispersion medium is water. For Example, Starch, etc.
  • Alcosol: These colloids contain alcohol as the dispersion medium. 
  • Acrosol: These colloids have air as the dispersed phase. e.g. mist, etc.

Preparation of Colloids

The colloids which are stable are known as lyophilic sols, in these, the strong forces of attraction take place between the dispersed phase and dispersion medium. Some of the major methods to prepare colloids are as follows:

Condensation Method 

Small solute particles are condensed in this process to create a dispersed phase particle.

  • By Oxidation: We can obtain colloidal Sulphur by passing oxygen gas through the solution of Hydrogen Sulphides. HNO3, H3Br2, etc are used as oxidizing agents in this process.

2H2S + O2  ⇢  2H2O + 2S

  • By Reduction: In the process, suitable reducing agents such as formaldehyde, hydrogen peroxide, stannous chloride, etc are reacted with the aqueous solution of these salts to obtain metals like gold, silver, and platinum in the colloidal state.

2AuCl3 + 3SnCl2  ⇢  3SnCl4 + 2Au

  • By Double Decomposition: In this process hydrogen Sulphide is passed through a cold arsenic oxide solution in water and a solution of arsenic Sulphide is obtained.

As2O3 + 3H2S   ⇢  As2S3 + 3H2O

  • By Hydrolysis: Salt solutions are hydrolyzed by boiling their respective dilute solutions. For example, ferric hydroxide is obtained by hydrolysis of its corresponding salt.

FeCl3 + 3H2O ⇢ Fe(OH)3 + 3HCl

Dispersion Methods

Large particles of material (suspension) are broken down into smaller particles using these procedures. The procedures listed below are used:

  • Mechanical Dispersion: In this method, the substances are grounded to coarse particles and then mixed with a dispersion medium to get a suspension. Then, it is ground in a colloidal mill consisting of two metallic dyes rotating in opposite directions. This method is used to obtain colloidal solutions of black ink paint varnished dyes, etc.
  • Electrical or Bredig’S Arc method: In this method, the metal to be changed into solution is made as two-electrode which is immersed in a dispersion medium. An electric arc is used in between the electrodes and ice to keep the dispersion medium cool. Here, the excessive amount of heat gives a colloidal solute substance.
  • Peptization: The process of changing a freshly prepared precipitate into a colloidal solution is called peptization. A small amount of electrolyte is added as a peptization agent. Some of the important peptizing agents are electrolyte and sugar gem gelatin. For example, a freshly prepared ferric hydroxide can be changed into a colloidal solution through this method by shaking it with water containing FeCl3.

Fe(OH)3 + FeCl3 → (Fe(OH)1 Fe)+3 + 3Cl

Properties of Colloids

  • Filterability: Colloids require specialized filters known as ultrafilters for filtration. They easily flow through standard filter sheets without leaving any trace.
  • Stability: Colloids are relatively stable in nature. The particles of the dispersed phase are in constant motion and stay suspended in the solution.
  • Heterogeneous Nature: Colloids are heterogeneous in nature because they have two phases, the dispersed phase, and the dispersion medium.
  • Homogeneous Appearance: Despite the fact that colloids include suspended particles and are heterogeneous in nature, they appear to be a homogeneous solution. This is due to the fact that the suspended particles are so small that they cannot be seen by the naked eye.

Tyndall Effect: The Optical Properties of Colloidal Solutions

Colloids show a phenomenon known as the Tyndall effect, which John Tyndall identified in 1869. When we shine a bright converging beam of light through a dark colloidal solution, the path of the beam is lit with a blue glow. The scattering of light by colloidal particles is referred to as the Tyndall effect, and the lighted route is referred to as the Tyndall cone. The distributed colloidal particles scatter the light that falls on them, producing emissions similar to ultraviolet and visible radiations. These dispersed radiations are lit.

Tyndall Effect


Brownian Movement: The Mechanical Properties of Colloidal Particles

The Brownian movement is a highly significant characteristic of scattered particles in a colloidal solution. When a colloidal solution is examined via an ultramicroscope, the colloidal particles may be seen moving in a zigzag pattern. The colloidal particles are constantly bombarded from all directions by the moving molecules of the dispersion medium.

Thus, this gives momentum to the particles, causing them to travel ahead and collide with another particle. Collisions cause the colloidal particle to travel in a random zigzag pattern.

Application of Colloids

Colloids have wide applications in numerous ways in industries, medical and domestic applications like:

  • In food items: Syrup, Halwa, and Soup are examples of colloidal systems in cuisine.
  • Medicines: Colloidal silver, also known as Argyrols, works as an antiseptic for eye infections.
  • In Cottrell precipitator air purification: This technique includes coagulation of solution particles. When dust or smoke flows through the inlet of an electrified chamber with a central electrical plate that is charged with the opposite charge of a dent or a smoke particle, the particles coagulate and clean air is passed via another exit.
  • Refinishing of leather: Animal skins are very soft; when submerged in a tannin solution with the opposite charge of the animal skin, particles agglomerate and the skin hardens; this is known as the tanning of leather.

FAQs on Colloids

Question 1: Why is Milk a colloid?


Due to the charged gap particles that remain suspended in the liquid, milk is a colloid. Milk is a colloid, despite having the appearance of being a homogeneous mixture, since it contains tiny globules of protein and fat that do not separate apart after standing due to the (often negatively) charged particles.

Question 2: What are the changes that may occur in a substance or matter?


The changes that occur in the matter are physical and chemical changes. The physical changes are about changes in the state of matter and other external factors whereas, chemical changes are concerned with the chemical composition of the matter.

Question 3: What is the precaution that should be taken during the dialysis process?


During the process of dialysis, the distilled water in the container should be frequently changed to prevent the accumulation of crystalloids. Otherwise, the impurities will get diffused back.

Question 4: What is the application of colloids in industrial products?


The application of colloid in industrial products are as follows:

  1. Colloids are used as a thickening agent for the products like lubricants, lotions, toothpaste, etc.
  2. It is also used in manufacturing inks and paints.
  3. It is used in medicines as an antiseptic.
  4. And, as stabilizers in food industries to manufacture foods items like jam, ice cream, etc.

Question 5: Which substance is used to reduce the pore size of filter paper for ultrafiltration?


As filter paper have a large pore size which is not efficient for the filtration of colloid so, to reduce the size of pores collodion solution (cellulose nitrate solution) is used.

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Last Updated : 25 Oct, 2022
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