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Colloids – Definition, Types, Properties, Applications

  • Last Updated : 16 Sep, 2021

As a kid, you may have prepared suspensions such as mud and water, flour and water, or tempera paint, which is a suspension of solid colors in the water. These suspensions are heterogeneous mixes of rather big visible particles (or that can be seen with a magnifying glass). They are hazy, and after mixing, the suspended particles settle out. When we form a solution, on the other hand, we create a homogeneous mixture in which no settling happens and the dissolved species are molecules or ions. Suspensions behave substantially differently than solutions. A solution can be colored, but it is clear, the molecules or ions are undetectable, and they do not settle out when left to stand.

Colloids (or colloidal dispersions) are a class of mixes with characteristics that fall between suspensions and solutions. A colloid’s particles are bigger than those of most simple molecules; nevertheless, colloidal particles are tiny enough that they do not settle out while standing.

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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.

Types of Colloids

There are mainly two types of colloids that are:

  1. 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.
  2. Macromolecular Colloids: The biomolecular such as enzymes or protein which are bigger in size when immersed in a proper dispersion is known as macromolecular colloids. e.g. rubber, cellulose, starch, etc.

Preparation of Colloidal Solutions

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 follow:

(1) 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 (Arsenic solution)

  • 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 (Colloidal solution of Ferrric Hydroxide)

(2) Dispersion Methods

Large particles of a 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 consist of two metallic dyes rotating in opposite directions. This method is used to obtain colloidal solutions of black ink paint varnishes dyes, etc.

  • Electrical or Bredig’S Arc method: In this method, the metal to be changed into solution are 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 

Physical Properties of Colloidal Solutions

  • Filterability: Colloids require specialised 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 with the naked eye.

Tyndall Effect: The Optical Properties of Colloidal Solutions

Colloids show a phenomena known as the Tyndall effect, which 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.

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 particle. 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 tanning of leather.

Sample Problems

Problem 1: 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.

Problem 2: 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.

Problem 3: What is the application of colloid 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.

Problem 4: Which method is applicable for the preparation of metallic sols?


Bredig’s Arc dispersion method is used for the preparation of metallic sols. In this method, metallic particles are broken into tiny-sized sol particles and immersed in a dispersion medium accordingly.

Problem 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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