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Colloids - Definition, Classification, Preparation, Applications, Practice Problems and FAQ

Let's try an experiment. Turn your phone's flashlight on and place it upside down. Take a glass of water and a glass of milk. Place each glass on the top of the flashlight and observe.

What did you observe? 

The light easily passes through the glass of water but when the glass of milk was placed, the light was not able to pass through. 

Why was this phenomenon observed?

Well, the reason is pretty simple. The constituent particle size of milk is very large as compared to the constituent particles of water. Due to this, the movement of light was hindered. Though water and milk are both liquids, they are not similar in composition. 

We can think of several solutions and solvents which are basically liquids, but have different compositions and hence, different properties. 

Colloids is a type of a mixture in which the particles of one substance (dispersed phase) are dispersed in another substance (dispersion medium). 

In this concept page, we will understand how colloids are different from other solutions, their classification, properties and applications. 

TABLE OF CONTENTS

  • What is a Colloid?
  • Classification of Colloids
  • Preparation of Colloids
  • Applications of Colloids
  • Practice Problems
  • Frequently Asked Questions - FAQ

What is a colloid?

The term colloid is derived from the Greek word ‘Kola’, meaning ‘glue-like’ colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium. In a colloid, the dispersed phase may consist of particles of a single macromolecule or an aggregate of many atoms, molecules or ions. 

Colloidal particles have an enormous surface area per unit mass as a result of their small size. Due to this enormous surface area, they exhibit some special properties.

Solution vs Colloid vs Suspension

Solutions

Colloids 

Suspensions

Particle size

The constituent particles of solutions are ions or small molecules having a particle size in the range of 0.01 to 1 nm

Particle size

The constituent particles of colloids are large molecules or aggregates having a particle size in the range of 1 to 1000 nm

Particle size

The constituent particles of solutions are large molecules or aggregates having a particle size in the range of over 1000 nm

Filterability

Solutions can not be separated by filtration 

Filterability

Colloids can not be separated by filtration 

Filterability

Suspensions can be separated by filtration 

Settling

Solutions do not separate on standing 

Settling

Colloids do not separate on standing

Settling

Particles of Suspensions settle down on standing

Visibility 

Particles are so small that it is not possible to see through naked eyes.

Visibility 

Particles of colloids are bigger than that of the solution but smaller enough to be unseen by naked eyes.

Visibility 

Particles of suspension can be seen easily.

Transmission of lights

Light easily passes through the solution.

Transmission of lights

Light passes through colloids up to a certain extent and shows the Tyndal effect

Transmission of lights

Light can not pass through the suspension due to bigger particles. 

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Classification of Colloids

Colloids can be classified based on the following criteria:

  1. Physical state of the dispersed phase and the dispersion medium
  2. Nature of interaction between the dispersed phase and the dispersion medium
  3. Type of particles of the dispersed phase
  • Physical state of the dispersed phase and the dispersion medium

Depending on whether the dispersed phase and the dispersion medium are solids, liquids, or gases, colloidal systems can be classified into eight different types. A colloidal system is one in which a gas is combined with another gas to generate a homogenous mixture.

Depending upon the nature of the dispersion medium, colloidal solutions have specific names as shown in the table below.

Dispersion medium

Name of colloidal system

Water 

Hydrosol or aquasol

Alcohol 

Alcosols

Benzene

Benzosols

Air

Aerosols

 

2.  Nature of interaction between the dispersed phase and the dispersion medium

Colloidal sols are characterised as lyophilic (solvent loving) or lyophobic (solvent hating) based on the nature of the interaction between the dispersed phase and the dispersion medium. When water is the dispersion medium, the terms hydrophilic (love of water) and hydrophobic (hate of water) are used (water hating)

a.  Lyophilic colloids: The word lyophilic means liquid loving. They are prepared by directly mixing substances like rubber, gum, starch, gelatin, egg albumin, etc., with a suitable dispersion medium. 

Since they directly form colloidal sols, they are also referred to as intrinsic sols. One of the important characteristics of this type of sol is that even when the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply mixing with the dispersion medium. Due to this property, they are also called reversible sols. These sols are quite stable and hence cannot be easily coagulated.

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b.  Lyophobic colloids: The word lyophobic means liquid hating. Substances such as metals and their sulphides, etc., when simply mixed with the dispersion medium do not form the colloidal sol. These sols are called lyophobic sols. 

Their colloidal sols can be only prepared by special methods. As their colloidal sols cannot be prepared by simple mixing, they are also called extrinsic colloids. These sols are readily coagulated by adding a suitable electrolyte or heating and shaking. Hence, they are not stable. Once lyophobic sols are precipitated, they cannot be reconstituted by simply mixing with the dispersion medium. Therefore, they are also referred to as irreversible sols. These sols required stabilising agents for their preservation.

Lyophilic Colloids

Lyophobic Colloids

Ease of preparation

Lyophilic colloids can be prepared directly from colloidal sols. Hence, are also referred to as intrinsic sols.

Ease of preparation

Lyophobic colloids can not be prepared directly from colloidal sols. They require a special method for preparation.

Stability

These solutions are quite stable and can not be easily coagulated or precipitated.

Stability

These solutions are less stable and can be easily coagulated or precipitated.

Reversibility

These are reversible colloids.

Reversibility

These are irreversible colloids.

Solvation

A layer of dispersion medium covers highly solvated, scattered particles.

Solvation

Solvation does not occur in dispersed particles.

Viscosity

Highly viscous; viscosity is more than that of the medium.

Viscosity

The viscosity of the dispersion medium is the same.

Surface Tension

In most cases, it is less than the medium.

Surface Tension

In most cases, it is the same as the medium.

Examples

Dissolved starch, protein, gelatin, gum, etc., in water

Examples

Metals, metal sulphides, and metal oxides are all examples of lyophilic sols.

 

  • Type of particles of the dispersed phase

Based on the type of particles of the dispersed phase, colloids are classified into three types namely,

  1. Multimolecular colloids
  2. Macromolecular colloids
  3. Associated colloids

Preparation of Colloids

There are two major methods for the preparation of colloids. They are

  1. Dispersion methods 
  2. Condensation methods
  • Dispersion methods: This method involves the breaking of bigger particles into smaller colloidal particles. The three prominent methods in this type of preparation of colloids are 
  1. Mechanical disintegration
  2. Electrical disintegration (or) Bredig’s arc method
  3. Peptisation
  • Mechanical disintegration

The material is initially ground into coarse particles in this procedure. The suspension is then made by mixing it with dispersion media. A colloidal mill is then used to grind the suspension.

It comprises two metallic dyes that are practically touching and rotating at a high speed of 7000 revolutions per minute in the opposite direction. The mill's spacing between the dyes is controlled such that coarse suspension is subjected to a high shearing force, resulting in colloidal particles. This process produces a colloidal solution of black ink, paints, varnishes, and dyes.

  • Electrical disintegration (or) Bredig’s arc method:

The process involves both dispersion and condensation. Electrodes of the metal are suspended in the dispersion medium. An electric arc is struck between metal electrodes immersed in the dispersion medium. 

The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.


This method is used in the preparation of colloidal sols of metals such as gold, silver, platinum, etc., 

  • Peptisation:

The process of converting a precipitate into colloidal sol by shaking with a dispersion medium in the presence of a small amount of electrolyte, known as a peptising agent, is called peptisation. Generally, it is used to convert a freshly prepared precipitate into a colloidal sol. 

Example: Freshly precipitated Fe(OH)3 is shaken with an aqueous solution of FeCl3.




2. Condensation methods: In this method, smaller particles are condensed suitably to be of the colloidal size. The three prominent methods in this type of preparation of colloids are 

  1. By exchange of solvent
  2. By change of physical state
  3. Chemical methods
  • By exchange of solvent:

There are a variety of compounds that can be made into colloidal solutions by taking a solution of the substance in one solvent and pouring it into another solvent that is less soluble.

  • By change of physical state:

The vapours of certain elements, such as mercury and sulphur, are passed through cold water containing a stabiliser, such as ammonium salt of citrate, to produce a colloidal solution.

  • Chemical methods:

Colloids are prepared by chemical reactions like oxidation, reduction, etc. The product formed are molecules, aggregate and give sols.

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Application of Colloids

  • Electrical Precipitation of Smoke:

Smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc.

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The electric smoke precipitator is called the Cottrell smoke precipitator. Smoke is led through a chamber containing weight (plate) having a charge opposite to that carried by smoke particles. The smoke particles, on coming in contact with the plate lose their charge and get precipitated.


  • Thickening agent: Colloids are thickening substances used in lubricants, lotions, toothpaste, coatings, and other commercial items.
  • Industrial products: Colloids are important in the production of paints and inks. In ball-point pens, gel ink (liquid-solid colloid) is used.
  • Purification of water: Sulphates of aluminium (alum) and iron coagulate the suspended contaminants in natural water and make water fit for drinking.
  • Medicine: The majority of the medications are colloidal in nature. Colloidal gold and calcium are injected into the human body for muscular vitality. As an eye lotion, Argyrol (silver sol) is utilised. Albumin, hetastarch, and dextran are some of the other colloids used in medicine.

Practice Problems

Q 1. Match the following.
 

Column I (Dispersion medium)

Column II (Name of the colloidal system)

  1. Water 
  1. Aerosols
  1. Alcohol 
  1. Benzosols
  1. Benzene
  1. Alcosols
  1. Air
  1. Hydrosol or aquasol


a. 1 - I, 2 - II, 3 - III, 4 - IV
b. 1 - IV, 2 - III, 3 - II, 4 - I
c. 1 - I, 2 - III, 3 - II, 4 - IV
d. 1 - IV, 2 - II, 3 - III, 4 - I
Answer: For different dispersion mediums, there are different names for the colloidal system.

Column I (Dispersion medium)

Column II (Name of the colloidal system)

  1. Water 
  1. Hydrosol or aquasol
  1. Alcohol 
  1. Alcosols
  1. Benzene
  1. Benzosols
  1. Air
  1. Aerosols

So, option B) is the correct answer.

Q 2. Smoke is an example of

a. Solid in gas
b. Solid in liquid
c. Liquid in gas
d. None of the above
Answer: Smoke is an example of aerosol in which gas is the dispersion medium and the solid dust particles are the dispersed phase.

So, option A) is the correct answer.

Q 3. Which method is used for the preparation of gold sol?

a. Electrical disintegration
b. Peptization
c. Bredig’s arc method
d. Both A and C

Answer: D
Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared by the electrical disintegration method. This method is also known as Bredig’s arc method.

So, option D) is the correct answer.

Q 4. Which of the following methods involves a chemical method of preparation of colloids?\

a. Oxidation
b. Reduction
c. Double decomposition
d. All of the above
Answer: Colloids are prepared by chemical reactions like oxidation, reduction, double decomposition etc.

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So, option D) is the correct answer.

Frequently Asked Questions - FAQ

Q 1. Can we consider protein in water as a colloid?
Answer: 
Protein water is an example of a colloidal sol in which the sol particles have a hydrogen bonding affinity for the medium.

Q 2. Is it possible to convert a suspension into a colloid by high dilution?
Answer: 
Irrespective of whether a suspension is diluted or not, the particle size would not change, and if the constituent particles are bigger enough to see through the eyes, then the mixture will be considered a suspension.

Q 3. Why lyophilic sols are more stable?
Answer: 
The existence of charge and the solvation of colloidal particles contribute to the stability of lyophilic sols. Lyophobic sols, on the other hand, are only stable because of the presence of charge. As a result of the widespread solvation, the lyophilic sol is more stable than the lyophobic sol.

Q 4.Can we prepare whipped cream (a cream that is present on a birthday cake) from regular milk instantly?
Answer:
Cream is the fat-rich component of milk that rises to the top of the milk (or is driven there by centrifugation). Milk is a "colloid," which is a substance made up of minute, insoluble particles suspended in a liquid. Fat globules, small droplets of fat, distributed in a water-based solution are the particles in this scenario. The lighter-than-water fat globules will ultimately float to the top and congregate together, where they can be skimmed away from the "skim milk" left on the bottom if fresh, unhomogenized milk is left undisturbed. 

In regular milk, the fat percentages are not that much, hence, the preparation of whipped cream will take much longer time than usual.

Related Topics

Enzyme Catalysis

Adsorption

Properties of Colloids

Coagulation of Colloids

Emulsion

Purification of Colloids

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