Surface Chemistry

Properties of Colloidal Solutions

A colloid is a mixture in which one substance of microscopically dispersed insoluble particles is suspended throughout another substance. Owing to this peculiar structure of colloid, it has varied physical and chemical properties. Let us explore more about the physical, chemical, optical as well as electrical properties of colloidal solutions.

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Physical Properties of Colloidal Solutions

  • Stability: Colloids are relatively stable in nature. The particles of the dispersed phase are in a state of continuous motion and remain suspended in the solution.
  • Filterability:  Colloids require specialized filters known as ultrafilters for filtration. They readily pass through ordinary filter papers without yielding any residue.
  • Heterogenous nature: Since colloids consist of two phases, the dispersed phase as well as the dispersion medium, they are known as heterogeneous in nature.
  • Homogenous appearance: Even though colloids have suspended particles and are heterogeneous in nature, they appear as if it is a homogenous solution. This is because the suspended particles are so tiny that they are not visible by the naked eye.

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Colligative Properties

The particles of the dispersed phase come together to form associate molecules. The formation of these associate molecules renders the solution certain special properties such as

  • a decrease in vapor pressure
  • elevation in boiling point
  • depression in freezing point
  • a decrease in osmotic pressure

Read the Classification of Colloids here.

Optical Properties of Colloidal Solutions: Tyndall Effect

Properties of colloidal solutions

Colloids exhibit a phenomenon known as the Tyndall effect observed by Tyndall in 1869. When we pass an intense converging beam of light through a colloidal solution kept in dark, the path of the beam gets illuminated with a bluish light. This phenomenon of scattering of light by colloidal particles is called the Tyndall effect and the illuminated path is known as the Tyndall cone. The dispersed colloidal particles scatter the light falling on them resulting in emissions that are comparable to ultraviolet and visible radiations. These scattered radiations get illuminated.

The zone of scattered light is observed to be much larger than the particle itself. This makes the colloidal particles to appear as tiny bright spots when viewed under a microscope. This has to be done at right angles to the beam of light.

True solutions do not exhibit a Tyndall effect. This is because the size of particles (ions or molecules) present in a true solution are too small to scatter light. Thus, the Tyndall effect can be used to distinguish a colloidal solution from a true solution.

Learn different types of Emulsion and its properties here.

Mechanical Properties of Colloidal Particles: Brownian Movement

The dispersed particles present in a colloidal solution exhibit a very important property called the Brownian movement. When a colloidal solution is viewed under an ultramicroscope, the colloidal particles are seen continuously moving in a zigzag path.

There is a continuous bombardment of the moving molecules of the dispersion medium on the colloidal particles from all directions. This imparts a momentum to the particles to move in a forward direction where again it collides with another particle. These collisions result in the random zigzag movement of the colloidal particle.

The Brownian movement imparts stability to the sol. It opposes the gravitational force acting on colloidal particles and prevents them from settling down thus maintaining the stability of the sol.

Learn about Chemical Adsorption and its significance here.

Electrical Properties of Colloidal Solutions

The particles of the colloidal solution carry the same type of charge, while the dispersion medium carries an equal and opposite charge. The charge on the dispersion medium balances the charge on dispersed particles and the solution as a whole is electrically neutral.

The dispersed particles of a colloid repel each other since they carry similar charges and this prevents them from settling down thus maintaining the stability of the sol. Based on the nature of the charge, the colloidal sols may be classified as positively charged and negatively charged sols.

Solved Questions For You

Que: Which of these is not a property of colloids?

  1. Tyndall effect
  2. Brownian motion
  3. Heterogenous nature
  4. High instability

Ans: The correct option is “D”. High instability is not a property of colloids. Rather, colloids are quite stable.

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