A heterogeneous mixture in which solute particles are larger than molecules or ions but cannot be seen by naked eye is called colloidal solution. A system which shows properties of colloidal solution is called colloidal system. Its example is clay in water or starch in water. The solvent is called the dispersion medium in a colloidal solution. The solid solute particles are called dispersed medium in colloidal solution. The dispersion medium may be a liquid, gas or solid.

• Smoke is composed of a solid dispersed in gas.
• Milk is a liquid dispersed in liquid
• Pumice stone is gas dispersed in s solids There are two forms of colloidal systems:
• 1. Sol: A system composed of non-viscous colloidal solution is called sol. For example milk.

2. Gel: A system composed of viscous colloidal solution is called gel. For example butter.

The colloidal systems can be classified into two general classes on the basis of their affinity for liquids:

• Lyophilic Systems: The system in which dispersed phase and liquid dispersion medium attract each other is called lyophlic systems.
• Lyophobic Systems: The systems in which the dispersed phase and liquid dispersion phase repel each other is called lyophobic systems.Properties of Colloidal Systems
• The colloidal system shows following properties
  1. Adsorption: The tendency of molecules and ions to adhere to the surface of certain solids or liquids is called adsorption. Colloidal particles show a high tendency of adsorption. Thus, colloidal systems provide a large surface area for adsorption of molecules and ions.
  2. Brownian Movements: Robert Brown in 1927 observed that colloidal particles show random dancing movements. These movements were named Brownian movements.
  3. Tyndall Effect: The colloidal particles scatter light. This is called Tyndall effect. The path of light appears as a cone. It is known as Tyndall cone. This property helps to detect the presence of colloidal particles.
  4. Precipitation: The additions of an electrolyte remove the electrical double layer present around the colloidal particles. As a result the dispersed particles of a colloidal suspension will aggregate and precipitate.
  5. Electrical Properties: All colloidal particles carry same electric charge. This charge may be positive or negative. There is a adsorption of free ions in the dispersion medium. It produces an electrical double layer around the colloidal particles. The electric charges on the colloidal particles stabilize the colloidal systems.
  6. Filtration: The colloidal particles cannot pass through a parchment membrane. This property of colloidal dispersions is used to separate them from true solution by a process called dialysis.
  7. Phase Reversal: The sol and gel form of colloidal system can be interchanged due to change in certain conditions. Certain lyophilic sols form gels under certain conditions. For example, aqueous agar sols are cooled. It forms a jelly-like gel. The conversion of a sol to a gel is called gelation. If a gel of gelatin or agar is heated, it will convert back to a sol. This process is known as solation. The property of colloidal dispersions is called phase reversal.

  Biological significance of colloidal systems

  1. 1. Protoplasm a Colloidal system: Protoplasm is living, viscous (semi-fluid, jelly-like) substance. It is surrounded by cell wall. It is present in all prokaryotic and eukaryotic cells. Protoplasm is colloidal in nature. The small molecules and ions are true solute particles. But the larger particles remain suspended in
  2. Cyclosis and amboeid movements: The cyclosis occurs due to phase reversal of colloidal property. The cyclosis usually occurs in sol phase. Atnoeboid movements in amoeba occur due to colloidal properties.
  3. Fruits: ‘Turns store a large amount of proteins and starch. They also exhibit colloidal properties. These properties help in the storage of food in fruits.
  4. Blood: Hie plasm protein forms a colloidal system in blood. This system maintains the pH and osmotic concentration of blood.
  5. Milk: Milk is a perfect colloidal system. It contains all essential nutrients for young.