A colloidal solution (or colloid) is a heterogeneous mixture in which particles of size 1 to 1000 nanometres (1–1000 nm) are dispersed throughout a continuous medium. Colloidal particles are large enough to scatter light (producing the Tyndall effect) but small enough to remain suspended indefinitely without settling. Common examples include milk, fog, blood, smoke, and gelatin.
Colloidal particles have a size range of 1 to 1000 nm (nanometres).
Colloids show the Tyndall effect — scattering of a light beam — which true solutions do not.
Colloidal particles undergo Brownian motion (random zig-zag movement).
Colloids are heterogeneous mixtures but appear homogeneous to the naked eye.
Milk is an emulsion (oil-in-water colloid); blood is a sol (solid-in-liquid colloid).
Fog is an aerosol (liquid droplets dispersed in gas).
Coagulation of colloids occurs by adding electrolytes, which neutralise surface charge.
Colloidal particles carry electric charge and show electrophoresis under an electric field.
Mixtures are classified into three types based on dispersed particle size:
True Solution (homogeneous mixture):
Colloidal Solution (heterogeneous mixture):
Suspension (heterogeneous mixture):
The Tyndall effect is the scattering of light by colloidal particles, making the beam of light visible as it passes through the colloid. This was first observed by physicist John Tyndall in 1869.
Why it occurs: Colloidal particles (1–1000 nm) are comparable in size to the wavelengths of visible light (400–700 nm), causing them to scatter light effectively (Rayleigh/Tyndall scattering). True solutions (particle size < 1 nm) are too small to scatter light.
Examples of Tyndall effect in real life:
Tyndall effect is used to distinguish colloids from true solutions.
Colloids are classified based on the states of the dispersed phase and dispersion medium:
| Type | Dispersed Phase | Medium | Examples |
|---|---|---|---|
| Sol | Solid | Liquid | Paint, ink, blood, starch solution |
| Aerosol | Solid/Liquid | Gas | Smoke, fog, mist, clouds |
| Foam | Gas | Liquid | Whipped cream, soap lather |
| Emulsion | Liquid | Liquid | Milk, mayonnaise, butter |
| Gel | Liquid | Solid | Jelly, cheese, boot polish |
| Solid sol | Solid | Solid | Alloys (coloured glass) |
Hydrophilic colloids: water-attracting (e.g., gelatin, starch, protein) — easily hydrated and stable Hydrophobic colloids: water-repelling (e.g., Fe(OH)₃ sol, As₂S₃ sol) — less stable, require traces of electrolyte for stability
Tyndall effect: Scatters light beam (discussed above)
Brownian motion: Colloidal particles exhibit random, zig-zag motion due to collision with molecules of the dispersion medium. Discovered by Robert Brown. This keeps particles from settling.
Electrophoresis: Colloidal particles carry electric charge and migrate toward oppositely charged electrode under an applied electric field.
Coagulation (flocculation): Adding electrolytes neutralises the surface charge on colloidal particles, causing them to aggregate and precipitate. e.g., alum (KAl(SO₄)₂) coagulates clay colloids in river water — delta formation.
Adsorption: Colloidal particles have large surface area and can adsorb ions/molecules on their surface.
Osmotic pressure: Very low (due to large particle size compared to true solutions).
Biological and natural colloids:
Food colloids:
Industrial colloids:
A colloidal solution (colloid) is a heterogeneous mixture in which particles of size 1–1000 nm are dispersed in a medium. These particles are too small to settle but large enough to scatter light, producing the Tyndall effect. Examples include milk, blood, fog, and smoke.
The Tyndall effect is the scattering of a light beam by colloidal particles, making the path of light visible. It occurs because colloidal particles (1–1000 nm) are comparable in size to wavelengths of visible light. True solutions do not show this effect because their particles are too small (<1 nm).
In a true solution, particles are < 1 nm (ions/molecules), the mixture is transparent, and no Tyndall effect is seen. In a colloidal solution, particles are 1–1000 nm, it may appear translucent, and it shows the Tyndall effect. Both pass through ordinary filter paper, but only colloids are retained by ultrafiltration membranes.
Brownian motion is the continuous, random, zig-zag movement of colloidal particles caused by unequal bombardment by molecules of the dispersion medium. It prevents colloidal particles from settling under gravity and contributes to the stability of the colloid.
Five examples of colloidal solutions: (1) Milk — emulsion of fat in water; (2) Blood — sol of proteins and cells in plasma; (3) Fog — aerosol of water droplets in air; (4) Smoke — aerosol of solid particles in air; (5) Jelly — gel of liquid in solid gelatin matrix.
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