Colloidal Solution Characteristics

A mixture of starch in water, described as neither a true solution nor a suspension, exhibits properties typical of a colloidal solution. Colloids fall between true solutions and suspensions in terms of particle size and behaviour.

• Particle Size: Colloidal particles are larger than solute particles in true solutions but smaller than suspended particles. They range from about 1 nanometer (nm) to 1000 nm.
• Light Scattering: Due to their size, colloidal particles can scatter a beam of light that passes through the mixture. This effect, known as the Tyndall effect, makes the light's path visible. True solutions do not exhibit this.
• Stability: Colloidal particles generally do not settle out of the mixture upon standing, even under gravity. They remain dispersed due to factors like Brownian motion and electrostatic repulsion. Suspensions, conversely, have particles that settle over time.

Identifying the Correct Property

Evaluating the options based on colloidal properties:

• Option 1 (Particles visible to naked eye): Incorrect. Colloidal particles are typically too small to be seen without a microscope.
• Option 2 (Completely transparent, does not scatter light): Incorrect. This describes a true solution. Colloids scatter light.
• Option 3 (Particles scatter light, do not settle): Correct. This accurately combines the Tyndall effect (light scattering) and the stability characteristic of colloids.
• Option 4 (Particles settle down): Incorrect. This describes a suspension, not a stable colloidal solution.

The observation that the starch-water mixture scatters light (Tyndall effect) and its particles remain dispersed without settling confirms its nature as a colloidal solution.