Overview of Colloid Preparation
In Class 9 Science Practicals (Unit I), students learn to distinguish between true solutions, suspensions, and colloids. A colloid (or colloidal solution) is a mixture where one substance of microscopically dispersed insoluble particles is suspended throughout another substance. The particles are larger than molecules but smaller than suspension particles.
Key Preparation Methods
- Starch Sol: Prepared by adding a thin paste of starch and water into boiling water while stirring continuously. Heating is required to break down starch granules and stabilize the colloid.
- Egg Albumin/Milk Sol: Prepared by mixing egg white or milk with cold water (room temperature). Heating is avoided here because proteins in egg albumin coagulate (harden) upon heating, destroying the colloidal nature.
Properties Observed
| Property | Colloidal Solution |
|---|---|
| Stability | Stable (particles do not settle down on standing). |
| Filtration | Particles pass through ordinary filter paper. |
| Transparency | Translucent (semitransparent). |
| Tyndall Effect | Scatters a beam of light passing through it. |
Quick Revision Points
- Dispersed Phase: The solute-like component (e.g., starch particles).
- Dispersion Medium: The solvent-like component (e.g., water).
- Colloids appear homogeneous to the naked eye but are actually heterogeneous.
- Particle size ranges from 1 nm to 100 nm.
Extra Practice Questions
- Why is distilled water preferred for preparing these sols? (Answer: To avoid impurities/ions affecting stability).
- What happens if you add salt to a colloid? (Answer: Coagulation or precipitation may occur).
- Is fog a colloid? (Answer: Yes, liquid dispersed in gas).
- Can we see colloidal particles with a naked eye? (Answer: No).
- What is the specific name for a liquid-in-liquid colloid like milk? (Answer: Emulsion).
