.png)
Is Matter Around Us Pure? Exam Study Guide
Welcome back. This chapter is a favorite for examiners because it tests your practical understanding of chemistry. Here is your dissected study guide.
1. The Scientific Definition of "Pure"
Common View: Pure means no adulteration (e.g., pure milk).
Scientific View: Pure means the substance is made of only one type of particle (atoms or molecules).
- Is Milk pure? No. It is a mixture of water, fat, and proteins.
- Is Air pure? No. It is a mixture of gases.
- Is 24-carat Gold pure? Yes (only Gold atoms).
- Is 22-carat Gold pure? No (Mixed with Copper/Silver).
2. The Three Types of Mixtures (High Priority)
You must distinguish between Solutions, Suspensions, and Colloids.
A. True Solution (Homogeneous)
- Appearance: Clear, uniform.
- Particle Size: Extremely small (< 1nm). Invisible to naked eye.
- Light: Does NOT scatter light (No Tyndall Effect).
- Examples: Salt water, Sugar water.
B. Suspension (Heterogeneous)
- Appearance: Cloudy, visible particles.
- Stability: Unstable (particles settle if left undisturbed).
- Light: Scatters light (Tyndall Effect) until particles settle.
- Examples: Muddy water, Chalk powder in water.
C. Colloidal Solution (The Tricky One)
- Appearance: Looks Homogeneous but is actually Heterogeneous.
- Feature: Scatters light (Tyndall Effect).
- Structure: Dispersed Phase (solute-like) + Dispersion Medium (solvent-like).
Exam Cheat Sheet for Colloids (Memorize This):
| Dispersed Phase | Dispersion Medium | Type | Example (Exam Favorites) |
|---|---|---|---|
| Liquid | Gas | Aerosol | Fog, Clouds, Mist |
| Solid | Gas | Aerosol | Smoke, Car exhaust |
| Gas | Liquid | Foam | Shaving cream |
| Liquid | Liquid | Emulsion | Milk, Face cream |
| Solid | Liquid | Sol | Milk of Magnesia, Mud |
3. Separation Techniques
Examiners love matching questions here: "Which technique is used to separate X from Y?"
| Mixture | Technique | Principle / Notes |
|---|---|---|
| Cream from Milk | Centrifugation | Density: Spinning forces denser particles to bottom. |
| Oil and Water | Separating Funnel | Density: Immiscible liquids separate into layers. |
| Salt and Camphor | Sublimation | Volatility: Camphor turns to gas; Salt does not. |
| Drugs in Blood / Dyes | Chromatography | Solubility: Colors rise at different speeds on paper. |
| Acetone and Water | Distillation | Boiling Point: For miscible liquids (Diff > 25K). |
| Oxygen from Air | Fractional Distillation | Boiling Point: For liquids with Diff < 25K. |
| Impure Copper Sulphate | Crystallization | Better than evaporation (avoids burning the solid). |
4. Physical vs. Chemical Changes
- Physical Change: No new substance. Reversible. (Melting ice, glowing bulb).
- Chemical Change: New substance formed. Irreversible. (Rusting, cooking, souring milk).
Is burning a candle physical or chemical? Answer: Both.
1. Melting of wax = Physical.
2. Burning of wick/wax ($CO_2$ + Heat) = Chemical.
5. Elements and Compounds
- Element: Basic form (Iron, Oxygen).
- Metals: Lustrous, conductive. Exception: Mercury (Liquid).
- Non-Metals: Poor conductors. Exceptions: Bromine (Liquid), Iodine (Lustrous), Graphite (Conducts electricity).
- Compound: Elements chemically combined (Water, $H_2O$).
- Concept: Properties are totally different from constituents.
Hydrogen (Burns) + Oxygen (Supports Fire) = Water (Extinguishes Fire).
- Concept: Properties are totally different from constituents.
6. Mentor’s Final Drill
Q: Mixture of Naphthalene balls and Common Salt. How to separate?
A: Sublimation (Naphthalene turns to gas).
Q: Why is "Air" a mixture, not a compound?
A: No fixed formula. Gases retain their individual properties.
Q: What is the "Tyndall Effect"?
A: Scattering of a light beam by particles in a Colloid (e.g., sunlight in a forest).
Q: Identify solute and solvent in "Tincture of Iodine".
A: Solute = Iodine (Solid); Solvent = Alcohol (Liquid).
Revision Tip: Focus heavily on the Separation Techniques table and the Colloids table. These are the areas where factual errors happen most often. You are doing great!