Is Matter Around Us Pure — SOF NSO Study Notes

Overview

Most substances we encounter daily — milk, air, soil, ink — are not chemically pure. They are **mixtures** of two or more pure substances. The SOF NSO tests your ability to classify these mixtures, understand their properties, and apply appropriate separation techniques. Questions appear in both the main Science section (5–8 marks) and occasionally in the Achievers Section as application-based scenarios.

Mastery requires three skills: (1) distinguishing pure substances from mixtures by properties, (2) classifying mixtures as homogeneous or heterogeneous (solutions, suspensions, colloids), and (3) selecting the correct separation method based on physical properties like particle size, boiling point, solubility, or density. Diagram-based questions showing separation apparatus (distillation setup, centrifuge, filter) are common, as are real-world identification problems (classify milk, sea water, blood).

You must memorize the exact particle-size ranges and defining properties of solutions, suspensions, and colloids, plus the principle behind each separation method. The exam frequently presents a mixture and asks "Which method separates X from Y?" — requiring instant recall of technique applicability.

Key Concepts

• **Pure substance**: Single type of particle throughout; fixed composition and properties. Examples: distilled water (H₂O), pure gold, sugar crystals. Cannot be separated by physical methods.

• **Mixture**: Two or more substances physically combined; variable composition; components retain individual properties. Can be separated by physical methods without chemical reactions.

• **Homogeneous mixture (Solution)**: Uniform composition throughout; particle size less than 1 nm; transparent; does not scatter light (no Tyndall effect); never settles. Examples: salt water, air, vinegar.

• **Heterogeneous mixture**: Non-uniform composition; components visible or distinguishable. Subdivided into suspensions and colloids based on particle size.

• **Suspension**: Particle size greater than 100 nm; opaque; particles visible to naked eye; settles on standing; separable by filtration. Examples: muddy water, chalk-water mixture, sand in water.

• **Colloid**: Particle size 1–100 nm; translucent; shows Tyndall effect (scatters light beam); does not settle; cannot be filtered by ordinary filter paper. Examples: milk, fog, smoke, jelly.

• **Tyndall effect**: Scattering of light by colloidal particles, making the path of light visible. Used to distinguish colloids from true solutions.

• **Separation principle**: Exploit differences in physical properties — boiling point, solubility, particle size, density, magnetic behavior — without altering chemical identity.

Formulas / Key Facts

1. **Particle size classification**: Solution < 1 nm; Colloid 1–100 nm; Suspension > 100 nm. (Memorize these exact boundaries.)

2. **Concentration formula**: Mass by mass percentage = (Mass of solute / Mass of solution) × 100

3. **Saturated solution**: Maximum solute dissolved at given temperature; no more solute dissolves. Unsaturated: can dissolve more solute.

4. **Solubility definition**: Maximum grams of solute that dissolves in 100 g of solvent at specified temperature.

5. **Types of colloids**: Sol (solid in liquid), Gel (liquid in solid), Emulsion (liquid in liquid), Foam (gas in liquid), Aerosol (solid or liquid in gas).

6. **Alloy**: Homogeneous mixture of metals (e.g., brass = copper + zinc) or metal + non-metal (steel = iron + carbon).

7. **Miscible vs immiscible**: Miscible liquids (e.g., alcohol-water) mix in all proportions; immiscible liquids (oil-water) form separate layers.

8. **Filtrate**: Liquid that passes through filter paper. Residue: solid left on filter paper.

Worked Examples

**Example 1: Classification** *Question*: Classify the following as solution, suspension, or colloid: (a) sugar water, (b) milk, (c) beach sand in water.

*Solution*: (a) Sugar water — particles < 1 nm, transparent, does not settle → **Solution** (b) Milk — particles 1–100 nm, shows Tyndall effect, does not settle → **Colloid** (c) Beach sand in water — particles > 100 nm, opaque, settles quickly → **Suspension**

**Example 2: Separation method selection** *Question*: Choose the appropriate method to separate: (a) salt from sea water, (b) chalk powder from water, (c) cream from milk.

*Solution*: (a) Salt from sea water — **Evaporation** (or distillation if water needed). Heat evaporates water; solid salt residue remains. (b) Chalk powder from water — **Filtration**. Chalk particles too large to pass through filter paper. (c) Cream from milk — **Centrifugation**. Spinning separates lighter fat (cream) from denser milk by density difference.

**Example 3: Distillation problem** *Question*: Acetone boils at 56°C; water boils at 100°C. Which boils off first during distillation of their mixture?

*Solution*: **Acetone** vaporizes first because it has the lower boiling point. In fractional distillation, the component with lowest boiling point evaporates, condenses and collects first in the receiver. Water remains in the distillation flask until temperature reaches 100°C.

Common Mistakes

1. **Confusing solution and colloid** → Students think milk is a solution because it looks uniform. **Fix**: Check for Tyndall effect; milk scatters light (colloid), sugar water does not (solution).

2. **Thinking filtration works for colloids** → Colloid particles (1–100 nm) pass through ordinary filter paper. **Fix**: Use centrifugation or special membranes (dialysis) for colloids; filtration only works for suspensions (> 100 nm).

3. **Misapplying evaporation vs distillation** → Using evaporation when both components are needed. **Fix**: Evaporation recovers only the solid (salt from sea water); distillation recovers both (pure water + salt).

4. **Ignoring solubility-temperature relationship** → Assuming solubility is constant. **Fix**: Most solid solutes become more soluble as temperature increases (used in crystallization); gases become less soluble at higher temperatures.

5. **Calling alloys as compounds** → Alloys are homogeneous mixtures, not compounds. **Fix**: Brass components (copper, zinc) retain individual properties and can be separated by physical methods; no fixed ratio.

Quick Reference

• **Mixture classification by appearance**: Homogeneous (uniform) vs Heterogeneous (non-uniform).
• **Mixture classification by particle size**: Solution < 1 nm < Colloid < 100 nm < Suspension.
• **Tyndall test**: Shine light through liquid; if beam visible → Colloid; if not → Solution.
• **Separation by particle size**: Filtration (suspensions), Centrifugation (colloids/emulsions).
• **Separation by boiling point**: Simple distillation (≥ 25°C difference), Fractional distillation (< 25°C difference).
• **Separation by solubility**: Crystallization (recover solid from solution by cooling).
• **Magnetic separation**: Removes magnetic substances (iron filings from sulfur).
• **Separating funnel**: Separates immiscible liquids by density (oil-water).