Is Matter Around Us Pure — Class 9 Physical Science

"Not everything that looks pure actually IS pure. Gold jewellery is never 100% gold. The 'pure' drinking water you buy has dissolved minerals. How do we tell what is truly pure?"

1. About the Chapter

This chapter explores what 'PURITY' means in a scientific sense — and how to SEPARATE mixtures into their components.

• Pure substances — elements and compounds with fixed composition
• Mixtures — homogeneous vs heterogeneous
• Solutions — types, concentration, solubility
• Suspensions and colloids — properties and differences
• Separation techniques — from simple evaporation to chromatography
• Physical vs chemical changes

Why This Chapter Matters

• EVERY separation technique here has real-world applications — from water purification to petroleum refining
• This is HIGH-WEIGHTAGE in AP Board exams (8-10 marks)
• Understanding mixtures and pure substances is ESSENTIAL for all higher chemistry (stoichiometry, reaction chemistry)

2. Pure Substances vs Mixtures

A PURE SUBSTANCE consists of a SINGLE type of particle (atoms or molecules) with a FIXED composition and DEFINITE properties. A MIXTURE consists of TWO or MORE substances mixed together in ANY proportion.

Common Misconception

'In everyday language, 'pure' means 'no adulteration'. In science, ONLY elements and compounds are pure. A 'pure' cold drink is NOT scientifically pure — it contains water, sugar, CO₂, flavours, and preservatives.'

3. Elements and Compounds

Element

A substance that CANNOT be broken down into simpler substances by CHEMICAL means. Contains only ONE kind of atom.

• Examples: Hydrogen (H), Oxygen (O), Gold (Au), Iron (Fe)
• Classification: Metals (Fe, Cu, Au), Non-metals (O, N, S), Metalloids (Si, Ge, As)

Compound

A substance formed by TWO or MORE elements combined CHEMICALLY in a FIXED ratio by mass.

• Examples: Water (H₂O — always 2:1 ratio), NaCl (always 1:1), CO₂ (C:O = 12:32 = 3:8)
• Properties: A compound has properties DIFFERENT from its constituent elements. 'Sodium (Na) is a violent metal. Chlorine (Cl) is a poisonous gas. But NaCl — common salt — is a harmless, edible crystal.'

4. Types of Mixtures

Homogeneous Mixture (Solution)

Uniform composition throughout. Particles are NOT visible. The mixture looks the SAME everywhere.

• Examples: Salt water, sugar solution, air, brass (alloy), vinegar

Heterogeneous Mixture

Non-uniform composition. Different components are VISIBLE.

• Examples: Sand and salt, oil and water, soil, a bowl of mixed fruits

'Alloys (like brass, bronze, steel) are HOMOGENEOUS mixtures of metals. Air is a homogeneous mixture of gases (N₂, O₂, Ar, CO₂, etc.).'

5. Solutions

A solution is a HOMOGENEOUS mixture of two or more substances.

Components

• Solute: The substance that DISSOLVES (smaller amount)
• Solvent: The substance that DOES the dissolving (larger amount)
• Example: In salt water, salt = solute, water = solvent

Types of Solutions Based on Solvent

• Aqueous: Water is the solvent (salt water, sugar water)
• Non-aqueous: Solvent is NOT water (iodine in alcohol — tincture iodine; sulphur in carbon disulphide)

Concentration of Solutions

Mass by mass percentage: (Mass of solute / Mass of solution) × 100 Mass by volume percentage: (Mass of solute / Volume of solution) × 100

Worked Example — Concentration

'10 g of sugar is dissolved in 190 g of water. Find the concentration (mass by mass percentage).'

Mass of solution = 10 + 190 = 200 g. Concentration = (10/200) × 100 = 5%.

Saturated, Unsaturated, Supersaturated

• Unsaturated solution: More solute CAN still dissolve at that temperature
• Saturated solution: MAXIMUM solute has dissolved. No more dissolves.
• Supersaturated: Contains MORE solute than normally possible at that temperature (achieved by heating then carefully cooling). 'Supersaturated solutions are UNSTABLE — adding a single crystal causes immediate crystallisation.'

Factors Affecting Solubility

1. Temperature: For MOST solids, solubility INCREASES with temperature
2. Pressure: Affects solubility of GASES — increases with pressure (soda bottles)
3. Nature of solute and solvent: 'Like dissolves like' — polar dissolves polar, non-polar dissolves non-polar

6. Suspensions and Colloids

Suspension

A HETEROGENEOUS mixture where particles are VISIBLE and settle down over time.

• Particle size: > 10⁻⁵ m (or 1000 nm)
• Properties: Particles settle on standing. Can be separated by filtration. Scatter light (opaque).
• Example: Muddy water, chalk powder in water, flour in water

Colloid

A HETEROGENEOUS mixture with particles of intermediate size that DO NOT settle.

• Particle size: 1 nm to 1000 nm (10⁻⁹ to 10⁻⁶ m)
• Properties: Particles do NOT settle. CANNOT be separated by ordinary filtration (need ultracentrifuge). Shows Tyndall effect.
• Examples: Milk, blood, jelly, smoke, fog, mayonnaise, cheese

The Tyndall Effect

'When a beam of light passes through a colloid, the path of the beam becomes VISIBLE. This is because colloidal particles are large enough to SCATTER light. This is the Tyndall effect.'

Examples: The visible beam of sunlight through a dusty room (dust is a colloid). Headlight beams through fog. The blue colour of the sky is due to scattering of sunlight by colloidal dust particles in the atmosphere.

Types of Colloids

7. Separation Techniques

7.1 Evaporation (Simple)

Used to separate a SOLID dissolved in a LIQUID. Heat the solution — liquid evaporates, solid remains.

• Example: Getting salt from seawater. 'AP's salt pans near Kakinada and Nellore use solar evaporation — seawater is trapped in shallow ponds, the sun evaporates water, and salt crystals are left behind.'

7.2 Crystallisation

A PURER form of evaporation. The solution is heated to form a SATURATED solution, then cooled slowly to form CRYSTALS. Better than evaporation because impurities remain in solution.

• Example: Obtaining pure copper sulphate crystals from impure samples
• 'Crystallisation gives PURE crystals. Simple evaporation can leave behind impurities that were also dissolved.'

7.3 Centrifugation

Uses CENTRIFUGAL FORCE to separate lighter and heavier components in a mixture. The denser particles settle at the bottom under rapid spinning.

• Examples: Separating cream from milk. Drying water from wet clothes in a washing machine spinner. Blood separation in labs (RBCs settle, plasma stays top).

7.4 Chromatography

Separates components of a mixture based on their DIFFERENT RATES of movement through a medium.

• Principle: Different substances travel at different speeds in a solvent (mobile phase) as they move through a stationary phase (e.g., filter paper)
• Applications: Separating colours in ink or dye. Testing purity of drugs. Forensic analysis. Separating pigments from leaves (chlorophyll, xanthophyll, carotene)

7.5 Distillation (Simple)

Used to separate a MIXTURE OF MISCIBLE LIQUIDS with a BOILING POINT DIFFERENCE of more than 25°C, or a solid from a liquid.

• Process: Heat the mixture in a distillation flask. The liquid with LOWER boiling point evaporates first, passes through a condenser, and is collected as DISTILLATE.
• Example: Obtaining pure (distilled) water from tap water. 'Distilled water is used in batteries and laboratories because it has NO dissolved salts.'

7.6 Fractional Distillation

Used to separate a MIXTURE of MISCIBLE LIQUIDS with CLOSE boiling points (difference < 25°C).

• Key: A fractionating column is placed between the flask and condenser. It provides MANY surfaces for condensation-evaporation cycles, allowing BETTER separation.
• Example: Separation of crude oil into petroleum fractions (petrol, kerosene, diesel, etc.). Separation of air into oxygen, nitrogen, and argon.

7.7 Separating Funnel

Used to separate TWO IMMISCIBLE LIQUIDS (liquids that do NOT mix).

• Process: Pour the mixture into a separating funnel. Let it settle — the denser liquid forms the LOWER layer. Open the stopcock to drain the lower layer first.
• Examples: Separating oil from water. Separating kerosene from water.

8. Physical and Chemical Changes

'Melting of wax is a PHYSICAL change (wax → liquid wax → wax). Burning of wax is a CHEMICAL change (wax + O₂ → CO₂ + H₂O + heat + light).'

9. Common Mistakes to Avoid

1. 'Air is a compound' — Air is a MIXTURE (homogeneous) of gases with variable composition. Compounds have FIXED composition by mass.
2. 'Salt solution is a compound' — Salt solution is a MIXTURE. Salt (NaCl) is a compound, but dissolving it in water creates a mixture. The salt can be RECOVERED by evaporation.
3. 'All homogeneous mixtures are solutions' — Yes, solutions are a type of homogeneous mixture. But not all homogeneous mixtures are called solutions (e.g., air is a homogeneous mixture of gases).
4. 'Tyndall effect = colloidal solution' — While it is a characteristic of colloids, very dilute suspensions and even some solutions can show weak Tyndall effect. The ABSENCE of Tyndall effect does not guarantee a pure solution.
5. 'Fractional distillation and simple distillation are the same' — Fractional distillation uses a fractionating column for LIQUIDS with close boiling points. Simple distillation is for large boiling point differences.

10. AP SSC Exam Focus

Key Tip

'For separation technique questions, FIRST identify the TYPE of mixture (solid-solid, solid-liquid, liquid-liquid, miscible/immiscible). THEN choose the appropriate method. For liquid-liquid miscible mixtures: distillation. For immiscible: separating funnel.'

AP-Specific Application

'Andhra Pradesh has a long coastline (visakhapatnam, Kakinada, Machilipatnam) — salt production by solar evaporation is a MAJOR industry. The Godavari and Krishna delta regions are known for fertile soil, and centrifugation is used in dairy cooperatives across the state.'

11. Quick Self-Test

1. 'Classify the following as elements, compounds, or mixtures: Diamond, Seawater, Bronze, Glucose, Air.' Answer: Diamond — element (carbon). Seawater — mixture. Bronze — mixture (alloy of Cu + Sn). Glucose — compound (C₆H₁₂O₆). Air — mixture.

2. 'What is the difference between a colloid and a suspension?' Answer: Colloid particles (1-1000 nm) do NOT settle on standing and pass through filter paper. Suspension particles (> 1000 nm) SETTLE on standing and are STOPPED by filter paper. Colloids show the Tyndall effect strongly.

3. 'A solution contains 20 g of salt dissolved in 180 g of water. Calculate the concentration (mass by mass percentage).' Answer: Mass of solution = 20 + 180 = 200 g. Concentration = (20/200) × 100 = 10%.

4. 'How will you separate a mixture of oil and water?' Answer: Use a SEPARATING FUNNEL. Oil and water are immiscible. Pour the mixture into the funnel, let it settle. Water (denser) forms the lower layer. Drain water out through the stopcock. Oil remains in the funnel.

5. 'Explain why the path of light becomes visible when it passes through milk (a colloid).' Answer: Milk is a colloid with particles of size 1-1000 nm. These particles SCATTER light falling on them — this is the Tyndall effect. The scattered light reaches our eyes, making the beam path visible.

6. 'A student thinks that crystallisation and evaporation are the same. Explain the difference.' Answer: In evaporation, the liquid is heated until it completely vaporises, leaving the solid behind — but impurities also remain. In crystallisation, the solution is saturated and then SLOWLY cooled — pure crystals form while impurities stay in the solution. Crystals are PURER.