BharatNotes
Overview
Chemistry in everyday life extends far beyond the laboratory — from the baking soda in your kitchen to the bleach that disinfects your water and the LPG that fuels your stove. Understanding solutions, colloids, and common chemical reactions is essential for UPSC Prelims, where questions frequently test practical applications of chemistry such as fire extinguisher types, water purification methods, and the chemistry behind household products.
Exam Strategy: Focus on the distinction between true solutions, colloids, and suspensions (especially the Tyndall effect as a distinguishing test), types of fire extinguishers and which fire class they address, and the chemistry of household substances (baking soda, bleach, hard water). Tables comparing properties are excellent for quick revision.
Types of Mixtures — Solutions, Colloids, and Suspensions
| Property | True Solution | Colloidal Solution | Suspension |
|---|---|---|---|
| Particle Size | <1 nm | 1–1000 nm | >1000 nm |
| Visibility | Particles not visible (even under microscope) | Visible under ultramicroscope | Visible to naked eye |
| Tyndall Effect | Not shown | Shown (scatters light) | May show (particles too large for true scattering) |
| Filterability | Passes through filter paper and semi-permeable membrane | Passes through filter paper but NOT through semi-permeable membrane | Does not pass through filter paper |
| Stability | Stable — does not settle | Relatively stable — does not settle easily | Unstable — settles on standing |
| Homogeneity | Homogeneous | Appears homogeneous but is heterogeneous | Heterogeneous |
| Examples | Salt water, sugar water, alcohol in water | Milk, fog, ink, blood, starch solution | Muddy water, chalk in water, flour in water |
Solutions — Concepts and Concentration
Types of Solutions
| Solute State | Solvent State | Example |
|---|---|---|
| Gas in Gas | Gas | Air (O2 in N2) |
| Gas in Liquid | Liquid | Soda water (CO2 in water) |
| Liquid in Liquid | Liquid | Alcohol in water |
| Solid in Liquid | Liquid | Salt in water |
| Solid in Solid | Solid | Alloys (brass — zinc in copper) |
| Gas in Solid | Solid | Hydrogen in palladium |
Solubility
- Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature
- Saturated solution — contains the maximum amount of dissolved solute at a given temperature
- Supersaturated solution — contains more solute than a saturated solution (unstable — solute precipitates on disturbance)
- Generally, solubility of solids in water increases with temperature; solubility of gases in water decreases with temperature
Concentration Units
| Unit | Definition | Formula |
|---|---|---|
| Molarity (M) | Moles of solute per litre of solution | M = moles of solute / volume of solution (L) |
| Molality (m) | Moles of solute per kilogram of solvent | m = moles of solute / mass of solvent (kg) |
| Mass percentage | Mass of solute per 100 g of solution | (mass of solute / mass of solution) x 100 |
| Parts per million (ppm) | Parts of solute per million parts of solution | Used for very dilute solutions (e.g., pollutants in water) |
Prelims Tip: Molarity changes with temperature (because volume changes), but molality does not (because mass is independent of temperature). This is why molality is preferred for precise scientific measurements.
Colloids — Properties and Types
Properties of Colloids
| Property | Description |
|---|---|
| Tyndall Effect | Scattering of light by colloidal particles, making the beam visible when passed through the colloid (e.g., light beam visible in fog or smoke) |
| Brownian Motion | Random, zigzag movement of colloidal particles caused by unequal bombardment by molecules of the dispersion medium; prevents settling and contributes to stability |
| Coagulation (Flocculation) | Settling of colloidal particles by addition of electrolytes, which neutralise the charge on colloidal particles (e.g., alum purifying muddy water) |
| Dialysis | Separation of colloidal particles from dissolved ions/molecules using a semi-permeable membrane (e.g., kidney dialysis removes waste from blood) |
| Electrophoresis | Movement of charged colloidal particles towards an electrode under an electric field |
Key Fact: The Tyndall effect is the simplest test to distinguish a colloid from a true solution. When a beam of light passes through a true solution (e.g., salt water), the path is not visible. When it passes through a colloid (e.g., milk diluted in water), the light beam becomes clearly visible due to scattering by colloidal particles.
Types of Colloids
| Type | Dispersed Phase | Dispersion Medium | Example |
|---|---|---|---|
| Sol | Solid | Liquid | Ink, paint, blood |
| Gel | Liquid | Solid | Butter, jelly, cheese |
| Emulsion | Liquid | Liquid | Milk, mayonnaise |
| Foam | Gas | Liquid | Shaving cream, whipped cream |
| Solid Foam | Gas | Solid | Pumice, sponge, bread |
| Aerosol (Liquid) | Liquid | Gas | Fog, mist, clouds |
| Aerosol (Solid) | Solid | Gas | Smoke, dust |
| Solid Sol | Solid | Solid | Ruby glass, gemstones |
Emulsions
An emulsion is a colloid of two immiscible liquids, one dispersed in the other as tiny droplets.
| Type | Dispersed Phase | Dispersion Medium | Examples |
|---|---|---|---|
| Oil-in-Water (O/W) | Oil droplets | Water | Milk, vanishing cream, mayonnaise |
| Water-in-Oil (W/O) | Water droplets | Oil | Butter, cold cream, petroleum |
Emulsifiers
Emulsifiers (or emulsifying agents) are substances that stabilise emulsions by reducing the surface tension between the two immiscible liquids.
| Emulsifier | Use |
|---|---|
| Soap/Detergent | Cleaning — emulsifies grease in water |
| Lecithin | Found in egg yolk — stabilises mayonnaise |
| Casein | Protein in milk — keeps fat droplets dispersed |
| Gum arabic | Stabilises food emulsions and inks |
Household Chemistry
Baking Soda vs Baking Powder
| Property | Baking Soda | Baking Powder |
|---|---|---|
| Chemical Name | Sodium bicarbonate (NaHCO3) | Mixture of baking soda + acid salt (cream of tartar) + starch |
| Reaction | Needs an external acid (vinegar, lemon, buttermilk) to produce CO2 | Self-contained — the acid salt reacts with baking soda when wet/heated |
| Use | Recipes with acidic ingredients | Recipes without acidic ingredients (cakes, biscuits) |
| How it works | NaHCO3 + acid produces CO2 gas, causing batter to rise | Double-acting — releases CO2 when mixed and again when heated |
Common Household Chemicals
| Substance | Chemical Name/Formula | Use |
|---|---|---|
| Vinegar | Acetic acid (CH3COOH), 5–8% solution | Cooking, pickling, cleaning |
| Bleach | Sodium hypochlorite (NaOCl) | Disinfection, whitening clothes, water purification |
| Washing Soda | Sodium carbonate (Na2CO3) | Softening hard water, cleaning agent |
| Caustic Soda | Sodium hydroxide (NaOH) | Soap making, drain cleaner |
| Plaster of Paris | Calcium sulphate hemihydrate (CaSO4.1/2H2O) | Casts for broken bones, moulds, construction |
| Slaked Lime | Calcium hydroxide (Ca(OH)2) | Whitewashing, water treatment |
| Quick Lime | Calcium oxide (CaO) | Cement manufacture, disinfectant |
| Epsom Salt | Magnesium sulphate (MgSO4.7H2O) | Pain relief (bath soak), laxative |
| Blue Vitriol | Copper sulphate (CuSO4.5H2O) | Fungicide, electroplating |
Exam Tip: A frequently tested distinction: washing soda (Na2CO3) is used for softening water and cleaning, while baking soda (NaHCO3) is used in cooking and as an antacid. Caustic soda (NaOH) is a strong base used in soap making — do not confuse these three "sodas."
Hard Water and Soft Water
| Property | Hard Water | Soft Water |
|---|---|---|
| Cause | Contains dissolved calcium and magnesium salts (Ca2+, Mg2+ ions) | Free from excess calcium/magnesium salts |
| Lather | Does not form lather easily with soap | Forms lather easily |
| Types of Hardness | Temporary (bicarbonates — removed by boiling) and Permanent (sulphates/chlorides — not removed by boiling) | N/A |
| Removal Methods | Boiling (temporary), washing soda, ion exchange, distillation, RO | Already soft |
Water Purification Methods
| Method | Principle | Removes |
|---|---|---|
| Alum (Phitkari) | Coagulation — neutralises charges on colloidal impurities, causing them to settle | Suspended particles, turbidity |
| Chlorination | Chlorine (Cl2 or NaOCl) kills bacteria by disrupting cell processes | Bacteria, viruses |
| Boiling | Heat kills pathogens at 100 degrees C | Bacteria, viruses, parasites |
| RO (Reverse Osmosis) | Forces water through a semi-permeable membrane under pressure | Dissolved salts, heavy metals, bacteria, viruses |
| UV Purification | UV light damages DNA of microorganisms, preventing reproduction | Bacteria, viruses |
| Distillation | Boiling and condensing — removes all dissolved and suspended impurities | All impurities (produces purest water) |
Prelims Tip: RO removes dissolved salts (TDS) and heavy metals, while UV kills microorganisms but does not remove dissolved impurities. Many household purifiers combine RO + UV + UF for comprehensive purification.
Fire Extinguishers
Classes of Fire
| Fire Class | Fuel Type | Examples |
|---|---|---|
| Class A | Ordinary combustibles | Wood, paper, cloth, rubber, plastic |
| Class B | Flammable liquids | Petrol, oil, grease, paint, solvents |
| Class C | Energised electrical equipment | Wiring, circuit panels, motors, transformers |
| Class D | Combustible metals | Magnesium, titanium, sodium, lithium |
| Class K | Cooking oils and fats | Vegetable oil, animal fat, commercial kitchen fires |
Types of Fire Extinguishers
| Extinguisher Type | Agent | How It Works | Suitable For |
|---|---|---|---|
| Water | Water | Cools the burning material below ignition temperature | Class A only (NEVER on electrical or oil fires) |
| CO2 | Carbon dioxide | Displaces oxygen, suffocating the fire; leaves no residue | Class B, Class C (electrical fires) |
| Foam (AFFF) | Aqueous film-forming foam | Smothers fire by forming a blanket over the fuel, cutting off oxygen | Class A, Class B |
| Dry Chemical Powder (DCP) | Monoammonium phosphate or sodium bicarbonate | Coats fuel with powder, interrupting the chemical chain reaction | Class A, B, C (multi-purpose) |
| Wet Chemical | Potassium acetate | Creates a soapy foam blanket that cools and seals the burning surface | Class K (cooking oils) |
| Dry Powder (special) | Sodium chloride, graphite, or copper-based | Smothers metal fires without reacting | Class D (metal fires) |
Key Fact: Water must NEVER be used on electrical fires (risk of electrocution) or oil/grease fires (water vaporises instantly, causing a violent steam explosion that spreads burning oil). CO2 extinguishers are preferred for electrical fires because CO2 is non-conductive.
LPG — Liquefied Petroleum Gas
| Property | Details |
|---|---|
| Composition | Primarily propane (C3H8) and butane (C4H10), with small amounts of isobutane and propylene |
| Natural odour | Odourless and colourless |
| Added odorant | Ethyl mercaptan (ethanethiol, C2H5SH) — a pungent-smelling sulphur compound added for leak detection |
| Calorific value | Approximately 50 MJ/kg (high energy per unit mass) |
| Storage | Stored as liquid under moderate pressure in sealed cylinders |
| Safety | Heavier than air — leaking LPG settles at floor level (fire/explosion risk); always check for leaks before lighting |
Prelims Tip: The distinctive smell of cooking gas (LPG) is NOT natural — it is artificially added ethyl mercaptan (ethanethiol). This is done purely as a safety measure so that leaks can be detected quickly by smell.
Matches — Chemistry of Ignition
Safety Match Composition
| Component | Location | Chemical Composition |
|---|---|---|
| Match head | Tip of the matchstick | Potassium chlorate (KClO3) — oxidiser, antimony trisulphide (Sb2S3) — fuel, sulphur, glue |
| Striking surface | Side of the matchbox | Red phosphorus, powdered glass (friction agent), glue |
How it works: Friction between the match head and the striking surface converts a small amount of red phosphorus to white phosphorus. White phosphorus ignites at a very low temperature (~30 degrees C in air), producing enough heat to decompose KClO3, which releases oxygen. This oxygen ignites the antimony trisulphide and sulphur, which in turn light the matchstick.
Batteries in Daily Life
| Battery Type | Chemistry | Voltage | Rechargeable? | Common Use |
|---|---|---|---|---|
| Zinc-Carbon (Dry Cell) | Zinc anode, carbon cathode, MnO2 + NH4Cl paste | 1.5 V | No | Torches, remote controls, clocks |
| Alkaline Battery | Zinc anode, MnO2 cathode, KOH electrolyte | 1.5 V | No (some rechargeable variants exist) | Toys, cameras, portable devices |
| Lithium-Ion (Li-ion) | Lithium cobalt oxide cathode, graphite anode, Li-salt electrolyte | 3.7 V | Yes | Smartphones, laptops, electric vehicles |
| Lead-Acid | Lead anode, lead dioxide cathode, H2SO4 electrolyte | 2 V per cell (12 V battery = 6 cells) | Yes | Car batteries, UPS systems, inverters |
| Nickel-Metal Hydride (NiMH) | Metal hydride anode, nickel oxyhydroxide cathode | 1.2 V | Yes | Rechargeable AA/AAA batteries, hybrid cars |
Adhesives, Paints, and Everyday Products
| Product | Key Chemistry | How It Works |
|---|---|---|
| Fevicol / White Glue | Polyvinyl acetate (PVA) emulsion | Water evaporates, leaving a strong polymer bond |
| Super Glue | Cyanoacrylate | Polymerises rapidly in presence of moisture (sets in seconds) |
| Paints | Pigment (colour) + binder (resin) + solvent (thinner) | Solvent evaporates, binder hardens, pigment provides colour |
| Varnish | Resin dissolved in solvent (no pigment) | Dries to form a hard, transparent protective film |
| Soap | Sodium/potassium salts of fatty acids | Hydrophilic head bonds with water, hydrophobic tail bonds with grease — forms micelles that lift dirt |
| Detergent | Sodium salts of long-chain sulphonic acids | Similar to soap but works in hard water (does not form scum with Ca2+/Mg2+) |
Key Distinction: Soap forms insoluble precipitates (scum) with calcium and magnesium ions in hard water, making it less effective. Detergents do not form scum and work equally well in hard and soft water — this is why detergents have largely replaced soap for laundry.
Frequently Asked Questions (Prelims Pattern)
| Question | Answer |
|---|---|
| What is the Tyndall effect? | Scattering of light by colloidal particles, making the beam visible |
| What is Brownian motion? | Random zigzag motion of colloidal particles due to molecular bombardment |
| How does alum purify water? | Coagulation — neutralises charges on suspended particles, causing them to clump and settle |
| What chemical is added to LPG for leak detection? | Ethyl mercaptan (ethanethiol) |
| Which extinguisher is used for electrical fires? | CO2 extinguisher (non-conductive) |
| Why should water not be used on oil fires? | Water vaporises instantly, causing violent splattering of burning oil |
| What is the chemical name of baking soda? | Sodium bicarbonate (NaHCO3) |
| What causes hardness in water? | Dissolved calcium and magnesium salts |
| What is the principle of RO purification? | Forcing water through a semi-permeable membrane under pressure to remove dissolved salts |
| What type of battery is used in smartphones? | Lithium-ion (Li-ion) battery |
| What is the difference between soap and detergent? | Detergents work in hard water (no scum); soaps form scum with Ca2+/Mg2+ |
Key Terms for Quick Revision
| Term | Meaning |
|---|---|
| True Solution | Homogeneous mixture with particle size <1 nm; does not show Tyndall effect |
| Colloid | Heterogeneous mixture with particle size 1–1000 nm; shows Tyndall effect |
| Suspension | Heterogeneous mixture with particle size >1000 nm; settles on standing |
| Tyndall Effect | Scattering of light by colloidal particles making the beam visible |
| Brownian Motion | Random movement of colloidal particles due to bombardment by dispersion medium molecules |
| Coagulation | Settling of colloidal particles by addition of electrolytes |
| Emulsion | Colloid of two immiscible liquids (oil-in-water or water-in-oil) |
| Emulsifier | Substance that stabilises an emulsion (e.g., lecithin in mayonnaise) |
| Molarity | Moles of solute per litre of solution |
| Molality | Moles of solute per kilogram of solvent |
| Hard Water | Water containing dissolved Ca2+ and Mg2+ ions |
| Ethyl Mercaptan | Odorant (C2H5SH) added to LPG for leak detection |
| Micelle | Spherical arrangement of soap/detergent molecules around a grease droplet in water |
Sources: Chemistry LibreTexts; Wikipedia (Liquefied petroleum gas, Ethanethiol, Optical fiber); Cleveland Clinic (endocrine system); NFPA (fire extinguisher types); ELGAS (LPG safety); CliffsNotes (colloids); GeeksforGeeks (Tyndall effect).
