सामान्य रसायन · पाठ — सामान्य ज्ञान · 10वीं स्तर

General Chemistry — the science hiding in your kitchen

◆ Open your kitchen shelf. The white powder that makes pakoras fluffy is baking soda, NaHCO3. The salt in your dal is sodium chloride, NaCl. The curd in the fridge is sour because of lactic acid, and the vinegar in the pickle jar is a 5–8% solution of acetic acid. You have been doing chemistry every single day without knowing its name. In this chapter we give names to what you already see — and that is exactly what exam questions ask.

What you will master today (Learning Objectives)

Matter: states, changes of state, and how to separate mixtures

The atom: who discovered what, atomic number, mass number, isotopes, valency

The Periodic Table: from Döbereiner's triads to Moseley's atomic number

Chemical reactions: the five types, oxidation and reduction, corrosion, rancidity

Acids, bases and salts: indicators, pH, and the famous salts of daily life

Metals, non-metals and carbon compounds: properties, exceptions, alloys, fuels, soaps

The traps examiners set — and how to walk around them

1. Matter and Its States

Anything that has mass and occupies space is matter. It exists as solid, liquid or gas. Particles of matter have spaces between them, keep moving, and attract each other.

Temperature in kelvin = temperature in °C + 273. Ice melts at 273 K (0 °C); water boils at 373 K (100 °C).

Sublimation: a solid changes directly into vapour without becoming liquid — camphor, naphthalene, ammonium chloride. Solid carbon dioxide is called dry ice.

Evaporation causes cooling. That is why water in an earthen pot (matka) stays cool. Evaporation increases with surface area, temperature and wind speed, and decreases with humidity.

Two more states: plasma (the glow of fluorescent tubes and stars) and Bose–Einstein condensate (formed at extremely low temperatures; named after Satyendra Nath Bose and Albert Einstein).

🧠 Active Recall: What is 25 °C in kelvin? (298 K) Name two substances that sublime. (Camphor, ammonium chloride)

2. Pure Substances and Mixtures

Element: made of only one kind of atom (iron, oxygen).
Compound: two or more elements combined chemically in a fixed ratio (water, H2O). Its properties differ from its elements.
Mixture: substances simply mixed, with no fixed ratio. Air, sea water and brass are mixtures. Alloys are homogeneous mixtures, not compounds.

Solution, colloid and suspension: in a true solution (salt or sugar in water) particles are too small to scatter light. Colloids such as milk and fog scatter a beam of light — this is the Tyndall effect.

Separation techniques — exam favourites:

Cream from milk → Centrifugation

Dyes in ink → Chromatography

Oil (kerosene) and water → Separating funnel

Salt and ammonium chloride → Sublimation

Gases of air → Fractional distillation of liquid air

Mnemonic: "Cream Circles, Colours Climb, Camphor Climbs Away" — Cream by Centrifuge, Colours by Chromatography, sublimable solids by Sublimation.

🧠 Active Recall: Is brass an element, compound or mixture? (Homogeneous mixture) Which colloid in your house shows the Tyndall effect? (Milk)

3. Inside the Atom

John Dalton said matter is made of tiny indivisible atoms. Later, scientists broke the atom open:

J.J. Thomson — discovered the electron; his model looked like a Christmas pudding (electrons embedded in a positive sphere).

E. Goldstein — discovered canal rays (1886), which led to the proton.

Ernest Rutherford — alpha particles on a thin gold foil; most of the atom is empty, with a tiny positive nucleus at the centre.

Niels Bohr — electrons move in fixed orbits (shells K, L, M, N).

James Chadwick — discovered the neutron (1932).

Mnemonic: "Thomson Tagged Electrons, Goldstein Got Protons, Chadwick Chose Neutrons."

Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons. Neutrons = A − Z.

Maximum electrons in a shell = 2n² (K = 2, L = 8, M = 18), but the outermost shell holds at most 8.

Valency: the combining capacity. Sodium (2, 8, 1) loses 1 electron → valency 1. Chlorine (2, 8, 7) gains 1 → valency 1. Aluminium (2, 8, 3) → valency 3.

Isotopes: same atomic number, different mass number — protium, deuterium and tritium (hydrogen); carbon-12 and carbon-14; chlorine-35 and chlorine-37. Uses: an isotope of uranium is fuel in nuclear reactors, an isotope of cobalt treats cancer, an isotope of iodine treats goitre.

Isobars: different elements, same mass number — argon-40 and calcium-40.

Mnemonic: "IsotoPes keep the same Protons; IsobArs keep the same mAss."

One atomic mass unit (u) = one-twelfth of the mass of one carbon-12 atom.

🧠 Active Recall: Sodium has Z = 11, A = 23. How many neutrons? (12) Who discovered the neutron? (Chadwick)

4. The Periodic Table — a story of four scientists

Döbereiner (triads): groups of three similar elements — Li, Na, K; Ca, Sr, Ba; Cl, Br, I.

Newlands (law of octaves): every eighth element resembles the first, like musical notes — worked only up to calcium.

Mendeleev: arranged elements by increasing atomic mass and left gaps. He predicted eka-boron (scandium), eka-aluminium (gallium) and eka-silicon (germanium).

Moseley (1913): atomic number is the true basis. The Modern Periodic Law says properties of elements are a periodic function of their atomic number.

The Modern Periodic Table has 18 groups (vertical) and 7 periods (horizontal). Across a period, atomic size decreases; down a group, it increases. Noble gases are in group 18.

How to place an element: number of shells = period; valence electrons give the group (1 → group 1, 2 → group 2, 3 to 8 → group 10 + valence electrons). Chlorine (2, 8, 7) → period 3, group 17.

🧠 Active Recall: Eka-aluminium is which element? (Gallium) How many groups are there? (18)

5. Chemical Reactions

A chemical change makes a new substance — rusting, burning, curdling of milk. Melting and dissolving are physical changes.

Five types to know:

Combination: CaO + H2O → Ca(OH)2 (quicklime to slaked lime; releases heat).

Decomposition: CaCO3 → CaO + CO2 on heating; 2AgCl → 2Ag + Cl2 in sunlight (used in black-and-white photography); electrolysis of water gives hydrogen and oxygen in 2 : 1 by volume.

Displacement: Fe + CuSO4 → FeSO4 + Cu (blue solution turns light green).

Double displacement: Na2SO4 + BaCl2 → BaSO4 (white precipitate) + 2NaCl.

Redox: CuO + H2 → Cu + H2O. Hydrogen gains oxygen (oxidised); copper oxide loses oxygen (reduced).

Exothermic reactions give out heat (respiration, burning). Endothermic reactions absorb heat (decomposition of calcium carbonate).

Corrosion: iron rusts when both air and water are present; rust is hydrated iron(III) oxide. Silver turns black (silver sulphide); copper turns green (basic copper carbonate).

Rancidity: oils and fats get oxidised and smell bad. Chips packets are flushed with nitrogen to prevent it.

Whitewash: slaked lime reacts with carbon dioxide in air to form calcium carbonate, which gives the shiny finish.

🧠 Active Recall: In CuO + H2 → Cu + H2O, which substance is reduced? (CuO) Which gas is filled in chips packets? (Nitrogen)

6. Acids, Bases and Salts

Acids are sour and turn blue litmus red. Bases are bitter, soapy and turn red litmus blue. Litmus comes from lichen.

Indicators: phenolphthalein — colourless in acid, pink in base. Methyl orange — red in acid, yellow in base. Turmeric turns reddish-brown in base. Onion, vanilla and clove oil are olfactory (smell) indicators.

pH scale: 0 to 14. Below 7 acidic, 7 neutral, above 7 basic. Our body works within pH 7.0 to 7.8. Tooth decay starts when the pH of the mouth falls below 5.5.

Natural acids (very high-yield):

Vinegar → Acetic acid

Orange, Lemon → Citric acid

Tamarind → Tartaric acid

Tomato → Oxalic acid

Sour milk (curd) → Lactic acid

Ant sting, Nettle sting → Methanoic acid

Mnemonic: "Tamarind-Tartaric, TOmato-Oxalic" — the two T-fruits are the ones students mix up.

A bee or ant sting is acidic, so a mild base like baking soda gives relief. Milk of magnesia (magnesium hydroxide) is an antacid. Always add acid to water slowly, never water to acid.

Salts of daily life:

Common salt — NaCl

Baking soda — NaHCO3 (antacid, baking; baking powder = baking soda + tartaric acid)

Washing soda — Na2CO3·10H2O (removes permanent hardness of water)

Bleaching powder — CaOCl2 (chlorine on dry slaked lime; disinfects drinking water)

Plaster of Paris — CaSO4·½H2O (gypsum heated at 373 K; sets hard with water)

Gypsum — CaSO4·2H2O

Blue vitriol — CuSO4·5H2O (blue because of water of crystallisation)

Chlor-alkali process: electrolysis of brine gives sodium hydroxide, chlorine and hydrogen.

Mnemonic for hydrates: "Gypsum drinks 2 glasses, Plaster of Paris only half; Blue vitriol 5, Washing soda 10."

🧠 Active Recall: Which acid is in tamarind? (Tartaric) What is the formula of Plaster of Paris? (CaSO4·½H2O)

7. Metals and Non-metals

Metals are lustrous, malleable, ductile, sonorous and good conductors. Non-metals are generally the opposite. Exams love the exceptions:

Mercury — the only liquid metal. Bromine — the liquid non-metal.

Gallium and caesium melt on your palm.

Sodium, potassium and lithium are soft enough to be cut with a knife; sodium and potassium are stored in kerosene. White phosphorus is stored in water.

Iodine is a lustrous non-metal. Graphite is a non-metal that conducts electricity. Diamond is the hardest natural substance.

Silver and copper are the best conductors of heat; lead and mercury are poor conductors. Gold and silver are the most malleable and ductile.

Reactivity series (NCERT): K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au.

Mnemonic: "Please Send Cats, Monkeys And Zebras In Large Hired Cages Made of Silver and Gold."

Metals above hydrogen give hydrogen with dilute acids; copper, mercury, silver and gold do not. Metal oxides are basic; non-metal oxides are acidic; Al2O3 and ZnO are amphoteric. Aqua regia (3 parts conc. HCl : 1 part conc. HNO3) dissolves gold.

Extraction: highly reactive metals (Na, Mg, Al) by electrolysis; middle metals (Zn, Fe, Pb) by reduction with carbon; low metals like mercury by heating alone (cinnabar, HgS). The thermit reaction (Fe2O3 + 2Al) is used to join railway tracks.

Alloys: Brass = copper + zinc; Bronze = copper + tin; Solder = lead + tin; Stainless steel = iron + nickel + chromium. 22-carat gold = 22 parts gold + 2 parts copper or silver.

Protection from rusting: painting, oiling, galvanising (zinc coat), alloying. Anodising forms a thick oxide layer on aluminium.

🧠 Active Recall: Which non-metal conducts electricity? (Graphite) Bronze is copper plus what? (Tin)

8. Carbon and Its Compounds

Carbon has valency 4 (tetravalency) and can link with other carbon atoms in long chains and rings (catenation). That is why it forms so many compounds.

Allotropes: diamond (hardest), graphite (conductor) and fullerene C-60 (football shape).

Hydrocarbons: alkanes CnH2n+2 (methane CH4, ethane C2H6), alkenes CnH2n (ethene C2H4), alkynes CnH2n−2 (ethyne C2H2). Successive members of a homologous series differ by CH2 (14 u).

Saturated hydrocarbons burn with a clean blue flame; unsaturated ones give a yellow sooty flame. Methane is a major component of biogas and CNG.

Ethanol (C2H5OH): used in medicines; industrial ethanol mixed with poisonous methanol is denatured alcohol.

Ethanoic acid (CH3COOH): 5–8% solution is vinegar. Pure ethanoic acid melts at 290 K and freezes in cold winters — glacial acetic acid.

Key reactions: esterification (ethanol + ethanoic acid → sweet-smelling ester), saponification (ester + NaOH → soap), hydrogenation (vegetable oil + hydrogen with nickel catalyst → vanaspati).

Soaps are sodium or potassium salts of long-chain carboxylic acids. In hard water they form scum with calcium and magnesium salts; detergents work even in hard water.

🧠 Active Recall: What is the formula of ethene? (C2H4) Which catalyst is used for hydrogenation of oils? (Nickel)

⚠️ Caution: Examiner's Trap

Trap 1: Isotopes vs Isobars
Type: Similar Name
Trap question: "Argon-40 and calcium-40 are isotopes." — False. They are isobars (same mass number, different elements).
Smart move: isotoPes = same Protons; isobArs = same mAss.

Trap 2: Who is oxidised?
Type: Reverse Causation
Trap question: In CuO + H2 → Cu + H2O, "copper oxide is oxidised." — False. Copper oxide loses oxygen, so it is reduced; hydrogen is oxidised.
Smart move: Whoever GAINS oxygen is oxidised; whoever LOSES oxygen is reduced.

Trap 3: The "all" statements
Type: Almost Correct Statement / Negative Phrasing
Trap question: "All metals are solids at room temperature." or "No non-metal conducts electricity." — Both false (mercury; graphite).
Smart move: The moment you read "all", "no" or "only", think of the exception list: mercury, bromine, gallium, iodine, graphite, diamond.

Trap 4: Gypsum vs Plaster of Paris
Type: Almost Correct Statement
Trap question: "Plaster of Paris is CaSO4·2H2O." — False. That is gypsum; Plaster of Paris is CaSO4·½H2O.

🔁 Linguistic Bridge: In Hindi we say this, in English we say that

परमाणु क्रमांक — Atomic number

द्रव्यमान संख्या — Mass number

समस्थानिक / समभारिक — Isotopes / Isobars

संयोजकता — Valency

ऊर्ध्वपातन — Sublimation

अपकेंद्रण / वर्णलेखन — Centrifugation / Chromatography

ऑक्सीकरण / अपचयन — Oxidation / Reduction

संक्षारण / विकृतगंधिता — Corrosion / Rancidity

खाने का सोडा / धावन सोडा — Baking soda / Washing soda

विरंजक चूर्ण — Bleaching powder

बिना बुझा चूना / बुझा चूना — Quicklime / Slaked lime

नीला थोथा / हरा कसीस — Blue vitriol / Green vitriol

यशदलेपन — Galvanisation

मिश्रधातु / अयस्क — Alloy / Ore

अम्लराज — Aqua regia

🔗 Static-Dynamic Link

The chlor-alkali process you read about here is the basis of large industries that make caustic soda and chlorine, and chlorine is used to disinfect drinking water. CNG (mainly methane) is the same fuel that runs many city buses and autos — when a question links "clean fuel for vehicles" to a gas, the answer is methane, not butane (LPG).

📊 High-Yield Facts & Figures (Quick Glance)

📋 Summary Table

Topic | Must-remember point
Matter | K = °C + 273; sublimation; evaporation cools
Mixtures | Centrifuge-cream, chromatography-ink, sublimation-NH4Cl
Atom | Thomson-electron, Goldstein-canal rays, Chadwick-neutron
Atomic numbers | Isotopes same Z; isobars same A
Periodic table | Moseley, atomic number, 18 groups, 7 periods
Reactions | 5 types; gain of oxygen = oxidation
Acids & bases | pH 7 neutral; natural acids list; indicator colours
Salts | NaHCO3, Na2CO3·10H2O, CaOCl2, CaSO4·½H2O
Metals | Exceptions list; reactivity series; alloys
Carbon | Catenation, allotropes, CnH2n+2 / CnH2n / CnH2n−2, soap

📅 Revision Planner: Three-Level Blurting

🔴 After 2 days: On a blank page, write the reactivity series, the natural acids list and the formulas of five salts. Check and correct.

🟡 Weekly (Day 7): From memory, draw the atom timeline (Dalton → Thomson → Goldstein → Rutherford → Bohr → Chadwick) and the periodic table timeline (Döbereiner → Newlands → Mendeleev → Moseley). Write all four traps.

🟢 Monthly (Day 30): Solve one practice set without notes. For every wrong answer, write the trap type. Then blurt the whole chapter in 10 minutes.

Review this chapter after 2 days. Chemistry for exams is not about memorising every formula — it is about knowing the core facts that keep coming back, and the exceptions that catch everyone else. You now know both.