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General Chemistry

The Hook: The Element That Changed Two World Wars

In 1918, a German chemist named Fritz Haber received the Nobel Prize for a process that pulls nitrogen out of thin air and turns it into ammonia — the Haber process. Ammonia made this way became the raw material for nitrogen fertilisers that feed billions; the same man also directed Germany's poison gas programme in World War I. One discovery fed billions; the other killed thousands. Chemistry is never just formulas on a page — it is the science that decides what we eat, breathe, build and even fight with. And for your exam, it is one of the most scoring areas of General Knowledge, because the facts are fixed: an acid is an acid, pH 7 is neutral, and iron rusts — yesterday, today, and on your exam day.

Learning Objectives

By the end of this chapter you will be able to:

1. Classify matter into elements, compounds and mixtures, and use atomic structure terms correctly.
2. Read the periodic table — groups, periods, and the behaviour of key elements.
3. Distinguish acids, bases and salts, and use the pH scale confidently.
4. Explain everyday chemistry: rusting, soaps, baking soda, fuels, alloys.
5. Spot and defeat the examiner's favourite traps in chemistry questions.

1. Matter and Its Building Blocks

Everything around you is matter — anything that has mass and occupies space. Matter exists mainly as solid, liquid and gas (plasma and Bose-Einstein condensate are the exotic extras). The smallest unit of an element that keeps its identity is the atom; the smallest unit of a compound that can exist freely is the molecule.

An atom has a dense nucleus (protons + neutrons) with electrons revolving around it. Three numbers define an element:

• Atomic number (Z) = number of protons = number of electrons (in a neutral atom).
• Mass number (A) = protons + neutrons.
• Isotopes = same Z, different A (e.g., hydrogen's three isotopes: protium, deuterium, tritium). Isobars = same A, different Z. Isotones = same number of neutrons.

🧠 Active Recall: Close the book. What is the difference between atomic number and mass number? What do isotopes share, and what do they differ in?

Mnemonic 1 — Isotopes vs Isobars: "Isotopes — same Protons" (the 'p' in tope reminds you of protons). Isobars share mass number instead.

2. The Periodic Table — Chemistry's Map

Dmitri Mendeleev (1869) arranged elements by atomic mass and famously left gaps, predicting elements like gallium and germanium before they were found. The modern periodic law (Moseley, 1913) says properties are a periodic function of atomic number, not mass.

• 7 periods (horizontal rows), 18 groups (vertical columns).
• Group 1: alkali metals (soft, reactive, stored in kerosene). Group 2: alkaline earth metals.
• Group 17: halogens (most reactive non-metals; fluorine is the most reactive of all).
• Group 18: noble gases (inert; helium, neon, argon…).
• Metalloids (B, Si, Ge, As, Sb, Te) sit on the staircase line between metals and non-metals.
• Lanthanides and actinides are the two f-block rows placed below the main table.

Trends to remember: metallic character increases down a group and decreases left to right across a period. Atomic size follows the same pattern.

🧠 Active Recall: Which group holds the noble gases? Why was Moseley's atomic-number basis better than Mendeleev's atomic-mass basis?

Mnemonic 2 — Group 1 in order: "Little Naughty Kids Rb Csilly Frogs" → Li, Na, K, Rb, Cs, Fr.

3. Acids, Bases and Salts — The Examiner's Favourite

Acids taste sour, turn blue litmus red, and release H⁺ ions in water (Arrhenius concept). Bases taste bitter, feel soapy, turn red litmus blue, and release OH⁻ ions. Salts form when an acid neutralises a base: acid + base → salt + water.

The pH scale (0–14, introduced by Sørensen) measures acidity:

• pH < 7 → acidic (lower = stronger acid)
• pH = 7 → neutral (pure water)
• pH > 7 → basic (higher = stronger base)

High-yield everyday values: gastric juice ≈ 1.5–2, lemon juice ≈ 2.2, vinegar ≈ 2.5–3, blood ≈ 7.4 (slightly basic), milk of magnesia ≈ 10.5.

Key compounds you must know cold:

🧠 Active Recall: Without looking — what is the formula of washing soda, and how many water molecules does gypsum carry? Is blood acidic or basic?

Mnemonic 3 — POP vs Gypsum: "POP is Poor in water — only Half" → CaSO₄·½H₂O; gypsum is the full 2H₂O.

4. Metals, Non-Metals and Everyday Chemistry

Reactivity series (top = most reactive): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. Metals above hydrogen displace it from dilute acids; gold and platinum resist almost everything (aqua regia — 3 parts HCl : 1 part HNO₃ — dissolves gold).

Rusting of iron needs both oxygen and moisture — a classic trap. Prevention: painting, galvanisation (zinc coating), oiling.

Important alloys:

• Brass = Cu + Zn | Bronze = Cu + Sn | Solder = Pb + Sn
• Stainless steel = Fe + Cr (+ Ni) | Duralumin = Al + Cu + Mg + Mn (aircraft)
• Amalgam = any alloy of mercury

Soaps and detergents: Soap is the sodium/potassium salt of a fatty acid; it fails in hard water (forms scum with Ca²⁺/Mg²⁺). Synthetic detergents work even in hard water.

Gases in the news and in exams: laughing gas = N₂O; marsh gas = CH₄; water gas = CO + H₂; producer gas = CO + N₂; LPG is mainly propane + butane; CNG is mainly methane.

🧠 Active Recall: Why does galvanised iron resist rust even when scratched? Which gas is called laughing gas?

5. Carbon, Fuels and a Few Special Facts

Carbon shows allotropy — same element, different forms: diamond (hardest natural substance, non-conductor), graphite (soft, conducts electricity, used in pencils and as a lubricant), fullerenes (C₆₀, "buckyballs"). Charcoal, coke and coal are impure forms.

Fuels: the quality of petrol is rated by octane number; diesel by cetane number. Incomplete combustion of carbon fuels produces poisonous carbon monoxide (CO), which binds haemoglobin far more strongly than oxygen — this is why coal stoves in closed rooms kill.

A few one-liner facts examiners love:

• Heavy water = D₂O (used as moderator in nuclear reactors).
• Dry ice = solid CO₂.
• Chile saltpetre = NaNO₃; Indian saltpetre = KNO₃.
• The chemical used in photography's fixing bath = sodium thiosulphate (hypo).
• Ozone (O₃) layer absorbs harmful UV radiation; CFCs deplete it.

🧠 Active Recall: Diamond and graphite are both carbon — why does only graphite conduct electricity? What is dry ice?

🔗 Static-Dynamic Link

Chemistry stays in the news quietly. India's National Green Hydrogen Mission (approved 2023) targets making green hydrogen — produced by splitting water using renewable electricity — a major fuel; the underlying chemistry (electrolysis: 2H₂O → 2H₂ + O₂) is pure static GK. Similarly, debates on ethanol blending in petrol and lithium mining (lithium is the lightest metal, Group 1) all trace back to this chapter's basics.

⚠️ Caution: Examiner's Trap

Trap 1 — "लगभग सही कथन" (Almost-true statement): "Rusting of iron requires only oxygen." Sounds plausible, but rusting needs both oxygen and water. Examiners drop one condition to catch skimmers. Similarly: "Plaster of Paris is CaSO₄·2H₂O" — wrong by half a water molecule; that is gypsum.

Trap 2 — "नकारात्मक वाक्यांश" (Hidden negative): "Which of the following is NOT an allotrope of carbon?" with options diamond, graphite, fullerene, coke. Students who miss the NOT pick diamond instantly. Coke is an impure form, not a true allotrope. Circle the NOT before reading options.

Trap 3 — "समान नाम/विपरीत कार्य-कारण" (Similar names / reversed logic): Baking soda vs washing soda vs caustic soda — three "sodas", three different compounds (NaHCO₃, Na₂CO₃·10H₂O, NaOH). Another favourite: "Hard water is hard because it is difficult to freeze" — reversed causation; it is "hard" because of dissolved Ca/Mg salts that resist lathering with soap.

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

📊 High-Yield Facts Table

📅 Revision Planner

• After 2 days: Blurt-test yourself — write from memory all 10 compounds from the acids-bases-salts table with formulas, then check. Redo the high-yield table aloud.
• Weekly: Take a blank page and redraw the periodic table's group map (Groups 1, 2, 17, 18 + metalloids) and the reactivity series. Attempt 25 practice MCQs and log every error by trap type.
• Monthly (blurting): Write everything you remember about (a) acids/bases/salts, (b) alloys, (c) gases, in 10 minutes flat. Compare with this chapter; whatever you missed goes onto a flashcard for the next cycle.

Chemistry rewards the reviser, not the crammer. Two days from now, this chapter should still be alive in your head — make sure it is.