Chemistry — Matter, Periodic Table, Acids/Bases & Metals

Chemistry is the science of what things are MADE of and how they REACT. SSC exams test everyday chemistry: why bread rises (CO₂ from baking soda + acid), why iron rusts (Fe + O₂ + H₂O), what makes vinegar sour (acetic acid, CH₃COOH), and what pH means for your stomach (1–2), blood (7.35–7.45), and swimming pool (7.2–7.8). Master the pH table, periodic table shortcuts, reactivity series, alloys, and common-name-to-chemical mappings — that is 80% of SSC chemistry.

MATTER & STATES

Everything around us is matter — it occupies space and has mass. Ordinary matter exists in three familiar states: Solid (fixed shape, fixed volume, very strong intermolecular forces — ice, iron, diamond), Liquid (no fixed shape but fixed volume, moderate forces — water, mercury, alcohol), and Gas (no fixed shape, no fixed volume, very weak forces — air, steam, LPG). Two more states are tested in SSC: Plasma (ionised gas at extremely high temperature — found in stars, lightning, neon signs) and Bose-Einstein Condensate / BEC (gas cooled to near absolute zero where atoms behave as one quantum unit — the "5th state"). In a gas, particles move fastest; in a solid, slowest. Temperature does NOT change during a change of state — all incoming heat goes into breaking intermolecular bonds (called latent heat).

When matter changes state, the process has a specific name. Melting (solid → liquid), Freezing (liquid → solid), Vaporisation/Boiling (liquid → gas), Condensation (gas → liquid), Sublimation (solid → gas directly — no liquid stage), and Deposition (gas → solid directly). Sublimation is a favourite SSC topic: dry ice (CO₂), camphor, naphthalene (moth balls), and iodine crystals all sublime. The boiling point of water decreases at high altitude (less atmospheric pressure), which is why food takes longer to cook in the mountains.

ATOMS & MOLECULES

The atom is the smallest particle of an element that retains its chemical identity. Understanding evolved through four landmark models: Dalton (1808) — atoms are solid, indivisible billiard-ball spheres; Thomson (1897) — discovered the electron; proposed "plum pudding model" (negative electrons embedded in positive dough). Rutherford (1911) — gold foil experiment showed atoms have a tiny, dense, positive nucleus with electrons orbiting it (nuclear model). Bohr (1913) — electrons orbit in fixed energy levels (shells: K, L, M, N), and can jump between shells by emitting/absorbing light. The atom number (protons) defines the element; mass number = protons + neutrons. Electrons count = protons (in a neutral atom).

Valency is the combining capacity of an atom — how many bonds it can form. H and Na have valency 1; O and Mg have valency 2; N and Al have valency 3; C and Si have valency 4. Atoms bond to achieve a full outer electron shell (stable configuration). Two main types: Ionic bond — one atom donates electrons to another, forming oppositely charged ions (e.g., Na⁺ and Cl⁻ bond to form NaCl — table salt). Covalent bond — atoms share electrons (e.g., H₂O: oxygen shares two electrons, one with each H; CO₂: C shares with each O). Ionic compounds have high melting points and conduct electricity in solution. Covalent compounds generally do not conduct electricity and have lower melting points.

PERIODIC TABLE

Mendeleev (1869) arranged 63 known elements in increasing order of atomic mass — the first Periodic Table. He left gaps for undiscovered elements and predicted their properties (later confirmed with Gallium and Germanium). Henry Moseley (1913) discovered that atomic number (proton count) is the true basis of periodicity — the Modern Periodic Law: properties of elements are periodic functions of their atomic number. Today's PT has 118 elements in 18 groups (vertical columns) and 7 periods (horizontal rows). Element 118 = Oganesson (Og), the most recently confirmed element, synthesised in 2002.

Periods (horizontal rows): as you move left → right, atomic number increases by 1; atomic radius decreases (more protons pull electrons closer); electronegativity increases; metallic character decreases. Period 1 has 2 elements (H, He); Period 2 has 8; Period 6 has 32. Groups (vertical columns): elements in the same group have the same number of valence electrons and similar chemical properties. Group 1 = Alkali Metals (Li, Na, K, Rb, Cs, Fr) — most reactive metals; Group 2 = Alkaline Earth Metals (Be, Mg, Ca); Group 17 = Halogens (F, Cl, Br, I) — most reactive non-metals; Group 18 = Noble Gases (He, Ne, Ar, Kr, Xe, Rn) — least reactive of all. Groups 3–12 = Transition Metals (Fe, Cu, Zn, Ag, Au, Hg, Cr, Ni).

ACIDS, BASES & SALTS

The pH scale (0–14) measures how acidic or basic a solution is. pH stands for "power of Hydrogen" and measures the concentration of H⁺ ions. pH 0–6.9 = Acidic (higher H⁺ concentration); pH 7 = Neutral (pure water at 25°C); pH 7.1–14 = Basic/Alkaline (higher OH⁻ concentration). Every 1-unit drop in pH means the solution is 10× more acidic — pH 1 is 100× more acidic than pH 3. Indicators: Red litmus turns blue in base; Blue litmus turns red in acid. Phenolphthalein is colourless in acid and pink in base. Why pH matters for SSC: stomach (pH 1–2 for protein digestion by pepsin), blood (pH 7.35–7.45 — outside 6.8–7.8 is fatal), rainwater (pH ≈ 5.6 due to dissolved CO₂), acid rain (pH < 5.6 due to SO₂ and NO₂ from pollution).

Acids

Acids produce H⁺ (hydrogen ions) in water, taste sour, and turn blue litmus red. Strong acids (HCl, H₂SO₄, HNO₃) fully ionise in water; weak acids (CH₃COOH, H₂CO₃, citric acid) only partially ionise. Key reactions: Acid + Metal → Salt + H₂↑ (hydrogen gas); Acid + Base → Salt + Water (neutralisation). H₂SO₄ is called the "King of Chemicals" — it is the most widely produced industrial chemical in the world, used in batteries, fertilisers, and refining. Concentrated H₂SO₄ is a strong dehydrating agent (it absorbs water violently — always add acid to water, never water to acid).

Bases

Bases produce OH⁻ (hydroxide ions) in water, taste bitter, feel soapy, and turn red litmus blue. A base that dissolves in water is called an alkali (e.g., NaOH, KOH, Ca(OH)₂, NH₄OH). Strong bases (NaOH, KOH) fully ionise; weak bases (NH₄OH, Mg(OH)₂) partially ionise. Antacids are mild bases: Milk of Magnesia (Mg(OH)₂), baking soda (NaHCO₃), and sodium bicarbonate tablets neutralise excess stomach acid. The relief you feel after taking an antacid is acid-base chemistry: Base + HCl (stomach acid) → Salt + H₂O (+ CO₂ if a carbonate).

Neutralisation: Acid + Base → Salt + Water. This reaction releases heat (exothermic). Classic SSC examples: HCl + NaOH → NaCl + H₂O (stomach acid + antacid = salt water + relief); NaHCO₃ + HCl → NaCl + H₂O + CO₂ (the burp after antacid tablet!). Adding lime (Ca(OH)₂) to acidic soil — a farmer neutralising soil acidity — is the same reaction. If a factory discharges acidic waste, it is treated with lime before release into rivers.

Important Salts

METALS & NON-METALS

Metals (~90 elements) are generally: lustrous (shiny when polished), good conductors of heat and electricity, malleable (beaten into sheets), ductile (drawn into wires), solid at room temperature — exception: Mercury (Hg) is a liquid metal. Gold is the most malleable metal (1g of Au can be beaten into a 1 m² sheet). Silver is the best electrical conductor; copper is used commercially because it is far cheaper. Non-metals (~17 elements) are generally: poor conductors (exception: graphite conducts electricity), brittle when solid, non-lustrous (exception: iodine has a metallic sheen), can be solid/liquid/gas. Key exception: Carbon (non-metal) as diamond is the hardest natural substance, and as graphite it conducts electricity. Bromine is the only liquid non-metal at room temperature.

The Reactivity Series (Activity Series) ranks metals from most reactive (top) to least reactive (bottom). The more reactive a metal, the more easily it loses electrons and reacts with water/acids. Potassium (K) ignites spontaneously in air and explodes with cold water (K + H₂O → KOH + H₂ + fire). Sodium (Na) reacts vigorously with cold water (floats, skids, melts into ball). Gold (Au) does not react even with strong acids — it needs aqua regia (3 parts HCl + 1 part HNO₃, a fuming yellow liquid) to dissolve. A more reactive metal displaces a less reactive metal from its salt solution in water (displacement reaction).

Alloys

An alloy is a homogeneous mixture of two or more metals (or a metal + non-metal) designed to have superior properties — alloys are NOT pure compounds. Steel (Fe + C) is harder and stronger than pure iron. Bronze (Cu + Sn) is harder than pure copper. Stainless steel resists rusting because chromium (Cr) forms a thin, adherent oxide layer (Cr₂O₃) that prevents oxygen and water from reaching the underlying iron. Alloys are the foundation of construction, transportation, electronics, and medicine.

Corrosion & Rusting

Corrosion is the gradual deterioration of metals by reaction with environmental agents (O₂, H₂O, CO₂, acids). Only iron undergoes rusting specifically: Fe + O₂ + H₂O → Fe₂O₃·xH₂O (hydrated ferric oxide = rust, reddish-brown flaky layer). Rusting requires BOTH oxygen AND water — iron rusted in dry air or pure water alone does NOT rust. NaCl and CO₂ accelerate rusting. Aluminium also oxidises but the Al₂O₃ layer formed is thin, transparent, hard, and self-sealing — it does NOT flake off, so aluminium does not visibly corrode. Similarly, copper forms a green patina (Cu(OH)₂·CuCO₃) that protects it. Corrosion prevention: Galvanisation (coating iron with Zn); Painting/Varnishing; Electroplating (Cr, Ni); Alloying (stainless steel); Cathodic protection (used in ships and pipelines — connect iron to a more reactive metal like Zn or Mg which corrodes first instead).

CHEMICAL REACTIONS

A chemical reaction converts reactants into products with new chemical properties. Law of Conservation of Mass (Lavoisier): total mass of reactants = total mass of products (atoms are rearranged, never created or destroyed). Reactions involve breaking old bonds (energy in) and forming new bonds (energy out). If net energy is released = exothermic (burning, rusting, neutralisation, respiration); if net energy is absorbed = endothermic (photosynthesis, cooking an egg, melting ice, electrolysis of water). SSC tests five main reaction types plus redox.

Oxidation is the loss of electrons (or gain of oxygen, or loss of hydrogen). Reduction is the gain of electrons (or loss of oxygen, or gain of hydrogen). These ALWAYS occur simultaneously — you cannot have one without the other; together they are called a redox reaction. The substance that gets oxidised (loses electrons) is called the reducing agent (it reduces the other). The substance that gets reduced (gains electrons) is the oxidising agent. Example: In rusting, Fe → Fe³⁺ + 3e⁻ (iron is OXIDISED = is the reducing agent); O₂ + 4e⁻ → 2O²⁻ (oxygen is REDUCED = is the oxidising agent). In the thermite reaction (2Al + Fe₂O₃ → Al₂O₃ + 2Fe), Al is oxidised; Fe₂O₃ is reduced — the iron produced is molten hot (used to weld railway tracks).

IMPORTANT EVERYDAY COMPOUNDS

SSC loves asking "What is the chemical name of washing soda?" or "Give the formula of Plaster of Paris." The key trick: soda compounds all contain sodium (Na); lime compounds contain calcium (Ca); potash = potassium (K). Always distinguish between the anhydrous (no water), monohydrate, dihydrate, and decahydrate forms of the same compound — they have different common names.

COLLOIDS — THE IN-BETWEEN STATE

Most things around us aren't pure solids or pure liquids — they're mixtures where tiny particles of one substance are scattered through another. When these particles are microscopic (between 1–1000 nanometres), we get a colloid. Milk is a colloid (fat droplets in water). Fog is a colloid (water droplets in air). Smoke is a colloid (solid particles in air). Colloids show two unique effects: Tyndall Effect (when you shine light through fog or dusty air and see the beam — that's light scattering off colloidal particles) and Brownian Motion (particles zig-zag randomly under a microscope due to constant molecular bombardment).

BATTERIES & ELECTROCHEMICAL CELLS

A battery converts chemical energy to electrical energy through a chemical reaction. Every battery has an anode (negative terminal, where oxidation occurs), a cathode (positive terminal, where reduction occurs), and an electrolyte (the medium that allows ions to flow between them). Primary batteries can't be recharged (your TV remote's alkaline cell). Secondary batteries can be recharged (your car battery, phone battery). This is directly tested in SSC as 'match battery type to its use'.

HARD WATER, HEAVY WATER & WATER CHEMISTRY

Not all water is the same. Hard water is water that contains dissolved calcium and magnesium salts — it doesn't lather well with soap (that's why you need more soap in some areas). Heavy water sounds exotic but it's just water made with a heavier form of hydrogen called deuterium. It's important as a moderator in nuclear reactors. These are small but directly tested topics in SSC.

FLAME, COAL & COMBUSTION

When something burns, it doesn't all burn at once or equally. A candle flame has three distinct zones — each at a different temperature. The OUTERMOST zone is the hottest (complete combustion), which is why goldsmiths heat gold with the outer blue flame of a blowpipe. Coal itself comes in four varieties with increasing carbon content — peat is the weakest, anthracite is the purest and hottest-burning.

INDUSTRIAL COMPOUNDS — NAME, FORMULA & USE

SSC consistently asks 'what is the chemical name of baking soda?' or 'which compound is called washing soda?' This table covers the 25 most tested industrial compounds by their common name, chemical name, and formula. Learn the common name → chemical name link — that's what gets asked.

QUICK-FIRE FACTS FOR SSC

COMMON EXAM TRAPS

1. Galvanisation = ZINC, not tin. Coating with tin = tinning. Zinc provides sacrificial protection even if scratched.
2. Only iron RUSTS. Aluminium oxidises but the Al₂O₃ coat is protective and invisible. "Aluminium rusts" is FALSE.
3. Baking soda ≠ Washing soda. NaHCO₃ (baking soda) is edible; Na₂CO₃·10H₂O (washing soda) is for laundry. Do not mix them up.
4. Most abundant element in Earth's CRUST is Oxygen (46%), NOT Silicon. Silicon is 2nd (28%).
5. Mercury is a METAL (liquid). Bromine is a NON-METAL (liquid). Both are liquid at room temperature — but they are different categories.
6. Mendeleev used ATOMIC MASS; Moseley (Modern PT) uses ATOMIC NUMBER. SSC almost always tests this distinction.
7. Chlorophyll contains MAGNESIUM; Haemoglobin contains IRON. Do not swap these — a classic trap.
8. pH below 7 = ACIDIC; above 7 = BASIC/ALKALINE. Many students accidentally say lower pH is more basic.
9. Plaster of Paris ABSORBS water to set (CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O / gypsum). It does NOT lose water.
10. LEO GER: Oxidation = Lose Electrons (not necessarily gain oxygen); Reduction = Gain Electrons (not necessarily lose oxygen). Oxidation does not require oxygen.