Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Stacking oranges in a grocery crate shows the 74% close-packing of fcc/hcp.”
Packing efficiency is the fraction of space filled by identical hard spheres in a lattice. fcc and hcp give π/√18 ≈ 0.74, bcc ≈ 0.68, simple cubic ≈ 0.52. Remember the value applies only to equal-sized spheres; ionic radii differences alter the effective packing.
Memory trick: 74% = fcc/hcp, 68% = bcc, 52% = sc.
“A few phosphorus atoms replace silicon in a crystal, supplying extra electrons that boost conductivity, like adding extra runners to speed a relay.”
Point defects include vacancies, interstitials, and substitutional atoms. Doping silicon with pentavalent phosphorus (donor) adds free electrons, while trivalent boron (acceptor) creates holes. Beware of swapping donor and acceptor roles or assuming any defect always reduces conductivity.
Memory trick: Donor adds e⁻, Acceptor adds holes.
“Adding table salt to water makes the water’s vapor pressure fall in direct proportion to the salt’s mole fraction.”
Raoult’s law: for an ideal solution with a non-volatile solute, Psolvent = Xsolvent·P°solvent. Since Xsolvent = 1-Xsolute, the pressure drop is proportional to the solute’s mole fraction. Trap: it applies only to ideal solutions and non-volatile solutes; volatile solutes need separate terms.
Memory trick: P = (1-Xsolute)·P° – ‘1-X’ is the solvent’s share.
“Sprinkling salt on ice makes it melt quicker, because the freezing point falls with more dissolved particles.”
Colligative properties (ΔTf, ΔTb, ΔPv, π) depend solely on the total number of solute particles, not their nature. ΔTf = i·Kf·m and ΔTb = i·Kb·m. Forgetting the van’t Hoff factor i for electrolytes leads to wrong numerical answers.
Memory trick: ΔT = i·K·m (count particles)
“A sealed soda bottle stays fizzy because the high CO₂ pressure forces the gas to stay dissolved.”
Henry's law: C = k_H·P, where C is the concentration of a gas in a liquid and P its partial pressure above the liquid. Valid for low pressures and constant temperature; k_H varies strongly with temperature, a common source of error.
Memory trick: C ∝ P (k_H constant)
“Dissolving 58 g NaCl in exactly 1 kg water gives a molality of 1 m, no matter the temperature.”
Molality (m) = moles of solute per kilogram of solvent. Because it uses mass, m is independent of temperature, unlike molarity which uses volume. Using solution volume instead of solvent mass is a frequent mistake.
m = mol/kg
Memory trick: m = n(solute)/kg solvent
“Dust clings to a car windshield, while water soaks into a sponge – surface vs bulk.”
Adsorption is the accumulation of molecules only on the outer surface of a solid; absorption involves the same species entering the interior of the material. Surface forces (van-der-Waals, chemisorption) limit adsorption, while absorption requires diffusion. In exams, never treat them as synonyms; the question will specify “surface” or “throughout”.
Memory trick: Adsorb = A (At surface); Absorb = B (Bulk).
“Sunlight becomes a visible beam when it passes through a glass of milk, a classic colloid.”
Colloids contain particles 1–1000 nm, large enough to scatter visible light, producing the Tyndall effect. True solutions have ≤1 nm particles and stay clear. In exams, a milky appearance or a visible light beam signals a colloid, not a true solution.
Memory trick: Colloid 1-1000 nm → Tyndall; ≤1 nm → clear.
“Baking limestone in a kiln turns it white – that’s calcination, while heating zinc sulfide in a furnace with oxygen “toasts” it – that’s roasting.”
Roasting: heat a sulphide ore in the presence of air/oxygen, producing metal oxide and SO₂. Calcination: heat a carbonate ore without a reducing/oxidising agent, releasing CO₂ and forming metal oxide. Exam trap – don’t mix the gas environment: air → roasting, no air → calcination.
Memory trick: Roast = O₂ present; Calcine = CO₂ released, O₂ absent.
“A double-lane road lets cars travel side-by-side, just like a double bond pulls atoms closer than a single lane.”
Bond order = (Nb – Na)/2, where Nb and Na are the numbers of electrons in bonding and antibonding molecular orbitals. It predicts bond strength and length: higher BO → stronger, shorter bond. BO = 0 means no bond; half-integer values are allowed.
Memory trick: BO = (Nb-Na)/2
“A water droplet clings to a charged glass rod because its bent shape gives a permanent dipole.”
Dipole moment μ is a vector equal to charge separation (μ = δ·r). Molecules like H₂O have a non-zero μ, while symmetric CO₂ has μ = 0 because bond dipoles cancel. Even with μ = 0 a molecule can be attracted to a charge via induced dipole, a common trap.
Memory trick: μ = charge × distance; symmetry → zero.
“Snapping a wooden stick needs energy – breaking each C–C bond costs its bond enthalpy.”
Bond enthalpy is the energy required to break one mole of a specific bond in the gas phase. Reaction ΔH ≈ Σ(bonds broken) – Σ(bonds formed). The trap: using average values for molecules with resonance or forgetting that breaking bonds is endothermic (+) and forming bonds is exothermic (−).
Memory trick: Break = +, Form = – (bond enthalpy)
“Like sugar crystals in tea stop dissolving once the cup is saturated, a sparingly soluble salt reaches its Ksp.”
Ksp is the equilibrium constant for MX(s) ⇌ M⁺ + X⁻ and equals [M⁺][X⁻] at saturation. The solid’s activity is taken as 1, so it does not appear in the expression. Forgetting to omit the solid term is a common exam error.
Memory trick: Ksp = [cation][anion]; solid omitted.
“A glass of water with sodium acetate tastes slightly basic, showing the acetate ion hydrolyzing water.”
Salts of a strong base and a weak acid hydrolyze: A⁻ + H₂O ⇌ HA + OH⁻, making the solution basic; the opposite case gives acidity. Always examine both ions – a salt is not automatically neutral.
Memory trick: Weak acid → basic; weak base → acidic.
“A pH meter measures the hydrogen electrode potential, which shifts with acid concentration via the Nernst equation.”
E = E° – (0.0592/n) log Q at 25 °C corrects the standard potential for actual ion activities. Changing concentrations alters the cell voltage accordingly. Trap: the log term is subtracted for reduction potentials; use the correct n (electrons transferred).
Memory trick: E = E° – (0.059/n)logQ (25 °C).
“The rust on an old bike chain is a galvanic cell formed between iron and atmospheric oxygen with moisture as electrolyte.”
Corrosion is the spontaneous oxidation of a metal in presence of an oxidizing agent, usually water and O₂, acting as an electrochemical cell. Iron acts as anode, losing electrons, while O₂ reduction occurs at cathodic sites. Trap: a protective coating breaks the cell, stopping rust.
Memory trick: Rust = unwanted galvanic cell.
“A fuel cell in a car’s power-train turns hydrogen gas into electricity just like a tiny battery that never runs out while fuel is supplied.”
A fuel cell converts the chemical energy of a fuel directly into electrical energy via redox reactions, without combustion, producing only water (or CO₂) as major by-product. It needs continuous fuel supply, unlike a one-time galvanic cell. In exams remember: fuel + cell = continuous electricity, not a finite battery.
Memory trick: Fuel + Cell = Fuel → Electricity, no flame
“Building a LEGO tower, the number of bricks that click in one move is the molecularity; how fast the tower grows is the reaction order.”
Molecularity is the count of reactant molecules in an elementary step and must be an integer (unimolecular, bimolecular, termolecular). Reaction order is obtained experimentally from the rate law and may be fractional, zero or integer. Trap: never equate order with molecularity for a multi-step mechanism.
Memory trick: Molecular = whole-number molecules; Order = observed rate, can be fraction.
“A radioactive carbon-14 sample loses half its activity in the same time no matter how much you start with, just like a first-order reaction.”
For a first-order reaction t½ = 0.693/k, independent of the initial concentration because ln([A]0/[A]) = kt gives ln2 when [A]=[A]0/2. The half-life stays constant throughout. Trap: applying this formula to zero- or second-order reactions yields wrong results.
t½ = 0.693/k
Memory trick: t½ = 0.693/k → 0.693 ≈ ln2, constant for first order.
“A ripe banana (soft, 3° carbon) easily splits into two pieces (carbocation) – SN1; a firm banana (1°) needs a swift, backside push – SN2.”
SN1 proceeds via a planar carbocation in two steps, so tertiary carbons, which can stabilize the positive charge, react fastest. SN2 is a single backside attack; less hindered primary carbons give the highest rate. Trap: a primary halide that can form a resonance-stabilized carbocation (e.g., benzyl) may still follow SN1.
Memory trick: 3° → SN1, 1° → SN2; polar protic = SN1, polar aprotic = SN2.
“A flat aromatic carbon is like a sealed window; a nucleophile can’t slip in from behind.”
In haloarenes the C–X bond is attached to an sp2 carbon, so backside attack for SN2 is geometrically impossible. An aryl carbocation needed for SN1 is extremely unstable, blocking that path. Only with strong ortho/para electron-withdrawing groups does SNAr (addition-elimination) work, and a benzyne route appears only under very harsh basic conditions.
Memory trick: sp2 blocks SN2; aryl⁺ unstable → no SN1; EW-G ortho/para needed for SNAr
“Your liver first turns drinking ethanol into acetaldehyde, then into acetic acid.”
Primary alcohols lose two H atoms: first to give an aldehyde, then to a carboxylic acid (strong oxidiser or excess). Secondary alcohols lose one H to form a ketone. Tertiary alcohols have no H on the carbon bearing –OH, so they resist oxidation. Beware: mild oxidants (PCC) stop primary alcohols at aldehyde; only strong/excess reagents give acid.
Memory trick: 1° → aldehyde → acid; 2° → ketone; 3° → no oxidation
“Phenol disinfectant feels sharper than rubbing alcohol because its –OH releases hydrogen more easily.”
Phenol is more acidic than aliphatic alcohols because the phenoxide ion formed after losing H is resonance-stabilised over the aromatic ring. This delocalisation lowers the energy of the conjugate base, giving a pKa around 10 versus 16-18 for alcohols. The myth that all –OH groups have similar acidity is false; resonance dominates.
Memory trick: Phenol: "Ph" sounds like "pH low" → stronger acid
“Whisking two pancake batter drops (carbonyls) together, the base links them at their α-hydrogens, forming a thin β-hydroxy “frosting” layer.”
Aldol condensation needs at least one carbonyl with an α-hydrogen. Base forms an enolate which attacks another carbonyl, giving a β-hydroxy carbonyl; on heating it dehydrates to an α,β-unsaturated carbonyl. Trap: if both partners lack α-H, no aldol forms.
Memory trick: Aldol = A (α-H) + d (dehydrate) → β-hydroxy → α,β-unsat.