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.
1,200 fundamentals
“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.
“In a lab, adding concentrated NaOH to formaldehyde (no α-H) instantly yields methanol and formate, just like the base that saponifies fats.”
Aldehydes that lack α-hydrogen atoms undergo the Cannizzaro reaction in strong base. OH⁻ adds to the carbonyl, forming a tetrahedral alkoxide; a hydride is transferred to a second aldehyde, producing an alcohol and a carboxylate ion. The trap: if any α-hydrogen is present the substrate follows aldol condensation, not Cannizzaro.
Memory trick: No α-H → Cannizzaro; α-H present → aldol.
“Adding HCl to an amine is like a lock catching a key, forming an ammonium salt that stays in solution.”
Amines act as bases because the nitrogen lone pair can accept a proton, forming R₃NH⁺. Electron-donating alkyl groups raise basicity (+I effect), while aromatic resonance withdraws electron density, lowering it. The exam trap: compare pKa of the conjugate acid, not pKb, and note tertiary aliphatic amines are strongest, aromatic amines are weakest.
Memory trick: More alkyl = stronger base (↑+I)
“Aryl diazonium salts are like chilled fireworks; kept at 0-5 °C they sit still, but a gentle warm-up makes them burst, losing N₂ and forming a new aromatic product.”
Diazonium salts are prepared at 0–5 °C in strong acid (NaNO₂/HCl) and are stable only in this cold, acidic medium. On warming they lose N₂; the resulting product depends on the added reagent (CuCl → chlorobenzene, CuBr → bromobenzene, H₂O → phenol). Assuming they always give phenol or bromide on heating is wrong—Sandmeyer reagents are required.
Memory trick: Diazo = Cold, N₂ flies; add Cu-X for X-substituted benzene.
“Proline is like a hinge in a paperclip; its nitrogen is part of a ring, not a free –NH₂.”
The 20 standard amino acids have a central α-carbon with –COOH and –NH₂, but proline’s –NH is secondary, forming a pyrrolidine ring, so it lacks a free –NH₂. Remember this when drawing structures or counting –NH₂ groups. Exam trap: assuming every amino acid has a primary –NH₂ gives a wrong structure for proline.
Memory trick: Pro-line = ‘pro’ in a ring, no free NH₂.
“DNA is the library, and each nucleotide is a book containing a code for life.”
Nucleic acids are polymers of nucleotides, each consisting of a phosphate, a pentose sugar and a nitrogenous base. DNA’s deoxyribose lacks a 2′-OH, making it stable; RNA’s ribose has a 2′-OH, rendering it more reactive. Exams test the sugar difference and its impact on stability.
Memory trick: DNA = Deoxy, no 2′ OH; RNA = Ribose, has 2′ OH.
“When you bake a cake, the flour you add is like essential amino acids – you must supply them, the body can’t make them.”
Essential amino acids must be obtained from the diet; the body synthesises all other amino acids. For adults the nine are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine and lysine. A common trap is counting arginine as essential or forgetting histidine.
Memory trick: PVT TIM H L K
“When you wash a greasy spoon in hard water, the soap forms scum while detergent keeps it sparkling.”
Soap is the sodium or potassium salt of a fatty acid. In hard water Ca²⁺/Mg²⁺ precipitate with soap forming insoluble scum, reducing its cleaning power. Detergents contain sulfonate or sulfate groups that stay soluble, so they work even with Ca²⁺/Mg²⁺. Exam trap: assuming any surfactant works in hard water; remember only detergents avoid scum.
Memory trick: Soap + Ca = scum; Detergent ≠ scum
“A soap molecule is like a tiny magnet, one pole pulls water, the other pulls oil, lifting dirt into the rinse.”
A soap molecule has a polar head that hydrogen-bonds with water and a non-polar tail that dissolves grease. Above the critical micelle concentration the tails cluster, trapping oil droplets inside micelles that are washed away. Trap: forgetting the need for micelle formation; cleaning fails below CMC.
Memory trick: Head H2O, tail oil → micelle
“After a spicy curry, you sip an antacid tablet that fizzles, neutralizing the burning acid in your stomach.”
Antacids are basic salts such as Mg(OH)₂, CaCO₃ or Al(OH)₃ that react with excess H⁺ in gastric juice: H⁺ + OH⁻ → H₂O, raising pH and relieving acidity. They act locally in the stomach and do not alter systemic pH. Trap: confusing antacids with acid-suppressing drugs that act by inhibiting secretion.
Memory trick: Base + H⁺ → H₂O, less burn
“A penicillin capsule stops a bacterial infection but does nothing against the flu virus you caught.”
Antibiotics are agents that either kill bacteria (bactericidal) or stop their growth (bacteriostatic) by targeting cell-wall synthesis, protein synthesis, etc. Viruses lack these structures, so antibiotics have no effect on viral infections. Trap: prescribing antibiotics for colds; resistance develops.
Memory trick: Bacteria → kill; Virus → no effect
“In the Haber plant, iron filings act like a busy highway toll, letting nitrogen and hydrogen zip through to form ammonia faster.”
Iron (often promoted with K₂O, Al₂O₃) provides a surface where N₂ and H₂ adsorb, lowering the activation energy for N≡N bond breakage, thus increasing the rate without being consumed. Catalysts speed up reaching equilibrium but do not shift the equilibrium position. Trap: assuming a catalyst changes the equilibrium composition is wrong.
Memory trick: Catalyst = speed-up, not equilibrium shift.
“Squeezing lemon into water releases H⁺ – an Arrhenius acid that also donates a proton, satisfying the Brønsted definition.”
Arrhenius acids increase [H⁺] in water; Brønsted acids are proton donors; Lewis acids accept electron pairs. Every Arrhenius acid is a Brønsted acid, but many Brønsted acids work outside aqueous media. Lewis is the broadest definition, yet not all Lewis acids are Brønsted.
Memory trick: Arrhenius = H⁺ in water; Brønsted = proton donor; Lewis = electron-pair acceptor.