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
“A buffer is a seesaw with a weight on each side; a small added load only tilts it a little, keeping the balance (pH) steady.”
A buffer contains a weak acid and its conjugate base (or weak base and its conjugate acid). It neutralises added H⁺ or OH⁻, limiting pH change, described by the Henderson-Hasselbalch equation. Trap: ignoring the finite buffer capacity; large additions break the buffer.
Memory trick: Acid + its salt = buffer.
“Adding extra salt to lemonade reduces its sourness, just as a common ion added to a solution suppresses further dissociation.”
When an ion already present in an equilibrium is added, the equilibrium shifts toward the side that consumes that ion, decreasing the degree of ionisation (common-ion effect). Trap: assuming solubility rises with added common ion; it actually falls.
Memory trick: Common ion → shift left, less dissociation.
“When a battery powers a torch, zinc atoms lose electrons (oxidation) while copper ions gain them (reduction).”
Oxidation is the loss of electrons; reduction is the gain of electrons. OIL RIG helps remember this. A common trap is to mix up the direction of electron flow, especially when oxidation numbers increase but electrons are actually lost.
Memory trick: Ox = Lose, Red = Gain
“In bleach, chlorine pulls electrons from stains, acting as the oxidising agent.”
An oxidising agent accepts electrons and is itself reduced; a reducing agent donates electrons and is oxidised. The name describes the opposite change of the species. Beware assuming the agent’s own oxidation state changes in the same direction as its name.
Memory trick: Agent does opposite of its label
“A battery powering a flashlight is a galvanic cell, while charging that same battery uses an electrolytic cell.”
In a galvanic cell the redox reaction is spontaneous, so electrons flow and we get electric current. In an electrolytic cell the reaction is non-spontaneous; an external source pushes electrons opposite to the natural direction. Exam trap: ΔG is negative (E°cell>0) for galvanic and positive (E°cell<0) for electrolytic.
Memory trick: Galvanic gives, electrolytic needs power.
“In a zinc-copper battery, zinc (anode) loses electrons while copper (cathode) gains them.”
Oxidation always occurs at the anode and reduction at the cathode, irrespective of cell type. In a galvanic cell the anode is negative, while in an electrolytic cell it becomes positive because the external source reverses polarity. Trap: don’t mix electrode sign with reaction direction.
Memory trick: A nOx = anode, CRed = cathode.
“When you drop a copper strip into a solution of silver nitrate, silver plates the copper because Ag⁺ is higher in the series.”
The electrochemical series orders metals by standard electrode potential; a more positive E° means a stronger tendency to be reduced. Metals with more negative potentials lose electrons easily and act as good anodes. Trap: series is listed from most negative to most positive potentials.
Memory trick: More negative = more willing to give electrons.
“Electroplating a jewelry piece: the thickness of gold depends on how long you run the current.”
First Faraday law: mass deposited ∝ total charge (Q = It). Second law: the same charge deposits equivalent moles of electrons (1 F = 96 485 C) regardless of the ion. Exam trap: include the ion’s valency n in m = (Q·M)/(n·F).
Memory trick: Mass ∝ Charge; same Q → same electrons.
“When you stir sugar into tea, the faster the spoon moves, the quicker the sweetness spreads.”
Rate = change in concentration per unit time (-d[A]/dt or d[P]/dt). It is the slope of the concentration-time curve. Beware mixing average rate over an interval with the instantaneous rate needed for kinetic equations.
Memory trick: Rate = ΔC/Δt → steeper slope, faster.
“When you double the amount of yeast, the dough may rise faster or slower depending on the reaction order.”
Order is the exponent of a reactant’s concentration in the rate law, found experimentally, not from the balanced equation. For complex steps, molecularity ≠ order. Do not assume stoichiometric coefficients give the order.
Memory trick: Order = power of [reactant] in rate law.
“A ball needs a push to roll over a hill before it can speed down the other side.”
Activation energy (Ea) is the minimum energy barrier that reactants must overcome to form the activated complex. It appears in k = A e^(-Ea/RT). Higher Ea slows a reaction, but temperature can compensate.
Eₐ
Memory trick: Ea = height of the energy hill.
“Enzyme in your saliva speeds up starch breakdown without being eaten up.”
A catalyst offers an alternative pathway with lower Ea, increasing the rate while remaining unchanged after the reaction. It does not alter the equilibrium constant. Remember: it speeds, not shifts equilibrium.
Memory trick: Catalyst = lower hill, unchanged after.
“Your phone battery drains faster on a hot day because reactions run quicker.”
Increasing temperature raises the rate constant k (Arrhenius: k ∝ e^(-Ea/RT)). A common rule of thumb is roughly double the rate for each 10 °C rise, though the exact factor depends on Ea.
Memory trick: ≈ double rate each +10 °C.
“A carbon chain is like a string of paperclips, each clip (atom) firmly holding the next without slipping.”
Carbon atoms form strong C–C single bonds (bond energy ~350 kJ mol⁻¹) and have four valence orbitals, allowing long chains and rings. This stability makes catenation a hallmark of organic chemistry. Exam trap: remember that catenation drops down the group; elements heavier than carbon have weaker M–M bonds and form shorter chains.
Memory trick: C = Strong, Stable, Single bond
“A smartphone’s camera module decides its photo ability, just as a functional group decides a molecule’s reactions.”
A functional group is a specific atom or group (-OH, -COOH, -NH2…) that governs the chemical behavior of the whole molecule. It defines the class and reactivity. Do not mix-up similar groups; -CHO is aldehyde, -COOH is acid.
Memory trick: Group = reaction guide
“Identical twins wearing different outfits – same DNA (formula) but different looks (structure).”
Isomers share the same molecular formula but differ in connectivity (structural) or spatial arrangement (stereoisomers). This leads to distinct physical and chemical properties. In stereochemistry, missing a chiral centre loses marks.
Memory trick: Same formula, different fashion
“Passing a basketball (electron pair) to a teammate is a nucleophile; the teammate who wants the ball is the electrophile.”
An electrophile is electron-poor and seeks a pair of electrons; a nucleophile is electron-rich and donates a pair to any electrophilic centre, not only carbon. Many bases are nucleophiles, but nucleophilicity depends on charge, solvent and hardness. Trap: don’t equate every base with a nucleophile or limit nucleophiles to carbon attacks.
Memory trick: E-electron-poor, N-electron-rich
“A blended smoothie combines fruits into one flavor, just as resonance blends several structures into one real molecule.”
Resonance delocalises π-electrons over two or more atoms, represented by contributing structures; the real molecule is a hybrid that is more stable than any single form. Keep total electrons constant across all contributors.
Memory trick: Resonance = electron sharing
“A crowded party stabilizes a guest; more alkyl groups around a carbocation spread the positive charge.”
Carbocation stability rises with alkyl substitution because hyperconjugation and inductive effects disperse the positive charge: 3° > 2° > 1° > methyl. Allylic and benzylic carbocations are even more stable than simple tertiary ones.
Memory trick: More alkyl = calmer cation
“Gasoline that runs a scooter is mainly alkanes, while ethene’s C=C double bond makes it a key plastic precursor.”
Alkanes contain only C–C single bonds (saturated) and follow CnH2n+2. Alkenes have one C=C double bond (unsaturated) with formula CnH2n. Alkynes have one C≡C triple bond with formula CnH2n-2. The suffix –ane, –ene, –yne tells the bond type; a common trap is forgetting the hydrogen count when a multiple bond is present.
Memory trick: -ane single, -ene double, -yne triple
“A fully inflated balloon (saturated hydrocarbon) has no extra space, while a deflated one (unsaturated) has a gap representing a double bond.”
Saturated hydrocarbons have only single C–C bonds, giving the maximum hydrogen count (CₙH₂ₙ₊₂). Unsaturated hydrocarbons contain one or more C=C or C≡C bonds, reducing hydrogen to CₙH₂ₙ or CₙH₂ₙ₋₂. Multiple bonds increase chemical reactivity.
Memory trick: No double = full; double/triple = missing H.
“A candle’s flame spreads faster on the thicker wick, just as H adds to the carbon already bearing more H atoms in HX addition.”
In electrophilic addition of HX to an unsymmetrical alkene, H attaches to the carbon that already has more hydrogens (the less-substituted carbon). This leaves a more substituted, more stable carbocation on the other carbon, so X ends up on that carbon. The common exam trap is to reverse the direction; remember the rule only flips in peroxide-initiated radical addition of HBr.
Memory trick: H to H-rich, X to C-rich (more substituted).
“Benzene is like a perfectly balanced round table where six friends (π electrons) share seats equally, making it stable and unwilling to let anyone sit extra (add).”
Benzene is a planar ring with six delocalised π electrons, satisfying Huckel’s 4n+2 rule (n=1). This aromatic stabilization forces electrophilic substitution rather than addition, preserving the aromatic sextet. Loss of planarity or electron count destroys aromaticity.
Memory trick: Aromatic → substitutes, not adds.
“A gas stove ignites methane (an alkane) and the flame shows carbon turning into invisible CO₂ while hydrogen becomes water droplets on the glass.”
Complete combustion of a hydrocarbon in excess O₂ gives CO₂ and H₂O with heat: CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O + ΔH. Incomplete combustion yields CO or soot. The O₂ amount follows from balancing the equation.
Memory trick: Fuel + O₂ → CO₂ + H₂O + energy.