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
“Water’s bent shape makes its O–H bonds point to different sides, so the molecule pulls water toward a glass rod.”
A molecule is polar when the vector sum of its bond dipoles is non-zero. Polar bonds alone are insufficient; a symmetric geometry (e.g., CO₂) cancels them. In exams, always examine both bond polarity and molecular shape before declaring a molecule non-polar.
Memory trick: Polar = dipole + asymmetry
“When you pump a bicycle tire, squeezing the air makes the gauge needle jump up.”
At constant temperature, pressure and volume of a fixed amount of gas are inversely related (PV = constant). Halving the volume doubles the pressure. The common trap: temperature must stay unchanged; mixing with Charles’s law gives wrong results.
PV = constant
Memory trick: Boyle: B = volume ↓ → pressure ↑
“A hot-air balloon swells as the burner heats the air, showing V ∝ T at constant pressure.”
At a fixed pressure, an ideal gas’s volume grows linearly with absolute temperature (Kelvin). Hence V1/T1 = V2/T2. The exam trap: forgetting to convert °C to K or assuming pressure changes gives a wrong answer.
V/T = constant
Memory trick: V∝T → V/T constant; keep P same, use K.
“Liquefying oxygen in a cryogenic tank shows ideal gas law breaks down at low temperature and high pressure.”
Real gases deviate from PV = nRT when molecules are close together; their finite size and intermolecular attractions matter, especially at high pressure or low temperature. The nuance: near the critical region you need van der Waals correction.
Memory trick: Real gas = R(e)al → high P, low T
“When a match burns, the flame heats the air, but the match’s chemical system loses heat, so ΔH is negative.”
ΔH is the heat exchanged by the system at constant pressure (ΔH = qp). Exothermic: system releases heat → ΔH < 0; Endothermic: system absorbs heat → ΔH > 0. The sign always refers to the system, not the surroundings – a frequent exam trap.
ΔH
Memory trick: ΔH negative = heat out of system; think “H = Hot out.”
“Ice melting into water spreads the molecules, increasing disorder – the rise in ΔS you can see.”
Entropy measures the degree of molecular disorder. A spontaneous process at constant T and P must have ΔS positive enough to make ΔG negative. Beware the trap of ignoring the temperature factor; ΔS can be negative yet ΔG still negative if TΔS is small.
ΔS
Memory trick: More ways → higher ΔS
“A battery delivering current converts chemical energy to electrical work, the ΔG of the cell reaction.”
ΔG = ΔH − TΔS predicts spontaneity at constant temperature and pressure; ΔG < 0 means the reaction proceeds spontaneously. A common mistake is using a plus sign before TΔS or neglecting the temperature dependence when ΔH and ΔS have opposite signs.
ΔG = ΔH − TΔS
Memory trick: ΔG = ΔH minus T × ΔS
“Making water directly from H₂ and O₂ or via H₂O₂ gives the same ΔH – like two routes to the same destination.”
Hess’s law follows from enthalpy being a state function: the total ΔH is independent of the reaction path. Use it by adding and reversing known equations, but remember to change the sign when a reaction is reversed and keep equations balanced.
Memory trick: State function → path irrelevant
“Like a busy highway where cars enter and exit at the same rate, forward and reverse reactions keep the concentration traffic steady.”
In a reversible reaction the forward and reverse rates become equal. When this happens the concentrations of all species stay constant, although both reactions continue. The exam trap: students often think the concentrations become equal, not just the rates.
Memory trick: Equal rates → steady traffic, not equal amounts.
“Adding water to a sponge makes it squeeze out air to keep its shape; a system shifts to oppose any disturbance in the same way.”
If concentration, pressure or temperature of a system at equilibrium is changed, the equilibrium moves to reduce that change. The direction follows the side that consumes the added component. Trap: temperature changes affect the position according to the reaction’s enthalpy, not just concentration.
Memory trick: Add → system consumes; heat acts like reactant or product.
“A lemon’s juice (pH≈2) feels sharp, while household soap (pH≈12) feels slippery, showing the ten-fold log scale of acidity.”
pH = −log[H⁺]; a decrease of 1 pH unit means [H⁺] rises tenfold. Below 7 the solution is acidic, above 7 basic, and at 25 °C neutral is 7. Trap: many forget that lower pH = higher [H⁺] and that neutrality shifts with temperature.
pH = −log[H⁺]
Memory trick: pH down → H⁺ up ×10
“Adding HCl to water makes the solution conduct electricity instantly, unlike acetic acid, which conducts only weakly.”
A strong acid dissociates completely (virtually 100 %) in dilute water, giving a large Ka (≫1). A weak acid ionises only partly, with Ka ≪1, but no fixed Ka cut-off defines weakness. Trap: don’t rely on a single Ka like 10⁻³; weakness is about degree of ionisation, not an arbitrary number.
Memory trick: Strong = full split; weak = holds back
“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.